Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!wuarchive!cs.utexas.edu!uunet!zephyr.ens.tek.com!tektronix!sequent!ccssrv!perry From: perry@ccssrv.UUCP (Perry Hutchison) Newsgroups: comp.dcom.modems Subject: Re: RS232 to modem connection Message-ID: <623@ccssrv.UUCP> Date: 15 Sep 89 03:26:06 GMT References: <346@galadriel.bt.co.uk> Reply-To: perry@ccssrv.UUCP (Perry Hutchison) Organization: Control-C Software, Inc., Beaverton, OR Lines: 56 >In article tjfs@tadtec.uucp (Tim Steele) writes: > 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. In article <346@galadriel.bt.co.uk> pcf@galadriel.bt.co.uk (Pete French) replies: > 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. Steele is right on both points here, but he later objects to > Machiavellian manufacturers who fit (usually) female connectors > instead of male ones ... 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". RS-232C specifies the pin numbers, but not the type of connector. Some years ago, I spent a long time looking for the connector specification, and finally found out (through other channels) that Western Electric had established the DB25 as a de-facto standard when they used it on their "data sets". RS-232C also specifies that DCE shall provide a female connector _mounted on the equipment_ and DTE shall provide a male connector _on a cable_. This means that the cable is considered part of the DTE and the manner in which it is connected to the circuitry is entirely up to the manufacturer. Even IBM's 9-pin PC-AT serial ports are not in violation of RS-232C. French continues: > 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. ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ The old W.E. 202 datasets (1200 baud, one-way at a time) had a "reverse channel" facility whereby assertion of "Secondary Request To Send" by the _receiving_ DTE would cause a "Secondary Carrier Detect" signal to appear at the _transmitting_ DTE. The reverse channel capacity was something like 50 baud, and it was intended only for use as a flow control mechanism. Now the real shocker: a "null modem", whether constructed as a cable or in a box, is at least potentially in full compliance with RS-232C! The standard says nothing about how DCE shall accomplish its function of transmitting data between DTE's.