W8SDZ@Simtel20.ARPA (04/11/84)
From: Keith Petersen <W8SDZ@Simtel20.ARPA> 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.
towson@Amsaa.ARPA (04/11/84)
From: David Towson (CSD) <towson@Amsaa.ARPA> Randolph - I enjoyed the guide to RS-232 connections. I wonder, however, why you gave such a nice description of the use of Ring Indicator, and then said to LEAVE IT OPEN. Huh? Also, I'd like to know how a modem can assert ANY RS-232 signal when it is off. It seems that the plus-and-minus three volts "forbidden zone" in the standard would make it impossible for signals to be asserted when the modem is off (unless the modem contains a battery). Or did you decide to slip April Fools Day to the fourth? Dave towson@amsaa