terryl (03/06/83)
Before we discovered the wonderful world of Ethernet, we here at Teklabs used to have an 11/44 connected to our 11/70 with an rs-232 line, BUT WE HAD LINE DRIVERS. Cbout reading some of the specs of rs-232 and find out what they specify. If memory serves me correctly(and it never does, especially at 4:30 in the morning), the safest distance for the cable is some- thing like 25 or 50 feet, max. With the line drivers, the above mentioned link was over 300 feet long (probably closer to 500, but I don't remember). The link was used almost exclusively for uucp traffic at 9600 baud, and we never had any serious problems with it. So if you want a reasonably fast and reliable rs-232 link for 2 computers, a line driver is almost a must. Terry Laskodi of Tektronix
helge (03/14/83)
My experience is that unless you want to use *really* long cables, there is no need for line drivers on rs232 lines. dz11's can drive at least 350m (>1000') of good cable without problems at 9600 baud. helge
sob (03/15/83)
we use rs-232 up to 700' with dz11s, dh11s and dh/dms over telco
type twisted pair wire. ( twist xd with a ground and twist rd with a ground)
the only terminals we have had problems with are old tektronics ones that
seem to drive only +5 volts as there "hi" voltage. ( most others use +12 )
for longer runs we use some home brew short hall modems ( 9600 at 10 miles)
scott bradner
harvard university
...decvax!genradb!wjh12!sobrf@wu1.UUCP (04/04/84)
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. From the asbestos mailbox of: Randolph Fritz UUCPnet: decvax!philabs!wu1!rf 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.
Drew.Anderson@CMU-CS-C.ARPA (Drew Anderson) (07/25/85)
One side comment is that the ranges for RS-232 are actually 3-25. I have only seen a very few occasions where the voltage ever went above ~14 (or below -14) but they do exist. I only mention this in case some one tries to use an opamp with +-15V supplies and no clamping diodes. Some people may consider +-15 a max/min since that is what the 1488 driver chips are rated at. Drew Anderson DDA@cmu-cs-c.arpa -------
henry@utzoo.UUCP (Henry Spencer) (07/26/85)
> One side comment is that the ranges for RS-232 are actually 3-25.
Yes and no. RS232B, which is obsolete, specified +-25 as the upper
limit of transmitter output voltages. RS232C, the modern standard,
specifies +-15 as the maximum transmitter output (whence the +-15
limit on the 1488 transmitter chips). However, the receivers still
have to be capable of withstanding +-25, for compatibility.
--
Henry Spencer @ U of Toronto Zoology
{allegra,ihnp4,linus,decvax}!utzoo!henrymikey@trsvax (07/30/85)
Doesn't RS-232-C also specify a load impedance in the spec that was never specified before? I seem to remember 1K or something? mikey at trsvax
hes@ecsvax.UUCP (Henry Schaffer) (08/02/85)
Section 2.4 The load impedance (RL and CL) of the terminator side of an interchange circuit shall have a dc resistance (RL) of not less than 3000 Ohms, measured with an applied voltage of not greater than 25 volts in magnitude, nor more than 7000 Ohms, measured with an applied voltage of 3 to 25 volts in magnitude. The effective shunt capacitance (CL) of the terminator side of an interchange circuit, measured at the interface point, shall not exceed 2500 picofarads. The reactive component of the load impedance shall not be inductive. ... --henry schaffer