Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!rutgers!usc!ginosko!uunet!mcsun!ukc!tadtec!tjfs From: tjfs@tadtec.uucp (Tim Steele) Newsgroups: comp.dcom.modems Subject: Re: RS232 to modem connection Message-ID: Date: 13 Sep 89 11:45:35 GMT References: Sender: tjfs@tadtec.uucp (Tim Steele) Organization: Tadpole Technology plc Lines: 69 In-reply-to: mz0l+@andrew.cmu.edu's message of 9 Sep 89 22:10:02 GMT Wow, this is my chance to attract a huge flame war! I have incredibly bigoted opinions on the RS232 "standard". The problem seems to be that the "standard" doesn't specify what connectors should be used, nor what kinds of equipment it may be used for. My mental model which has served me pretty well in many complex RS232 tangles is as follows: RS232 is really about how to connect an item of Data Communications Equipment (DCE) such as a modem to an item of Data Terminal Equipment (DTE) such as a terminal. Anything else is a "perversion" 8-) which involves bending the standard in some way. Although the standard doesn't say anything about connectors, there is a common "de facto" standard using 25 pin D connectors, with the female connector on the modem and the male connector on the terminal. Connecting cables are just like power extension cords; they're female at one end, male at the other and wired straight through (1-1, 2-2 ... 25-25). If everything was wired that way, RS232 connections would be trivial, and anything that could physically be plugged together would work. Unfortunately, there are some strange perversions around that make life hard. These are as follows: a) Cables that don't connect all 25 wires. Of course, the standard specifies more wires than you "really" need... indeed, with an intelligent modem such as a TrailBlazer you can get away with just Transmit & Receive Data and Signal Ground (that's 2, 3 and 7). Most of the other pins exist either so the DTE can tell the DCE something "extra" (like "Use high speed" or "I'm ready for your data") or vice versa (like "I can see carrier"). There are also a few pins for synchronous operation, and even some spare ones! If you have such a cable, you have to make sure that enough lines are connected... b) Misunderstandings between DCE and DTE about the use of various pins. For example, one end may expect to hardware handshake using RTS/CTS whereas the other end may be expecting to use XON/XOFF inband signalling. c) Machiavellian manufacturers who fit (usually) female connectors instead of male ones. They probably do this to improve their equipment reliability. For example, many terminals have a female connector on the back, wired just as if it were male. I call these "female wired as DTE". One (rather drastic) approach is to either change the connectors on such equipment or (more practically) permanently attach a cheap "gender bender" adaptor to make the sex right. d) "Perverted" equipment connection. The simplest example of this is connecting a terminal directly to a computer instead of via modems. Both the terminal and the computer are DTE devices (and should of course be male), so they usually end up being connected with a "null modem" cable, which is female-female wired straight through. e) Some mix and match version of the above. Nasty, isn't it? Thinks: how about a REAL standard! Tim -- tjfs@tadtec.uucp ..!uunet!mcvax!ukc!tadtec!tjfs Tadpole Technology plc, Titan House, Castle Park, CAMBRIDGE, CB3 0AY, UK Phone: +44-223-461000 Fax: +44-223-460727 Telex: TADTEC G