Arcane modem configuration (was Re: Modems on Ultrix)

[Carl S. Gutekunst]

In article <1149 at ncr-sd.UUCP> greg at ncr-sd.UUCP (Greg Noel) writes:
>About the only thing I do differently is to cross RTS and CTS instead
>of jumpering them.  This is useful on those terminals that will do
>hardware flow control (I'm not a fan of terminals that sent ^S and ^Q
>for flow control) and usually harmless otherwise.

Unfortunately, a lot of terminals use DTR for their flow control, and RTS as
'power on' (including all Wyse and Liberty terminals). For these you have to
either give up on hardware flow control (bad, especially when trying to use
Gosling EMACS) or break the rules, and wire RTS through to DCD and DSR (almost
as bad). When connecting to device that does DTR and RTS correctly, you get a
cable that isn't symetric. But since most terminals also use the wrong DB-25
connector (a DTE is supposed to be male), at least you can't plug it in the
wrong way. (*SIGH*) 

Note that using RTS and CTS for full-duplex flow control isn't technically
correct either, but that's a lot better than using DTR....

FYI: DEC and AT&T are the only terminal manufacturers I've worked with that
consistently get their RS232 connections correct (or close to).

>Other notes: Depending upon circumstances, you can frequently get by
>without connecting pin #1, or by jumpering it to pin #7 -- computer
>boards often connect them internally.

No! NEVER jumper #1 and #7 together!! I realize a lot of equipment does this,
but just because the machine is wrong it doesn't mean the mistake should be
duplicated in the cable. #1 is supposed to go to earth ground, the same as the
round pin on the power cable. Pin #7 is the signal ground, the same ground as
used by the equipment's DC power supplies. In properly designed equipment,
there will be no electrical connection between these grounds. Connecting them
introduces radiated noise problems, creates a possible shock hazard, and makes
the equipment much more vulnerable to nearby lightning strikes.

<csg>