Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.1 6/24/83; site redwood.UUCP Path: utzoo!watmath!clyde!burl!ulysses!mhuxr!ihnp4!zehntel!hplabs!hpda!fortune!rhino!redwood!rpw3 From: rpw3@redwood.UUCP (Rob Warnock) Newsgroups: net.micro.pc Subject: Re: Homemade AT Serial Port/RS232 Cable - (nf) Message-ID: <128@redwood.UUCP> Date: Wed, 16-Jan-85 23:04:59 EST Article-I.D.: redwood.128 Posted: Wed Jan 16 23:04:59 1985 Date-Received: Mon, 21-Jan-85 03:35:42 EST References: <816@pucc-i> <361@ahuta.UUCP> Organization: [Consultant], Foster City, CA Lines: 72 +--------------- | Thanks for posting the pin assignments. I'd like to make one small | correction, not in the pins but the terminology. A "null modem cable" | is NOT a modem cable without control leads. It's a cable that | REPLACES a modem, so that two back-to-back terminals (or terminal | and computer) can converse without benefit of a modem. | For example: | ... | Request to send ------------------------------ Clear to send | Clear to send ------------------------------ Request to send | ... | Dave Tutelman +--------------- Quite true, but unfortunately you blew it with your example, which does NOT emulate a pair of back-to-back modems. Request/Clear-To-Send (RTS/CTS) don't work that way. First, you only need to cross-connect them when emulating a half-duplex (or "controlled carrier") modem, since a full-duplex modem usually has clear-to-send true whenever carrier-detect is. Second, CTS is a response to the LOCAL RTS, not the remote's RTS. To properly emulate half-duplex use of RTS/CTS, you have to make a local RTS cause both a local CTS and a remote CD (carrier-detect). (The terms "remote" and "local" are symmetric.) Try it this way ("X" is a cross-over, "+" a join): 7 GND --------------- GND 2 TD ------\ /------- TD X 3 RD ------/ \------- RD 4 RTS --+--\ /----+-- RTS (I request to send; you get carrier) | X | 5 CTS --+ / \ +-- CTS / \ 8 CD ---/ \----- CD 6 DSR --+ +-- DSR (The dataset is ready when the terminal is) | | 20 DTR -+ +-- DTR Alternatively, to correctly emulate a pair of back-to-back full-duplex modems (of the 103/212 style), use: 7 GND --------------- GND 2 TD ------\ /------- TD X 3 RD ------/ \------- RD 4 RTS -(n/c) (n/c)- RTS 5 CTS --+ +-- CTS | | 8 CD --+---\ /---+-- CD X 6 DSR --+---/ \---+-- DSR | | 20 DTR -+ +-- DTR Many variations are possible, especially when non-standard flow-control methods are used, but hooking RTS on one side to CTS on the other is not one that normally makes any sense. Usually flow control is done by cross-connecting DTR and CTS; that is, if the local D(ata) T(erminal) is R(eady), the remote is C(lear) T(o) S(end). Rob Warnock Systems Architecture Consultant UUCP: {ihnp4,ucbvax!dual}!fortune!redwood!rpw3 DDD: (415)572-2607 USPS: 510 Trinidad Lane, Foster City, CA 94404