Path: utzoo!mnetor!uunet!mcvax!unido!tub!cabo From: cabo@tub.UUCP (Carsten Bormann) Newsgroups: comp.dcom.modems Subject: Re: INFO-MODEMS Digest V88 #52 Message-ID: <362@tub.UUCP> Date: 22 Feb 88 23:10:37 GMT References: <8802180550.AA09815@ucbvax.Berkeley.EDU> <40@marque.mu.edu> Reply-To: cabo@tub.UUCP (Carsten Bormann) Organization: Technical University of Berlin, Germany Lines: 115 Keywords: V.24 Input Ready Output Ready Summary: Learn V.24! In article <40@marque.mu.edu> brianb@marque.UUCP (Brian Bebeau) writes: () () It's been my experience that () most DTE devices specify DTR as being *always* on, therefore useless () for hardware flow control. If the DTE implements RS-232 properly, () RTS-CTS crossed over will work properly for hardware flow control. This is complete nonsense. Can you guys please read the standards before writing such postings? The internationally relevant CCITT recommendation V.24 (called RS-232 in the US) does not provide for flow-control signalling in any form. Period. (Warning: If you disagree, you don't have any idea about serial communications standards. No need to make another incorrect posting.) Now, as flow-control is sometimes desirable, some manufacturers have decided to bend the specification a little. What you need for flow-control is two additional lines: Let's call them Input Ready (IR) and Output Ready (OR), both from the DTE point of view. The lines for flow-control are not there in V.24 (and I bet in RS-232, although I don't have a copy of the latest version RS-232-D). Adding them to the boards (and to the chips) creates considerable costs. But you can take any pair of RS-232 signalling lines and REINTERPRET them as IR and OR. Now, what signalling lines look like they are not so useful and can thus be stolen? DTR and DCD are needed to signal that a connection is wanted and is present, resp., so typically we don't want to steal these lines (actually, the CCITT recommendation V.24 prescribes the use of DSR for the purpose for which DCD is used most of the time nowadays, but I don't want to pick nits here). RTS/CTS only really make sense in a half-duplex application, so in our full-duplex world they make good candidates for IR/OR. This becomes even more handy as CTS already has semantics somewhat similar to OR in the standard (but not for flow control, but for signalling from the modem to the terminal that the modem is in a sending state and has been in this state long enough that it is likely that the remote modem is in sync), so many chips already implicitly support OR semantics on the CTS pin. The only pin whose semantics are completely changed is the old RTS pin, which in its RTS semantics means that you want the modem to prepare for sending, and now in the IR semantics is used to signal the modem that you are ready for receiving! Modems that understand V.24 don't have OR/IR pins. Modems that have OR/IR pins (which is very useful for ``smart'' modems) on pin 5/4 don't support V.24 (at least not RTS/CTS). Do things become clearer? Let's close this up with a few examples. If you want to build a NULL MODEM (according to V.24), you wire: FGnd 1-------1 FGnd TxD 2--->---3 RxD RxD 3---<---2 TxD RTS 4\__>___8 DCD CTS 5/ DSR 6---<--20 DTR SGnd 7-------7 SGnd DCD 8___<__/4 RTS \5 CTS DTR 20--->---6 DSR This arrangement EXACTLY behaves like a pair of V.24-conforming modems (with the exceptions of missing time delays and no connection to pin 22, Ring Indicator, which I normally connect to pin 6). Now, most US modems use pin 8 (DCD) for what V.24 calls DSR (pin 6), so for US applications we modify this into: FGnd 1-------1 FGnd TxD 2--->---3 RxD RxD 3---<---2 TxD RTS 4\ CTS 5/ DCD' 8---<--20 DTR SGnd 7-------7 SGnd /4 RTS \5 CTS DTR 20--->---8 DCD' The name DCD' tries to express that this pin is not used in quite the same meaning as intended by V.24. If we want to include flow-control, we LEAVE THE REALM OF V.24, and get (assuming that we actually reinterpret pin 4 for IR and pin 5 for OR) the following wiring: FGnd 1-------1 FGnd TxD 2--->---3 RxD RxD 3---<---2 TxD IR 4--->---5 OR OR 5---<---4 IR DCD* 8---<--20 DTR SGnd 7-------7 SGnd DTR 20--->---8 DCD* Note that this picture does NOT include any pins labeled RTS or CTS, as there are no such pins in this picture. After reading comp.dcom.modems for some time, I get the impression this posting needs to be repeated every two weeks. Yours for better understanding of standards, --Carsten -- Carsten Bormann, Communications and Operating Systems Research Group Technical University of Berlin (West, of course...) Path: ...!pyramid!tub!cabo from the world, ...!unido!tub!cabo from Europe only.