Date: 4 Apr 84 12:19:06-PST (Wed)
From: decvax!mcnc!philabs!cubsvax!wu1!rf
Subject: RS-232

I recently prepared a guide to the RS-232 signals for competent,
unknowledgeable users.  It amazed and amused me, so I thought
some of you might like to see it.  The little notes "CONNECT" and
"LEAVE OPEN" are for people constructing a cable to be used with
an asynchronous modem.

                                Randolph Fritz

So, for the edification of the ignorant and the amusement of the
knowledgeable, I present:


   RANDOLPH FRITZ'S GUIDE TO RS-232 SIGNALS AND OTHER SICK JOKES.

Signal	DTE <-> DCE	Description
======	===========	===========

1 FG	    <-> 	Frame ground.  Sometimes connected to equipment
			chassis.  Where possible LEAVE OPEN.

2 TD		->	Transmitted data.  Data from computer (terminal) to
			modem.  CONNECT.

3 RD	<-		Received data.  Data from modem to computer
			(terminal).  CONNECT.

4 RTS		->	Request to send.  Turns on modem's transmit
			carrier.  CONNECT.

5 CTS	<-		Clear to send.  Indicates that modem's transmit
			carrier is on.  Some modems assert this all the
			time.  CONNECT.

6 DSR	<-		Data set ready.  Indicates that modem is on.
			Some modems assert this even when they're off.
			CONNECT.

7 SG	  <->		Signal ground.  Voltages of all other signals are
			referenced to this.  CONNECT.

8 DCD	<-		Data carrier detect.  Asserted when the modem
			hears a carrier.  Some modems assert this all the
			time, still others assert this in response to any
			received signal.  CONNECT.

9 	<-		In some Bell modems, +12V to power test equipment.
			If connected, can create smoke.  LEAVE OPEN.

10	<-		In some Bell modems, -12V to power test equipment.
			If connected, can create smoke.  LEAVE OPEN.

11 QM	<-		Equalizer mode.  Not RS-232.  Bell 208A 4800 bps
			modems only.  LEAVE OPEN.

12 HS	<-		High speed.  Not RS-232.  Hayes Smartmodem 1200
			only.  Asserted when 1200 bps connection
			established.  Mainly of use to auto-answer modem.
			LEAVE OPEN.

   or

12 SDCD <-		Secondary data carrier detect.  Used by only modems
			with a secondary channel.  LEAVE OPEN.

13 SCTS <-		Secondary clear to send.  Asserted when secondary
			transmit carrier is on.  LEAVE OPEN.

14 STD	<-		Secondary transmitted data.  LEAVE OPEN.

   or

14 NS		->	New sync.  Not RS-232.  Bell 208A 4800 bps
			modems only.  LEAVE OPEN.

15 TC	<-		Transmit clock.  Synchronous modems only.  LEAVE
			OPEN.

16 SRD	<-		Secondary received data.  LEAVE OPEN.

   or

16 DCT	<-		Divided clock, transmitter.  Not RS-232.  Bell
			208A 4800 bps modems only.  LEAVE OPEN.

17 RC	<-		Receive clock.  Synchronous modems only.  LEAVE
			OPEN.

18 DCR	<-		Divided clock, receiver.  Not RS-232.  Bell 208A
			4800 bps modems only.  LEAVE OPEN.

19 SRTS		->	Secondary request to send.  Turns on secondary
			carrier.  LEAVE OPEN.

20 DTR		->	Data terminal ready.  Asserted by computer (or
			terminal) when ready to use the modem.  RS-232
			specifies that all other signals from computer are
			meaningless when this is not asserted.  Most
			modems will not answer telephone when this is not
			asserted.  CONNECT.

21 SQ	<-		Signal quality.  Asserted by some fancy modems
			when line noise rises.  LEAVE OPEN.

22 RI	<-		Ring indicator.  The electrical equivalent of the
			telephone bell on an auto-answer modem.  The
			proper way to auto-answer is: wait for RI.  On
			second ring DTE should assert DTR.  The modem will
			then answer.  LEAVE OPEN.

23 HS		->	High speed.  Make a connection in high speed mode,
			if possible.  Used (if at all) by originate modem.
			LEAVE OPEN.

   or

23 HS	<-		High speed.  A connection has been made in high
			speed mode.  Used (if at all) by answer modem.
			LEAVE OPEN.

24 (TC)		->	Transmit clock from computer.  Unusual.  LEAVE
			OPEN.

25		->	Busy.  The line is busy.  Used (if at all) by
			auto-originate modem.  LEAVE OPEN.