From: hamilton@romulus.ehs.uiuc.edu (Wayne Hamilton)
Subject: Re: Null modem: 25 pin serial
Article-I.D.: news.C520Gs.Dyw
Organization: University of Illinois at Urbana
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Bruce Maynard (drac@uumeme.chi.il.us) wrote:
> In article <93083.215554MAL112@psuvm.psu.edu> Mitch Lewars <MAL112@psuvm.psu.edu> writes:
> >Can someone give me the specs
> >on a null modem cable, I ferget
> >which get crossed.... =-)
> >    mal112@psuvm.psu.edu

> That's an easy one... carry all 25 pins straight thru, with the exception of
> pins 2 & 3, which should be crossed...

as someone else already mentioned, don't "carry thru" the other 23 pins.

plan A, minimal null modem:
	SG(7)   -  SG(7)
	TD(2)   -  RD(3)
	RD(3)   -  TD(2)
if you're transfering files betwen 2 PCs, this ought to work (unless
you have a persnickity comm program).

plan B, null modem with modem control:
	SG(7)   -  SG(7)
	TD(2)   -  RD(3)
	RD(3)   -  TD(2)
	DCD(8)  -  DTR(20)
	DTR(20) -  DCD(8)
some comm software will wait for DCD before transferring anything.
such software will raise DTR to enable the modem to accept a call.
with this cable, each side's DTR will drive the other's DCD.

plan C, null modem with hardware flow control:
	SG(7)   -  SG(7)
	TD(2)   -  RD(3)
	RD(3)   -  TD(2)
	RTS(4)  -  CTS(5)
	CTS(5)  -  RTS(4)
for high-speed transfers, you want the flow control.

plan D, deluxe null modem (combine B and C):
	SG(7)   -  SG(7)
	TD(2)   -  RD(3)
	RD(3)   -  TD(2)
	RTS(4)  -  CTS(5)
	CTS(5)  -  RTS(4)
	DCD(8)  -  DTR(20)
	DTR(20) -  DCD(8)
this one is ideal.  it leaves out DSR and RI (rarely used anymore).
if you're really paranoid, or you just have an 8th wire to spend,
you might add:
	FG(1)   -  FG(1)

the pin numbers above are (obviously) for 25-pin connectors.
i don't have a 9-pin pinout handy.

--
	wayne hamilton
I'net:	hamilton@osiris.cso.uiuc.edu
Lowtek:	Box 476, Urbana, IL 61801; (217)384-4310(voice), -4311(BBS)