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RS-232 Guide

Keywords  RS232 Guide
File has pin callouts for usual 4-pin DIN and DB-25 versions of RS232-style
serial interface, with some notes on the Signal Set and some related notes.
80-column.  Date: 9-1-85
RS232 Guide:
-------- ---------- ------ ------- ------------------------------- ------------
printer  4-pin DIN  DB-25  common  modem                           flow
service  circuit #  cirk#  abbrev. service                         direx.
-------- ---------- ------ ------- ------------------------------- ------------
                     1      GND     Ground                           --
 DATA    *(4)*      *2*     TD      Transmit Data                   to Modem
 BUSY    *(2)*      *3*     RD      Receive Data                    to Terminal
                     4      RTS     Request To Send                 to Modem
                     5      CTS     Clear To Send                   to Terminal
                     6      DSR     Data Set Ready                  to Terminal
 Ground   (3)        7      L.GND   Logic Ground                     --
(no conn.)(1)        8      CD      Carrier Detect                  to Terminal
                     9      (reserved)
                    10      (reserved)
                    11      (not assigned)
                    12      SCD     Secondary Carrier Detect        to Terminal
                    13      SCTS    Secondary Clear To Send         to Terminal
                   *14*     STD     Secondary Transmit Data         to Modem
                    15      TCT     Transmit Clock (to terminal)    to Terminal
                   *16*     SRD     Secondary Receive Data          to Terminal
                    17      RC      Receive Clock                   to Terminal
                    18      (not assigned)
                    19      SRTS    Secondary Request To Send       to Modem
                    20      DTR     Data Terminal Ready             to Modem
                    21      SQD     Signal Quality Detect           to Terminal
                    22      RDT     Ring Detect                     to Terminal
                    23      DRS     Data Rate Select                to Terminal
                    24      TCC     Transmit Clock (to modem)       to Modem
                    25      (not assigned)
        Note: abbreviations and circuit names vary.
In Passing:
   The DB-25 version has pins assigned for a complete second data channel, and
such is used rather commonly in commercial work.
   In hobby work, multiple channels remains a topic to be exploited.  Multiplex
might become a hot topic in the next two or three years.
Logic Convention:
   Data lines TD, RD, STD, SRD, DATA, BUSY are Negative True Mark.  All others
are Positive True.
Level Convention:
   Spec signal is bipolar with Positive True High anywhere from +5V to +25V.
Positive True Low is from -5V to -25V.  The range from -5V to +5V is ignored.
   Spec notwithstanding, 5V single polar is used frequently on non-data lines,
and sometimes on data as well.  Manufacturers sometimes (shamelessly) mix
levels even inside the same unit.
   In fact, manufacturers interpret RS232 pretty stinking freely, using it more
nowadays as a Conventional Signal Set and point of departure than as a rigid
interface standard.  The usual connecting cable is wired pin-for-pin end-to-end
(pin 1 to pin 1, pin 2 to pin 2, and so forth).
   One end equipment must be set up "modem" (receives data on pin 2 TD, sends
data out on pin 3 RD etc.) and the other end equipment must be configured
"terminal" (receives data on pin 3 RD, sends data out on pin 2 TD, etc.) for
the signals to mesh right and for the connection to work.  Peripherals other
than modems are configured either as "terminal" or as "modem", according to the
manufacturer's ideas for the product.  This state of affairs gives rise to the
Like - Interface Problem.
   Connecting two like-configured end equipments (viz. a "terminal" to a
"terminal" or a "modem" to a "modem") together with a spec cable results in a
no-input-signal condition for both, and the connection would not work.  The
first step to a solution is to use a specially-wired connecting cable (often
called a Null Modem Adapter) which transposes TD and RD.  This will route the
correct data outputs to the correct data inputs.  Often, other supervisory
circuits must be tricked around to get the connection going.
Remarks on Signal Set:
   Data is moved on the data lines only.  Data lines are one-way in RS232.
Other lines carry control information and status signals, and their exact usage
varies somewhat among different manufacturers, and for different peripherals.
   Typically, only three to about seven of the twenty-odd circuits in the
Signal Set get used for most setups, even in commercial work.  Complicated
systems and complex peripherals use more lines, since they need more
sophisticated supervision.
   What counts for interfacing today is more the intent behind the original
RS232 Signal than the original task of the RS232 Signal Set signal itself.
Here, by the DB-25 pin numbers, are some notes covering the Signal Set items as
used in terminal-modem applications.  For other applications, a given signal
will usually be doing pretty much the same sort of job.
Signal Set Notes:
   1   GND   This is frame ground ONLY and is never supposed to be used for
circuit ground, though it is sometimes tied to it.  Frequently not connected.
   2   TD    Serial data outbound from computer or terminal. Supposed to be
   3   RD    Serial data inbound to computer or terminal.  Supposed to be
   4   RTS   Terminal query to modem, asking if ready to pass data outbound.
Commonly locked high.
   5   CTS   Affirmative reply to RTS.  N.B. CTS is supposed to latch high if
it goes high at all, assuming RTS is true.  Thereafter, it is not to be
clearable till after RTS clears.  This usage varies and CTS is commonly
strapped to #8 CD.
   6   DSR   This varies also nowadays from a simple power-on confirm to
fullfledged handshake with DTR to other things, but mostly it's just locked
high and forgotten.
   7   L.GND This is the correct common return for circuit currents. Always
   8   CD    High while modem is receiving carrier from far end.  Usually used
as circuit-go / circuit-no-go indicator.
   12  SCD   As above for CD, but for secondary channel.
   13  SCTS  As above for CTS.
   14  STD   As above for TD.
             "a complete second data channel is provided for in the Signal Set"
   15  TCT   The three clocks TCT, TCC, RC pass timing between the two
connected equipments.  For example, an entire terminal can be timed,
synchronized, and the return carrier synthesysed from and locked to the far-end
carrier.  Very handy for exotic low-error-rate and high-speed work.  Not found
in the hobby stuff.  Yet.
   16  SRD   As above for RD.
   17  RC    See #15.
   19  SRTS  As above for RTS.
   20  DTR   Sometimes used to handshake with DSR, sometimes even tells the
modem to go off-hook in reply to RDT.  Also used as a terminal-on pilot, and is
frequently ignored and locked high.
   21  SQD   Supposed to indicate the quality of incoming signal.  Usually
ignored in favor of CD.  Sometimes called QUAL.
   22  RDT   You have probably guessed by now this is supposed to go high when
the phone rings.
   23  DRS   Sets configuration for data rate, usually fast for speed, slow for
accuracy.  Sometimes used instead to switch encryption in and out, sometimes
selects protocol.
   24  TCC   See #15 TCT
   Neither the 4-pin DIN nor the DB-25 were set up to handle while powered.  So
unplugging, and particularly plugging in with power on, is worth avoiding.
   Again, manufacturers interpret RS232 to suit their needs, and information
here can only be considered a guide at best.
   September 1, 1985