Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!uwm.edu!cs.utexas.edu!uunet!mcsun!ukc!axion!galadriel!pcf From: pcf@galadriel.bt.co.uk (Pete French) Newsgroups: comp.dcom.modems Subject: Re: RS232 to modem connection Message-ID: <346@galadriel.bt.co.uk> Date: 14 Sep 89 08:05:37 GMT References: Organization: RT6115, BTRL, Martlesham Heath, England Lines: 70 From article , by tjfs@tadtec.uucp (Tim Steele): > 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. > True - it was designed for terminal -> modem connections. > 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. Are you talking about the V24 stabdard or the RS-232-C standard ? (V24 is the europeanb version). I dont now about V24 - I have never seen the spec - but RS-232-C certainly specifies that a 25-way D-connector should be used. It also specifies the pinout in great detail. Sorryto say, but the last bit is totally wrong, it should be the female on the terminal and the male connector on the modem. > a) Cables that don't connect all 25 wires. Of course, the standard > specifies more wires than you "really" need... I find that you need at least 8 to be useful. GND, TXD, RXD, DSR, DTR, RTS, CTS, CD. > synchronous operation, and even some spare ones! There are NO spare pins in RS-232-C. > d) "Perverted" equipment connection. The simplest example of this is > connecting a terminal directly to a computer instead of via modems. You are the only person I have ever met who thinks like me over this one. Most people assume that terminals connect to computers - not so. > Nasty, isn't it? Thinks: how about a REAL standard! It is, but people dont stick to it. Part of the problem lies in the bufferring. The standard only provides handshaking in one direction (DTE -> DCE) via the RTS/CTS lines. Any data travelling (DCE -> DTE) is expected to be always received. This is fine for terminal -> modem connections since the termninal should always be capable of printing or bufferring data received by the modem. A modem is not expected to stop sending to the terminal when the terminal cannot accept characters since it has no way of telling the remote end to stop transmitting. This is partly why you sometimes see the wrong connector on equipment. oI like RS-232-C , I have made many cables in my life and as far as I can remember the only "strainght thru" one was to connect a UNIX box to a modem. 8 wires and it works perfectly. For all those out there who are going to read this and disaggree with the fact that a computer should look like a terminal I would like to point out that such things as the production of a login banner when a call is answered and also the automatic logging out of a user when the call clears are two things that can only be provided by the DTE interface. Some terminals are now made to look like modems for connection to a computer directly. This will provide the above facilities - i.e. when you switch on the terminal you get a. login page immediately without hitting return and also when you switch the terminal off you are logged out automatically. -Pete. -- -Pete French. | British Telecom Research Labs. | "The carefree days are distant now, Martlesham Heath, East Anglia. | I wear my memories like a shroud..." All my own thoughts (of course) | -SIOUXSIE