Path: utzoo!mnetor!uunet!lll-winken!lll-lcc!ames!mailrus!tut.cis.ohio-state.edu!ut-sally!utah-cs!utah-gr!uplherc!sp7040!obie!wsccs!terry
From: terry@wsccs.UUCP (terry)
Newsgroups: comp.dcom.modems
Subject: Re: IBM-C128 hookup
Message-ID: <217@wsccs.UUCP>
Date: 28 Feb 88 02:03:21 GMT
References: <3154@cup.portal.com> <18600039@clio>
Lines: 48
Summary: More reliable cable

In article <18600039@clio>, berger@clio.las.uiuc.edu writes:
> 
> There are many ways to make a null modem cable, but here's one that
> should work for most applications.  I assume you have standard RS-232
> 25 pin D-type connectors on both ends.  Otherwise you'll have to
> translate the wire list for your own connectors.
> 
> 1 - 1
> 2 - 3
> 3 - 2
> 4 - 5
> 5 - 4
> 6,8 - 20
> 7 - 7
> 20 - 6,8

	Unfortunately, this neglects the fact that a number of modems do not
assert CTS unless they have DCD.  The above cable is fine for incoming calls
but lousy for outgoing.  Here is a more reliable cable:

 1 ----		/* connect pin one only on CPU side to avoid ground loops*/
 2 ---- 3
 3 ---- 2
 4 \  / 4	/* Forcing the self-satifaction at the cable end avoids wires,
 5 /  \ 5	   but you may need to wire it thru if the modem requires CTS
 6 -.		   but does not assert RTS until carrier, like Multitek.*/
 8 -+--20
     .- 6	/* If your call-out software opens the port right, it should
 20--+- 8	   not need DCD prior to talking to/reading from the port*/
 7 ---- 7

	The same cable should work from machine to machine (DTE-DTE, DCE-DCE)
and and from DCE machine to a modem, or DTE machine to a terminal, if the
terminal asserts DTR (DCD for terminals).

	A good book on this is titled 'THE RS-232 SOLOUTION'.  My cable comes
from supporting terminal emulation/file transfer software on UNIX boxes 9-6
5 days a week, and from participating in writing said software.  Note that the
same reservations apply on translation of this cable to 9-pin, 15-pin, or
NCR boxes (you CAN'T call out ttya or ttyb on an NCR box. ttyb is the console,
and ttya is the call-in-only maintenance port).  If you are talking about an
expander board for a UNIX or Xenix on an AT clone, it depends on the board.

| Terry Lambert           UUCP: ...!decvax!utah-cs!century!terry              |
| @ Century Software       or : ...utah-cs!uplherc!sp7040!obie!wsccs!terry    |
| SLC, Utah                                                                   |
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