Date : Wed, 11 Apr 1984 11:06:00 MST (Wed)
From : Keith Petersen <W8SDZ@Simtel20.ARPA>
Subject: RS-232 hookup info list
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.