robert@pvab.UUCP (Robert Claeson) (08/23/87)
This is maybe a dumb question, but what is the intended use of pin 6,
the one named DSR or Data Set Ready in the RS232 standard? I've seen
that some devices (especially modems) uses it, but most ignores it (such
as computers and terminals). Is it really needed for modem-control
signaling or can I simply connect it to pin 20 (DTR) at the other end?
How should it be connected in a null-modem cable when I want to connect
two DTE devices with modem control?
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SNAIL: Robert Claeson, PVAB, P.O. Box 4040, S-171 04 Solna, Sweden
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ARPA: enea!pvab!robert@seismo.arpajfh@killer.UUCP (The Beach Bum) (08/26/87)
In article <304@pvab.UUCP>, robert@pvab.UUCP (Robert Claeson) writes: > This is maybe a dumb question, but what is the intended use of pin 6, > the one named DSR or Data Set Ready in the RS232 standard? I've seen > that some devices (especially modems) uses it, but most ignores it (such > as computers and terminals). Is it really needed for modem-control > signaling or can I simply connect it to pin 20 (DTR) at the other end? > How should it be connected in a null-modem cable when I want to connect > two DTE devices with modem control? Below is the copy of a file I keep around for cable building. It lists the most common signals on a RS-232 DB-25 connector for Data Terminal Equipment. Might do you some good to save a copy of this file (maybe as /usr/pub/rs232) CONNECTOR SIGNALS DTE Wired Equipment 1 - Chassis Ground 2 - Transmit Data (output) 3 - Receive Data (input) 4 - Request to Send (output) 5 - Clear to Send (input) 6 - Data Set Ready (input) 7 - Signal Ground 8 - Carrier Detect (input) 20 - Data Terminal Ready (output) As you can see, DSR is an input signal. What normally goes on with this signal is you connect it to DCD for a modem (least ways I do) or DTR for a terminal. MULTIPLEXOR to PLEXUS P/60 CABLE P l e x u s P / 6 0 M u l t i p l e x o r 2 ----------------------------------------- 3 3 ----------------------------------------- 2 4 ----------------------------------------- 5 6 ----------------------------------------- 20 7 ----------------------------------------- 7 20 ----------------------------------------- 6 This would be how I would connect a Case Multiplexor Port to a serial port on the P/60 (The one we are getting rid of :-). Basically, RxD and TxD are swapped and DTR and DSR are also swapped. This works pretty well. One big suggetion - once you find a cable wiring that works, save it in a file someplace public (unless only you make cables at your site) so that the rest of the world won't have to go through the grief of figuring out how to wire an IBM/PC to an HP Plotter. Identifying equipment as DTE or DCE make swapping the numbers alot easier. ---------- END OF `CABLE IN COMPUTER SCIENCE' 101 -------- John. -- John F. Haugh II HECI Exploration Co. Inc. UUCP: ...!ihnp4!killer!jfh 11910 Greenville Ave, Suite 600 "Don't Have an Oil Well?" Dallas, TX. 75243 " ... Then Buy One!" (214) 231-0993
howard@COS.COM (Howard C. Berkowitz) (08/26/87)
In article <304@pvab.UUCP>, robert@pvab.UUCP (Robert Claeson) writes: > This is maybe a dumb question, but what is the intended use of pin 6, > the one named DSR or Data Set Ready in the RS232 standard? There is a difference between what the standard says and what most implementers do with it. The standard says: "Signals on this circuit are used to indicate the status of the local data set. The ON condition on this circuit is presented to indicate that a) the local data communications equipment is connected to a communication channel (OFF HOOK in switched service) AND b) the local DCE is not in test (local or remote), talk, or dial mode AND c) the local DCE has completed, where applicable, 1. any timing functions required by the switching system to complete call establishment, 2. the transmission of any discrete answer tone..." In practice, the signal does not mean that much on most modems; it usually means only that the modem has power. Carrier detect and Clear to Send usually mean, to a much greater extent, that you have a viable channel. I've seen > that some devices (especially modems) uses it, but most ignores it (such > as computers and terminals). Is it really needed for modem-control > signaling or can I simply connect it to pin 20 (DTR) at the other end? > How should it be connected in a null-modem cable when I want to connect > two DTE devices with modem control? Be sure you connect Pin 20 of end 1 to Pin 6 of end 2 and vice versa, or simply connect local Pin 20 to local Pin 6 at both ends. You may also want to tie Pin 8 to pin 20 at each end. -- -- howard(Howard C. Berkowitz) @cos.com {seismo!sundc, hadron, hqda-ai}!cos!howard (703) 883-2812 [ofc] (703) 998-5017 [home] DISCLAIMER: I explicitly identify COS official positions.
fyl@ssc.UUCP (Phil Hughes) (08/28/87)
DSR, Data Set Ready is used to allow a DCE (modem, for example) to
tell a DTE (terminal or computer) that it (the DCE) is in an on-line
state. This differs from the CD signal that says the modem is actually
connected to something. In other words, a DTE would check the DSR
signal to determine if the modem is out there before dialing.
In a direct connect situation between two DTEs, you should just be
able to connect DTR to DSR.
--
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