Path: utzoo!utgpu!watmath!xenitec!zswamp!p0.f171.n221.z1.fidonet.org!Geoffrey.Welsh From: Geoffrey.Welsh@p0.f171.n221.z1.fidonet.org (Geoffrey Welsh) Newsgroups: comp.dcom.modems Subject: Re: RS232 to modem connection Message-ID: <870.251007A2@zswamp.fidonet.org> Date: 14 Sep 89 19:57:34 GMT Sender: ufgate@zswamp.fidonet.org (newsout1.26) Organization: FidoNet node 1:221/171.0 - Izot's Swamp, Kitchener ON Lines: 83 > From: tjfs@tadtec.uucp (Tim Steele) > Message-ID: > I have incredibly bigoted opinions on the RS232 "standard". The We all do. Everyone who has worked extensively with RS-232 "conforming" devices does. > 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. That's funny; I recently explained that an RS-232 device has not only a gender (of its connector) but a sexual preference (DTE vs. DCE)... > 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. The practice of putting the male connector on the DTE came in with the IBM PC; all micros I saw before that had female RS-232 connectors, regardless of "sexual preference". > 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). That's the evolution of the de facto "standard". > a) Cables that don't connect all 25 wires. Of course, the standard They're not bad; what I hate are machines (e.g. Amiga 1000) which do stupid things with lines the designers THOUGHT wouldn't be used (all A1000 power supply voltages can be accessed through the serial port; this has blown several modems that I know of...) > 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! Ideally the TBit (and other high-speed modems) should be using the RTS and CTS lines for hardware handshaking, and all modems should have the DTR and DCD lines connected. These all provide unambiguous information more quickly than would be possible via characters in the data stream (XON/XOFF, NO CARRIER, the "+++" escape sequence, etc.). Ironically, I'm fairly sure that this is not how the EIA defined the RTS and CTS lines' use in the RS-232 spec. > 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. Easy solution: be prepared to do all. > c) Machiavellian manufacturers who fit (usually) female connectors > instead of male ones. See previous note. > 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. Actually, time was that host computers had front ends that were by definition DCE. The problem came when we demanded TTY lines to handle both terminals AND modems... -- Geoffrey Welsh - via FidoNet node 1:221/171 UUCP: {{uunet!}watmath!xenitec!}zswamp!171.0!Geoffrey.Welsh ARPA: Geoffrey.Welsh@p0.f171.n221.z1.fidonet.org