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XMODEM/YMODEM Protocol Reference

                                  - 1 -

                 A compendium of documents describing the
                            XMODEM and YMODEM
                         File Transfer Protocols
                   This document was formatted 9-11-86.
                         Edited by Chuck Forsberg
                 Please distribute as widely as possible.
                       Questions to Chuck Forsberg
                           Omen Technology Inc
                        17505-V Sauvie Island Road
                          Portland Oregon 97231
                           Voice: 503-621-3406
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                    UUCP: ...!tektronix!reed!omen!caf
                                  - 2 -
      Here are some definitions which reflect the current vernacular
      in the computer media. The attempt here is identify the file
      transfer protocol rather than specific programs.
      XMODEM refers to the original 1979 file transfer etiquette
       introduced by Ward Christensen's 1979 MODEM2 program.  It's also
       called the MODEM or MODEM2 protocol.  Some who are unaware of MODEM7's
       unusual batch file mode call it MODEM7.  Other aliases include "CP/M
       Users's Group" and "TERM II FTP 3".  This protocol is supported by
       every serious communications program because of its universality,
       simplicity, and reasonable performance.
      XMODEM/CRC replaces XMODEM's 1 byte checksum with a two byte Cyclical
       Redundancy Check (CRC-16), giving modern error detection
      XMODEM-1k Refers to the XMODEM/CRC protocol with 1024 byte data blocks.
      YMODEM refers to the XMODEM/CRC (optional 1k blocks) protocol with the
       batch transmission described below.
      ZMODEM uses familiar XMODEM/CRC and YMODEM technology in a
       new protocol that provides reliability, throughput, file
       management, and user amenities appropriate to contemporary
       data communications. 
      Since its development half a decade ago, the Ward Christensen modem
      protocol has enabled a wide variety of computer systems to interchange
      data.  There is hardly a communications program that doesn't at least
      claim to support this protocol.
      Recent advances in computing, modems and networking have
      revealed a number of weaknesses in the original protocol:
         + The short block length caused throughput to suffer when used with
           timesharing systems, packet switched networks, satellite circuits,
           and buffered (error correcting) modems.
         + The 8 bit arithmetic checksum and other aspects allowed line
           impairments to interfere with dependable, accurate transfers.
         + Only one file could be sent per command.  The file name had to be
           given twice, first to the sending program and then again to the
           receiving program.
         + The transmitted file could accumulate as many as 127 extraneous
      Chapter 2  
      X/YMODEM Protocol Reference        09-11-86                        3
         + The modification date of the file was lost.
      A number of other protocols have been developed over the
      years, but none  have displaced XMODEM to date:
         + Lack of public domain documentation and example programs have kept
           proprietary protocols such as MNP, Blast, and others
           tightly bound to the fortunes of their suppliers. 
         + Complexity discourages the widespread application of BISYNC, SDLC,
           HDLC, X.25, and X.PC protocols.
         + Performance compromises and moderate complexity have limited the
           popularity of the Kermit protocol, which was developed to allow file
           transfers in environments hostile to XMODEM.
      The XMODEM protocol extensions and YMODEM Batch address these
      weaknesses while maintaining XMODEM's simplicity.
      YMODEM is supported by the public domain programs YAM (CP/M),
      YAM(CP/M-86), YAM(CCPM-86), IMP (CP/M), KMD (CP/M), rz/sz (Unix, Xenix,
      VMS, Berkeley Unix, Venix, Xenix, Coherent, IDRIS, Regulus).  Commerical
      implementations include MIRROR, and Professional-YAM.[1] Communications
      programs supporting these extensions have been in use since 1981.
      The 1k packet length (XMODEM-1k) described below may be used in
      conjunction with YMODEM Batch Protocol, or with single file transfers
      identical to the XMODEM/CRC protocol except for minimal
       changes to support 1k packets.
      Another extension is simply called the g option.  It provides maximum
      throughput when used with end to end error correcting media,
      such as X.PC and error correcting modems, including the emerging
      9600 bps units by Electronic Vaults and others.
      To complete this tome, Ward Christensen's original protocol document and
      John Byrns's CRC-16 document are included for reference.
      References to the MODEM or MODEM7 protocol have been changed
      to XMODEM to accommodate the vernacular. In Australia, it is
      properly called the Christensen Protocol.
       1. Available for IBM PC,XT,AT, Unix and Xenix
      Chapter   2
      X/YMODEM Protocol Reference         09-11-86                      4
      2.1  Some Messages from the Pioneer
      #: 130940 S0/Communications 25-Apr-85  18:38:47
      Sb: my protocol
      Fm: Ward Christensen 76703,302 (EDITED)
      To: all
      Be aware the article[2] DID quote me correctly in terms of the
      phrases like "not robust", etc.
      It was a quick hack I threw together, very unplanned (like
      everything I do), to satisfy a personal need to communicate with
      "some other" people. 
      ONLY the fact that it was done in 8/77, and that I put it in the public
      domain immediately, made it become the standard that it is.
      I think its time for me to
      (1) document it; (people call me and say "my product is going
          to include  it - what can I 'reference'", or "I'm writing a
          paper on it, what do I put in the bibliography") and
      (2) propose an "incremental extension" to it, which might take
          "exactly" the form of Chuck Forsberg's YAM protocol.  He
           wrote YAM in C for CP/M and  put it in the public domain,
           and wrote a batch protocol for Unix[3] called rb and sb
           (receive batch, send batch), which was basically XMODEM with 
          (a) a record 0 containing filename date time and size
          (b) a 1K block size option
          (c) CRC-16.
      He did some clever programming to detect false ACK or EOT, but basically
      left them the same.
      People who suggest I make SIGNIFICANT changes to the protocol,
      such as "full duplex", "multiple outstanding blocks", "multiple
      destinations", etc etc don't understand that the incredible
      simplicity of the protocol is one of the reasons it survived to
      this day in as many machines and programs as it may be found in!
      Consider the PC-NET group back in '77 or so - documenting to
      beat the band 
      - THEY had a protocol, but it was "extremely complex", because
        it tried to be "all things to all people" - i.e. send binary
        files on a 7-bit system,  etc.  I was not that "benevolent". I
        (emphasize > I < ) had an 8-bit UART,  
       2. Infoworld April 29 p. 16
       3. VAX/VMS versions of these programs are also available.
      Chapter   2
      X/YMODEM Protocol Reference        09-11-86                        5
      so "my protocol was an 8-bit protocol", and I would just say "sorry" to
      people who were held back by 7-bit limitations.  ...
      Block size: Chuck Forsberg created an extension of my protocol,
      called YAM, which is also supported via his public domain
      programs for UNIX called rb and sb - receive batch and send
      batch.  They cleverly send a "block 0" which contains the
      filename, date, time, and size. Unfortunately, its UNIX style,
      and is abit weird[4] - octal numbers, etc. BUT, it is a nice way
      to overcome the kludgy "echo the chars of the name" introduced
      with MODEM7. Further, chuck uses CRC-16 and optional 1K blocks.
      Thus the record 0, 1K, and CRC, make it a "pretty slick new 
      protocol" which is not significantly different from my own.
      Also, there is a catchy name - YMODEM. That means to some
      that it is the  "next thing after XMODEM", and to others that it
      is the Y(am)MODEM protocol.  I don't want to emphasize that too
      much - out of fear that other mfgrs might think it is a
      "competitive" protocol, rather than an "unaffiliated" protocol.
      Chuck is currently selling a much-enhanced version of his
      CP/M-80 C program YAM, calling it Professional Yam, and its 
      for the PC - I'm using it right now.  VERY slick!  32K capture buffer,
      script, scrolling, previously captured text search, plus
      built-in commands for just about everything - directory (sorted
      every which way), XMODEM, YMODEM, KERMIT, and ASCII file
      upload/download, etc.  You can program it to "behave" with most
      any system - for example when trying a number for CIS it detects
      the "busy" string back from the modem and substitutes a diff
      phone # into the dialing string and branches back to try it. 
      This chapter discusses the protocol extensions to Ward Christensen's
      1982 XMODEM protocol description document.
      The original document recommends the user be asked whether to continue
      trying or abort after 10 retries.  Most programs no longer ask the
      operator whether he wishes to keep retrying.  Virtually all correctable
      errors are corrected within the first few retransmissions.  If
      the line is so bad that ten attempts are insufficient, there is
      a significant danger of undetected errors.  If the connection is
      that bad, it's better to redial for a better connection, or
      mail a floppy disk. 
       4. The file length, time, and file mode are optional.The pathname and
          file length may be sent alone if desired.
      Chapter   3                             XMODEM Protocol Enhancements
      X/YMODEM Protocol Reference        09-11-86                        6
      3.1  Graceful Abort
      The YAM and Professional-YAM X/YMODEM routines recognize a
      sequence of two consecutive CAN (Hex 18) characters without modem
      errors (overrun, framing, etc.) as a transfer abort command.  This
      sequence is recognized when YAM is waiting for the beginning of a
      packet or for an acknowledge to one that has been sent.  The
      check for two consecutive CAN characters virtually eliminates the
      possibility of a line hit aborting the transfer.  YAM sends
      eight CAN characters when it aborts an XMODEM, YMODEM, or ZMODEM
      protocol file transfer.  Pro-YAM then sends eight backspaces to delete
      the CAN characters from the remote's keyboard input buffer, in
      case the remote had already aborted the transfer and was
      awaiting a keyboarded command.
      3.2  CRC-16 Option
      The XMODEM protocol uses an optional two character CRC-16
      instead of the one character arithmetic checksum used by the original
      protocol and by most commercial implementations.  CRC-16
      guarantees detection of all single and double bit errors, all errors
      with an odd number of error bits, all burst errors of length 16 or
      less, 99.9969% of all 17-bit error bursts, and 99.9984 per cent
      of all possible longer error bursts.  By contrast, a double bit
      error, or a burst error of 9 bits or more can sneak past the
      XMODEM protocol arithmetic checksum.
      The XMODEM/CRC protocol is similar to the XMODEM protocol,
      except that the receiver specifies CRC-16 by sending C (Hex
      43) instead of NAK when requesting the FIRST packet.  A two
      byte CRC is sent in place of the one byte arithmetic checksum.
      YAM's c option to the r command enables CRC-16 in single file
      reception, corresponding to the original implementation in the
      MODEM7 series programs.  This remains the default because many
      commercial communications programs and bulletin board systems still do
      not support CRC-16, especially those written in Basic or Pascal.
      XMODEM protocol with CRC is accurate provided both sender and
      receiver both report a successful transmission. The protocol is robust
      in the presence of characters lost by buffer overloading on
      timesharing systems.
      The single character ACK/NAK responses generated by the
      receiving program adapt well to split speed modems, where the reverse
      channel is limited to ten per cent or less of he main channel's speed.
      XMODEM and YMODEM are half duplex protocols which do not attempt to
      transmit information and control signals in both directions at the same
      time.  This avoids buffer overrun problems that have been reported by
      users attempting to exploit full duplex asynchronous file transfer
      protocols such as Blast.
      Professional-YAM adds several proprietary logic enhancements to XMODEM's
      Chapter   3                                 XMODEM Protocol Enhancements
      X/YMODEM Protocol Reference        09-11-86                        7
      error detection and recovery.  These compatible enhancements eliminate
      most of the bad file transfers other programs make when using
      the XMODEM protocol under less than ideal conditions.
      3.3  XMODEM-1k 1024 Byte Packet
      The choice to use 1024 byte packets is expressed to the sending
      program on its command line or selection menu.[1] 1024 byte packets
      improve throughput in many applications, but some environments cannot
      accept 1024 byte bursts, especially minicomuters running 19.2kb
      An STX (02) replaces the SOH (01) at the beginning of the transmitted
      block to notify the receiver of the longer packet length.  The
      transmitted packet contains 1024 bytes of data.  The receiver
      should be able to accept any mixture of 128 and 1024 byte
      packets.  The packet number is incremented by one for each
      packet regardless of the packet length. 
      The sender must not change between 128 and 1024 byte packet lengths if it
      has not received a valid ACK for the current packet.  Failure to observe
      this restriction allows certain transmission errors to pass undetected.
      If 1024 byte packets are being used, it is possible for a file to "grow"
      up to the next multiple of 1024 bytes. This does not waste disk space if
      the allocation granularity is 1k or greater.  With YMODEM batch
      transmission, the optional file length transmitted in the file
      name packet allows the receiver to discard the padding,
      preserving the exact file length and contents.
      1024 byte packets may be used with batch file transmission or with single
      file transmission.  CRC-16 should be used with the k option to preserve
      data integrity over phone lines.  If a program wishes to enforce this
      recommendation, it should cancel the transfer, then issue an informative
      diagnostic message if the receiver requests checksum instead of CRC-16.
      Under no circumstances may a sending program use CRC-16 unless the
      receiver commands CRC-16.
      4.  YMODEM Batch File Transmission
      The YMODEM Batch protocol is an extension to the XMODEM/CRC protocol that
      allows 0 or more files to be transmitted with a single command.  (Zero
      files may be sent if none of the requested files is accessible.) The
      design approach of the YMODEM Batch protocol is to use the
      normal routines for sending and receiving XMODEM packets in a
      layered fashion similar to 
       1. See "KMD/IMP Exceptions to YMODEM" below.
      Chapter 4                                   XMODEM Protocol Enhancements
      X/YMODEM Protocol Reference         09-11-86                      8
         Figure 1.  1024 byte Packets
          SENDER                                  RECEIVER
                                                  "s -k"
          " open x.x minutes"
          STX 01 FE Data[1024] CRC CRC
          STX 02 FD Data[1024] CRC CRC
          STX 03 FC Data[1000] CPMEOF[24] CRC CRC
         Figure 2.  Mixed 1024 and 128 byte Packets
          SENDER                                  RECEIVER
                                                  "s -k"
          " open x.x minutes"
          STX 01 FE Data[1024] CRC CRC
          STX 02 FD Data[1024] CRC CRC
          SOH 03 FC Data[128] CRC CRC
          SOH 04 FB Data[100] CPMEOF[28] CRC CRC
      packet switching methods.
      Why was it necessary to design a new batch protocol when one already
      existed in MODEM7?[1] The batch file mode used by MODEM7 is unsuitable
      because it does not permit full pathnames, file length, file date, or
      other attribute information to be transmitted. Such a restrictive design,
      hastily implemented with only CP/M in mind, would not have permitted
      extensions to current areas of personal computing such as Unix, DOS, and
      object oriented systems.  In addition, the MODEM7 batch file mode is
      somewhat susceptible to transmission impairments.
       1. The MODEM7 batch protocol transmitted CP/M FCB bytes f1...f8 and
          t1...t3 one character at a time.  The receiver echoed these bytes as
          received, one at a time.
      Chapter  4                             XMODEM Protocol Enhancements
      X/YMODEM Protocol Reference        09-11-86                        9
      As in the case of single a file transfer, the receiver initiates batch
      file transmission by sending a "C" character (for CRC-16).
      The sender opens the first file and sends packet number 0 with the
      following information.[2]
      Only the pathname (file name) part is required for batch transfers.
      To maintain upwards compatibility, all unused bytes in packet 0 must be
      set to null.
      Pathname The pathname (conventionally, the file name) is sent as a null
           terminated ASCII string.  This is the filename format used by the
           handle oriented MSDOS(TM) functions and C library fopen functions.
           An assembly language example follows:
                      DB      '',0
           No spaces are included in the pathname.  Normally only the file name
           stem (no directory prefix) is transmitted unless the sender has
           selected YAM's f option to send the full pathname.  The source drive
           (A:, B:, etc.) is never sent.
           Filename Considerations:
        + File names should be translated to lower case unless the sending
         system supports upper/lower case file names. This is a
         convenience for users of systems (such as Unix) which store
         filenames in upper and lower case.
       + The receiver should accommodate file names in lower and upper
       + When transmitting files between different operating systems,
         file names must be acceptable to both the sender and receiving
         operating systems.
           If directories are included, they are delimited by /; i.e.,
           "subdir/foo" is acceptable, "subdir\foo" is not.
      Length The file length and each of the succeeding fields are optional.[3]
           The length field is stored in the packet as a decimal
           string counting the number of data bytes in the file.  The
           file length does not include any CPMEOF (^Z) or other
           garbage characters used to pad the last packet.
       2. Only the data part of the packet is described here.
       3. Fields may not be skipped.
      Chapter 4                               XMODEM Protocol Enhancements
      X/YMODEM Protocol Reference  09-11-86                             10
           If the file being transmitted is growing during transmission, the
           length field should be set to at least the final expected file
           length, or not sent.
           The receiver stores the specified number of characters, discarding
           any padding added by the sender to fill up the last packet.
      Modification Date The mod date is optional, and the filename and length
           may be sent without requiring the mod date to be sent.
           Iff the modification date is sent, a single space separates the
           modification date from the file length.
           The mod date is sent as an octal number giving the time the contents
           of the file were last changed, measured in seconds from Jan 1 1970
           Universal Coordinated Time (GMT). A date of 0 implies the
           modification date is unknown and should be left as the date the file
           is received.
           This standard format was chosen to eliminate ambiguities
           arising from transfers between different time zones.
           Two Microsoft blunders complicate the use of modification dates in
           file transfers with MSDOS(TM) systems.  The first is the lack of
           timezone standardization in MS-DOS.  A file's creation time can not
           be known unless the timezone of the system that wrote the file[4] is
           known.  Unix solved this problem (for planet Earth, anyway) by
           stamping files with Universal Time (GMT). Microsoft would have to
           include the timezone of origin in the directory entries, but does
           not.  Professional-YAM gets around this problem by using the z
           parameter which is set to the number of minutes local time lags GMT.
           For files known to originate from a different timezone, the -zT
           option may be used to specify T as the timezone for an individual
           file transfer.
           The second problem is the lack of a separate file creation date in
           DOS.  Since some backup schemes used with DOS rely on the file
           creation date to select files to be copied to the archive, back-
           dating the file modification date could interfere with the safety of
           the transferred files.  For this reason, Pro-YAM does not modify the
           date of received files with the header information unless the d
           numeric parameter is non zero.
       4. Not necessarily that of the transmitting system!
      Chapter   4                             XMODEM Protocol Enhancements
      X/YMODEM Protocol Reference        09-11-86                       11
      Mode Iff the file mode is sent, a single space separates the file mode
           from the modification date.  The file mode is stored as an octal
           string.  Unless the file originated from a Unix system, the
           file mode is set to 0.  rb(1) checks the file mode for the
           0x8000 bit which indicates a Unix type regular file.  Files
           with the 0x8000 bit set are assumed to have been sent from
           another Unix (or similar) system which uses the same file
           conventions.  Such files are not translated in any way.
      Serial Number Iff the serial number is sent, a single space separates the
           serial number from the file mode. The serial number of the
           transmitting program is stored as an octal string.
           Programs which do not have a serial number should omit this
           field, or set it to 0.  The receiver's use of this field is
      Other Fields YMODEM was designed to allow additional header fields to be
           added as above without creating compatibility problems with older
           YMODEM programs.  Please contact Omen Technology if other fields are
           needed for special application requirements.
      The rest of the packet is set to nulls.  This is essential to preserve
      upward compatibility.[5]
      If the filename packet is received with a CRC or other error, a
      restrnsmission is requested.  After the filename packet has been
      received, it is ACK'ed if the write open is successful.  If the
      file cannot be opened for writing, the receiver cancels the
      transfer with CAN characters as described above.
      The receiver then initiates transfer of the file contents
      according to the standard XMODEM/CRC protocol.
      After the file contents have been transmitted, the receiver
      again asks for the next pathname.  Transmission of a null
      pathname terminates batch file transmission.  Note that
      transmission of no files is not necessarily an error. This is
      possible if none of the files requested of the sender could be
      opened for reading. 
      In batch transmission, the receiver automatically requests CRC-16.
      The Unix programs sz(1) and rz(1) included in the source code file
       5. If, perchance, this information extends beyond 128 bytes (possible
          with Unix 4.2 BSD extended file names), the packet should be
          sent as a 1k packet as described above.
      Chapter   4                             XMODEM Protocol Enhancements
      X/YMODEM Protocol Reference        09-11-86                       12
      RZSZ[12].SHQ (, should answer other questions about
      YMODEM batch protocol.
         Figure 3.  YMODEM Batch Transmission Session
          SENDER                                  RECEIVER
                                                  "sb foo.*<CR>"
          "sending in batch mode etc."
                                                  C (command:rb)
          SOH 00 FF foo.c NUL[123] CRC CRC
          SOH 01 FE Data[128] CRC CRC
          STX 02 FD Data[1024] CRC CRC
          SOH 03 FC Data[128] CRC CRC
          SOH 04 FB Data[100] CPMEOF[28] CRC CRC
          SOH 00 FF NUL[128] CRC CRC
             Figure 4.  YMODEM Filename packet transmitted by sz
             -rw-r--r--  6347 Jun 17 1984 20:34 bbcsched.txt
             00 0100FF62 62637363 6865642E 74787400     |...bbcsched.txt.|
             10 36333437 20333331 34373432 35313320     |6347 3314742513 |
             20 31303036 34340000 00000000 00000000     |100644..........|
             30 00000000 00000000 00000000 00000000
             80 000000CA 56
      Chapter   4                             XMODEM Protocol Enhancements
      X/YMODEM Protocol Reference        09-11-86                       13
            Figure 5.    YMODEM Header Information used by various programs
      | Program   | Batch | Length | Date | Mode | S/N | 1k-Blk | g-Option |
      |Unix rz/sz | yes   | yes    | yes  | yes  | no  | yes    | sb only  |
      |VMS rb/sb  | yes   | yes    | no   | no   | no  | yes    | no       |
      |Pro-YAM    | yes   | yes    | yes  | no   | yes | yes    | yes      |
      |CP/M YAM   | yes   | no     | no   | no   | no  | yes    | no       |
      |KMD/IMP    | yes   | ?      | no   | no   | no  | yes    | no       |
      4.1  KMD/IMP Exceptions to YMODEM
      KMD and IMP character sequence emitted by the receiver (CK) to
      automatically trigger the use of 1024 byte packets as an alternative to
      specifying this option on this command line.  Although this two character
      sequence works well on single process micros in direct communication,
      timesharing systems and packet switched networks can separate the
      successive characters by several seconds, rendering this method
      Sending programs may detect the CK sequence if the operating enviornment
      does not preclude reliable implementation.
      Receiving programs should retain the option of sending the
      standard YMODEM C (not CK) trigger with the standard 10 second
      timeout to insure compatibility with newer forms of
      telecommunications technology. 
      5.  YMODEM-g File Transmission
      Developing technology is providing phone line data transmission at ever
      higher speeds using very specialized techniques.  These high
      speed modems, as well as session protocols such as X.PC, provide
      high speed, error free communications at the expense of
      considerably increased delay time. 
      This delay time is moderate compared to human interactions, but it
      cripples the throughput of most error correcting protocols.
      The g option to YMODEM has proven effective under these circumstances.
      The g option is driven by the receiver, which initiates the
      batch transfer by transmitting a G instead of C.  When the
      sender recognizes the G, it bypasses the usual wait for an ACK
      to each transmitted packet, sending succeeding packets at full
      speed, subject to XOFF/XON or other flow control exerted by the medium.
      Chapter   5                             XMODEM Protocol Enhancements
      X/YMODEM Protocol Reference        09-11-86                       14
      The sender expects an inital G to initiate the transmission of a
      particular file, and also expects an ACK on the EOT sent at the end of
      each file.  This synchronization allows the receiver time to open and
      close files as necessary.
      If an error is detected in a YMODEM-g transfer, the receiver aborts the
      transfer with the multiple CAN abort sequence. The ZMODEM protocol should
      be used in applications that require both streaming throughput and error
             Figure 6.  YMODEM-g Transmission Session
             SENDER                                      RECEIVER
                                                 "sb foo.*<CR>"
             "sending   in batch mode etc..."
                                                 G (command:rb -g)
             SOH 00 FF foo.c NUL[123]   CRC CRC
             SOH 01 FE Data[128] CRC CRC
             STX 02 FD Data[1024] CRC   CRC
             SOH 03 FC Data[128] CRC CRC
             SOH 04 FB Data[100] CPMEOF[28] CRC CRC
             SOH 00 FF NUL[128] CRC CRC
      8/9/82 by Ward Christensen.
      I will maintain a master copy of this. Please pass on changes or
      suggestions via CBBS/Chicago at (312) 545-8086, CBBS/CPMUG (312) 849-1132
      or by voice at (312) 849-6279.
      6.1  Definitions
        <soh>   01H
        <eot>   04H
        <ack>   06H
        <nak>   15H
        <can>   18H
        <C>     43H
      Chapter   6                                 Xmodem Protocol Overview
      X/YMODEM Protocol Reference        09-11-86                       15
      6.2  Transmission Medium Level Protocol
      Asynchronous, 8 data bits, no parity, one stop bit.
      The protocol imposes no restrictions on the contents of the data being
      transmitted.  No control characters are looked for in the 128-byte data
      messages.  Absolutely any kind of data may be sent - binary, ASCII, etc.
      The protocol has not formally been adopted to a 7-bit environment for the
      transmission of ASCII-only (or unpacked-hex) data , although it could be
      simply by having both ends agree to AND the protocol-dependent data with
      7F hex before validating it.  I specifically am referring to the
      checksum, and the block numbers and their ones- complement.
      Those wishing to maintain compatibility of the CP/M file structure, i.e.
      to allow modemming ASCII files to or from CP/M systems should follow this
      data format:
         + ASCII tabs used (09H); tabs set every 8.
         + Lines terminated by CR/LF (0DH 0AH)
         + End-of-file indicated by ^Z, 1AH. (one or more)
         + Data is variable length, i.e. should be considered a continuous
           stream of data bytes, broken into 128-byte chunks purely for the
           purpose of transmission.
         + A CP/M "peculiarity": If the data ends exactly on a 128-byte
           boundary, i.e. CR in 127, and LF in 128, a subsequent sector
           containing the ^Z EOF character(s) is optional, but is preferred.
           Some utilities or user programs still do not handle EOF without ^Zs.
         + The last block sent is no different from others, i.e.  there is no
           "short block".
              Figure 7.  XMODEM Message Block Level Protocol
      Each block of the transfer looks like:
            <SOH><blk #><255-blk #><--128 data bytes--><cksum>
      in which:
      <SOH>           = 01 hex
      <blk #>         = binary number, starts at 01 increments by 1, and
                wraps 0FFH to 00H (not to 01)
      <255-blk #>   =   blk # after going thru 8080 "CMA" instr, i.e.
                each bit complemented in the 8-bit block number.
                Formally, this is the "ones complement".
      <cksum>       = the sum of the data bytes only.  Toss any carry.
      Chapter   6                                 Xmodem Protocol Overview
      X/YMODEM Protocol Reference        09-11-86                       16
      6.3  File Level Protocol
      6.3.1  Common_to_Both_Sender_and_Receiver
      All errors are retried 10 times.  For versions running with an operator
      (i.e. NOT with XMODEM), a message is typed after 10 errors asking the
      operator whether to "retry or quit".
      Some versions of the protocol use <can>, ASCII ^X, to cancel
      transmission. This was never adopted as a standard, as having a
      single "abort" character makes the transmission susceptible to
      false termination due to an <ack> <nak> or <soh> being corrupted
      into a <can> and aborting transmission. 
      The protocol may be considered "receiver driven", that is, the
      sender need not automatically re-transmit, although it does in
      the current implementations.
      6.3.2  Receive_Program_Considerations
      The receiver has a 10-second timeout.  It sends a <nak> every time it
      times out.  The receiver's first timeout, which sends a <nak>,
      signals the transmitter to start.  Optionally, the receiver
      could send a <nak> immediately, in case the sender was ready.
      This would save the initial 10 second timeout.  However, the
      receiver MUST continue to timeout every 10 seconds in case the
      sender wasn't ready. 
      Once into a receiving a block, the receiver goes into a
      one-second timeout for each character and the checksum.  If the
      receiver wishes to <nak> a block for any reason (invalid header,
      timeout receiving data), it must wait for the line to clear.
      See "programming tips" for ideas.
      Synchronizing: If a valid block number is received, it will be: 1) the
      expected one, in which case everything is fine; or 2) a repeat of the
      previously received block.  This should be considered OK, and only
      indicates that the receivers <ack> got glitched, and the sender re-
      transmitted; 3) any other block number indicates a fatal loss of
      synchronization, such as the rare case of the sender getting a
      line-glitch that looked like an <ack>.  Abort the transmission,
      sending a <can> 
      6.3.3  Sending_program_considerations
      While waiting for transmission to begin, the sender has only a
      single very long timeout, say one minute.  In the current
      protocol, the sender has a 10 second timeout before retrying.  I
      suggest NOT doing this, and letting the protocol be completely
      receiver-driven.  This will be compatible with existing programs.
      When the sender has no more data, it sends an <eot>, and awaits an <ack>,
      resending the <eot> if it doesn't get one.  Again, the protocol could be
      receiver-driven, with the sender only having the high-level 1-minute
      timeout to abort.
      Chapter   6                                 Xmodem Protocol Overview
      X/YMODEM Protocol Reference        09-11-86                       17
      Here is a sample of the data flow, sending a 3-block message.
      It includes the two most common line hits - a garbaged block,
      and an <ack> reply getting garbaged.  <xx> represents the checksum byte.
             Figure 8.  Data flow including Error Recovery
      SENDER                            RECEIVER
                              times out after 10 seconds,
                              <---              <nak>
      <soh> 01 FE -data- <xx>         --->
                              <---              <ack>
      <soh> 02 FD -data- xx           --->       (data gets line hit)
                              <---              <nak>
      <soh> 02 FD -data- xx           --->
                              <---              <ack>
      <soh> 03 FC -data- xx           --->
      (ack gets garbaged)             <---              <ack>
      <soh> 03 FC -data- xx           --->              <ack>
      <eot>                           --->
                              <---       <anything except ack>
      <eot>                           --->
                              <---              <ack>
      6.4  Programming Tips
         + The character-receive subroutine should be called with a parameter
           specifying the number of seconds to wait. The receiver should first
           call it with a time of 10, then <nak> and try again, 10 times.
           After receiving the <soh>, the receiver should call the character
           receive subroutine with a 1-second timeout, for the remainder of the
           message and the <cksum>.  Since they are sent as a
           continuous stream, timing out of this implies a serious
           like glitch that caused, say, 127 characters to be seen
           instead of 128. 
         + When the receiver wishes to <nak>, it should call a "PURGE"
           subroutine, to wait for the line to clear.  Recall the sender tosses
           any characters in its UART buffer immediately upon
           completing sending a block, to ensure no glitches were mis- 
           The most common technique is for "PURGE" to call the character
           receive subroutine, specifying a 1-second timeout,[1] and looping
           back to PURGE until a timeout occurs.  The <nak> is then sent,
           ensuring the other end will see it.
       1. These times should be adjusted for use with timesharing systems.
      Chapter   6                                 Xmodem Protocol Overview
      X/YMODEM Protocol Reference        09-11-86                       18
         + You may wish to add code recommended by John Mahr to your character
           receive routine - to set an error flag if the UART shows framing
           error, or overrun.  This will help catch a few more glitches - the
           most common of which is a hit in the high bits of the byte in two
           consecutive bytes.  The <cksum> comes out OK since counting
           in 1-byte produces the same result of adding 80H + 80H as
           with adding 00H + 00H.
      7.  XMODEM/CRC Overview
      1/13/85 by John Byrns -- CRC option.
      Please pass on any reports of errors in this document or suggestions for
      improvement to me via Ward's/CBBS at (312) 849-1132, or by voice at (312)
      The CRC used in the Modem Protocol is an alternate form of block check
      which provides more robust error detection than the original checksum.
      Andrew S. Tanenbaum says in his book, Computer Networks, that the CRC-
      CCITT used by the Modem Protocol will detect all single and double bit
      errors, all errors with an odd number of bits, all burst errors of length
      16 or less, 99.997% of 17-bit error bursts, and 99.998% of 18-bit and
      longer bursts.
      The changes to the Modem Protocol to replace the checksum with
      the CRC are straight forward. If that were all that we did we
      would not be able to communicate between a program using the old
      checksum protocol and one using the new CRC protocol. An initial
      handshake was added to solve this problem. The handshake allows
      a receiving program with CRC capability to determine whether the
      sending program supports the CRC option, and to switch it to CRC
      mode if it does. This handshake is designed so that it will work
      properly with programs which implement only the original
      protocol. A description of this handshake is presented in section 10. 
           Figure 9.  Message Block Level Protocol, CRC mode
      Each block of the transfer in CRC mode looks like:
           <SOH><blk #><255-blk #><--128 data   bytes--><CRC hi><CRC lo>
      in which:
      <SOH>          = 01 hex
      <blk #>        = binary number, starts at 01 increments by 1, and
               wraps 0FFH to 00H (not to 01)
      <255-blk #>  = ones complement of blk #.
      <CRC hi>     = byte containing the 8 hi   order coefficients of the CRC.
      <CRC lo>     = byte containing the 8 lo   order coefficients of the CRC.
      Chapter   7                                 Xmodem Protocol Overview
      X/YMODEM Protocol Reference        09-11-86                       19
      7.1  CRC Calculation
      7.1.1  Formal_Definition
      To calculate the 16 bit   CRC the message bits are considered to be the
      coefficients of a polynomial. This message polynomial is first multiplied
      by X^16 and then divided by the generator polynomial (X^16 + X^12 + X^5 +
      1) using modulo two arithmetic. The remainder left after the division is
      the desired CRC. Since a message block in the Modem Protocol is 128 bytes
      or 1024 bits, the message polynomial will be of order X^1023.
      The hi order bit of the first byte of the message block is the
      coefficient of X^1023 in the message polynomial.  The lo order
      bit of the last byte of the message block is the coefficient of
      X^0 in the message polynomial. 
          Figure 10.  Example of CRC Calculation written in C
      UPDCRC update routine from "rbsb.c".  Refer to the source code for these
      programs (contained in RZSZ1.SHQ and RZSZ2.SHQ) for usage.  A fast table
      driven macro is also included in this file.
      /* update CRC */
      unsigned short
      updcrc(c, crc)
      register c;
      register unsigned crc;
        register count;
        for (count=8; --count>=0;) {
                if (crc & 0x8000) {
                        crc <<= 1;
                        crc += (((c<<=1) & 0400)  !=  0);
                        crc ^= 0x1021;
                else {
                        crc <<= 1;
                        crc += (((c<<=1) & 0400)  !=  0);
        return crc;
      7.2  CRC File Level Protocol Changes
      7.2.1  Common_to_Both_Sender_and_Receiver
      The only change to the File Level Protocol for the CRC option is the
      initial handshake which is used to determine if both the sending and the
      receiving programs support the CRC mode. All Modem Programs
      should support the checksum mode for compatibility with older
      versions.  A receiving program that wishes to receive in CRC
      mode implements the mode setting handshake by sending a <C> in
      place of the initial <nak>.  If the sending program supports CRC
      mode it will recognize the <C> and will set itself into CRC
      mode, and respond by sending the first block as if a <nak> had 
      been received. If the sending program does not support CRC mode it will
      Chapter   7                                 Xmodem Protocol Overview
      X/YMODEM Protocol Reference        09-11-86                       20
      not respond to the <C> at all. After the receiver has sent the
      <C> it will wait up to 3 seconds for the <soh> that starts the
      first block. If it receives a <soh> within 3 seconds it will
      assume the sender supports CRC mode and will proceed with the
      file exchange in CRC mode. If no <soh> is received within 3
      seconds the receiver will switch to checksum mode, send a <nak>,
      and proceed in checksum mode. If the receiver wishes to use 
      checksum mode it should send an initial <nak> and the sending program
      should respond to the <nak> as defined in the original Modem Protocol.
      After the mode has been set by the initial <C> or <nak> the protocol
      follows the original Modem Protocol and is identical whether the checksum
      or CRC is being used.
      7.2.2  Receive_Program_Considerations
      There are at least 4 things that can go wrong with the mode setting
        1.  the initial <C> can be garbled or lost.
        2.  the initial <soh> can be garbled.
        3.  the initial <C> can be changed to a <nak>.
        4.  the initial <nak> from a receiver which wants to receive
            in checksum can be changed to a <C>.
      The first problem can be solved if the receiver sends a second <C> after
      it times out the first time. This process can be repeated several times.
      It must not be repeated too many times before sending a <nak> and
      switching to checksum mode or a sending program without CRC support may
      time out and abort. Repeating the <C> will also fix the second problem if
      the sending program cooperates by responding as if a <nak> were received
      instead of ignoring the extra <C>.
      It is possible to fix problems 3 and 4 but probably not worth the trouble
      since they will occur very infrequently. They could be fixed by switching
      modes in either the sending or the receiving program after a large number
      of successive <nak>s. This solution would risk other problems however.
      7.2.3  Sending_Program_Considerations
      The sending program should start in the checksum mode. This will insure
      compatibility with checksum only receiving programs. Anytime a <C> is
      received before the first <nak> or <ack> the sending program should set
      itself into CRC mode and respond as if a <nak> were received. The sender
      should respond to additional <C>s as if they were <nak>s until the first
      <ack> is received. This will assist the receiving program in determining
      the correct mode when the <soh> is lost or garbled. After the first <ack>
      is received the sending program should ignore <C>s.
      Chapter 7                                   Xmodem Protocol Overview
      X/YMODEM Protocol Reference        09-11-86                       21
      7.3  Data Flow Examples with CRC Option
      Here is a data flow example for the case where the receiver requests
      transmission in the CRC mode but the sender does not support the CRC
      option. This example also includes various transmission errors.  <xx>
      represents the checksum byte.
            Figure 11.  Data Flow: Receiver has CRC Option, Sender Doesn't
      SENDER                                          RECEIVER
                        <---                <C>
                                times out after 3 seconds,
                        <---                <C>
                                times out after 3 seconds,
                        <---                <C>
                                times out after 3 seconds,
                        <---                <C>
                                times out after 3 seconds,
                        <---                <nak>
      <soh> 01 FE -data- <xx>   --->
                        <---                <ack>
      <soh> 02 FD -data- <xx>   --->        (data gets line hit)
                        <---                <nak>
      <soh> 02 FD -data- <xx>   --->
                        <---                <ack>
      <soh> 03 FC -data- <xx>   --->
         (ack   gets garbaged)  <---                <ack>
                                times out after 10 seconds,
                        <---                <nak>
      <soh> 03 FC -data- <xx>   --->
                        <---                <ack>
      <eot>                     --->
                        <---                <ack>
      Here is a data flow example for the case where the receiver requests
      transmission in the CRC mode and the sender supports the CRC
      option.  This example also includes various transmission errors.
      <xxxx> represents the  2 CRC bytes.
      Chapter 7                                   Xmodem Protocol Overview
      X/YMODEM Protocol Reference        09-11-86                       22
          Figure 12.  Receiver and Sender Both have CRC Option
      SENDER                                         RECEIVER
                          <---                 <C>
      <soh> 01 FE -data- <xxxx> --->
                          <---                 <ack>
      <soh> 02 FD -data- <xxxx> --->           (data gets line hit)
                          <---                 <nak>
      <soh> 02 FD -data- <xxxx> --->
                          <---                 <ack>
      <soh> 03 FC -data- <xxxx> --->
      (ack gets garbaged)         <---                 <ack>
                                     times out after 10 seconds,
                          <---                 <nak>
      <soh> 03 FC -data- <xxxx> --->
                          <---                 <ack>
      <eot>                       --->
                          <---                 <ack>
      More information may be obtained by calling Telegodzilla at 503-621-3746.
      Hit RETURNs for baud rate detection.Speed detection is automatic for 300,
      1200, and 2400 bps.
      A version this file with boldface, underlining, and superscripts for
      printing on Epson or Gemini printers is available on Telegodzilla as
      UUCP sites can obtain this file   with
             uucp omen!/usr/spool/uucppublic/ymodem.doc /tmp
      A continually updated list of available   files is stored in
      The following L.sys line calls Telegodzilla (Pro-YAM in host operation).
      Telegodzilla waits for carriage returns to determine the incoming speed.
      If none is detected, 1200 bps is assumed and a greeting is displayed.
      In response to "Name Please:" uucico gives the Pro-YAM "link"
      command as a user name.  The password (Giznoid) controls access
      to the Xenix system connected to the IBM PC's other serial port.
      Communications between Pro-YAM and Xenix use 9600 bps; YAM
      converts this to the caller's speed. 
      Finally, the calling uucico logs in as uucp.
      omen Any ACU 1200 1-503-621-3746 se:--se: link ord: Giznoid in:--in: uucp
      Contact omen!caf if you wish the troff source files.
      Chapter   8                                 Xmodem Protocol Overview
      X/YMODEM Protocol Reference        09-11-86                       23
      9.  Document Revisions
      The September 11 1986 edition clarifies nomenclature and some minor
      points.  The April 15 1986 edition clarifies some points concerning CRC
      calculations and spaces in the header.
      10.  YMODEM Programs
      A demonstration version of Professional-YAM is available as ZMODEM.ARC
      This may be used to test YMODEM amd ZMODEM implementations.
      Unix programs supporting the YMODEM protocol are available on
      Telegodzilla as RZSZ1.SHQ and RZSZ2.SHQ (SQueezed shell archives).
      Most Unix like systems are supported, including V7, Xenix, Sys III,
      4.2 BSD, SYS V, Idris, Coherent, and Regulus.
      A version for VAX-VMS is available in VRBSB.SHQ.
      Irv Hoff has added YMODEM 1k packets and basic YMODEM batch transfers to
      the KMD and IMP series programs, which replace the XMODEM and
      MODEM7/MDM7xx series respectively.  Overlays are available for a wide
      variety of CP/M systems.
      Questions about YMODEM, the Professional-YAM communications program, and
      requests for evaluation copies may be directed to:
           Chuck Forsberg
           Omen Technology Inc
           17505-V Sauvie Island Road
           Portland Oregon 97231
           Voice: 503-621-3406
           Modem: 503-621-3746 Speed: 2400,1200,300
           Usenet: ...!tektronix!reed!omen!caf
           Compuserve: 70007,2304
           Source: TCE022
      Chapter 10                                  Xmodem Protocol Overview
       1.  ROSETTA STONE....................................................  2
       2.  YET ANOTHER PROTOCOL?............................................  2
           2.1  Some Messages from the Pioneer..............................  4
       3.  XMODEM PROTOCOL ENHANCEMENTS.....................................  5
           3.1  Graceful Abort..............................................  6
           3.2  CRC-16 Option...............................................  6
           3.3  XMODEM-1k 1024 Byte Packet..................................  7
       4.  YMODEM Batch File Transmission...................................  7
           4.1  KMD/IMP Exceptions to YMODEM................................ 13
       5.  YMODEM-g File Transmission....................................... 13
       6.  XMODEM PROTOCOL OVERVIEW......................................... 14
           6.1  Definitions................................................. 14
           6.2  Transmission Medium Level Protocol.......................... 15
           6.3  File Level Protocol......................................... 16
           6.4  Programming Tips............................................ 17
       7.  XMODEM/CRC Overview.............................................. 18
           7.1  CRC Calculation............................................. 19
           7.2  CRC File Level Protocol Changes............................. 19
           7.3  Data Flow Examples with CRC Option.......................... 21
       8.  MORE INFORMATION................................................. 22
       9.  Document Revisions............................................... 23
      10.  YMODEM Programs.................................................. 23
                                   - i -
                             LIST OF FIGURES
       Figure 1.  1024 byte Packets.........................................  7
       Figure 2.  Mixed 1024 and 128 byte Packets...........................  7
       Figure 3.  YMODEM Batch Transmission Session......................... 12
       Figure 4.  YMODEM Filename packet transmitted by sz.................. 12
       Figure 5.  YMODEM Header Information used by various programs........ 13
       Figure 6.  YMODEM-g Transmission Session............................. 14
       Figure 7.  XMODEM Message Block Level Protocol....................... 15
       Figure 8.  Data flow including Error Recovery........................ 17
       Figure 9.  Message Block Level Protocol, CRC mode.................... 18
      Figure 10.  Example of CRC Calculation written in C................... 19
      Figure 11.  Data Flow: Receiver has CRC Option, Sender Doesn't........ 21
      Figure 12.  Receiver and Sender Both have CRC Option.................. 22
                                  - ii -