From: michael@jester.GUN.de (Michael Gerhards)
Distribution: world
Subject: Re: HOW is a Null Modem Cable?
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Kevin Kimmell - Computer Science/German Undergrad (ke_kimmell@vax.cns.muskingum.edu) wrote:

> 	I am interrested in the extrodinarily simple concept of the null modem
> cable. (Actually I have NO idea, so don't count that last statement.)  What I'm
> asking is what pins does it use (or what are it's specifications?)  I just want
> to solder one myself instead of buying one.  I don't even know what port is
> used.

DB-25	DB-9
Pin #	Pin #	Name	EIA	CCITT	DTE-DCE	Description
------------------------------------------------------------------------------
 1		FG	AA	 101	  ----	Frame ground
 2	3	TD	BA	 103	  --->	Transmitted data
 3	2	RD	BB	 104	  <---	Received data
 4	7	RTS	CA	 105	  --->	Request to send
 5	8	CTS	CB	 106	  <---	Clear to send
 6	6	DSR	CC	 107	  <---	Data set ready
 7	5	SG	AB	 102	  ----	Signal ground
 8	1	DCD	CF	 109	  <---	Data carrier detect
 9		--	--	  -	    -	Positive DC test voltage
10		--	--	  -	    -	Negative DC test voltage
11		QM	--	  -	  <---	Equalizer mode
12		SDCD	SCF	 122	  <---	Secondary data carrier detect
13		SCTS	SCB	 121	  <---	Secondary clear to send
14		STD	SBA	 118	  --->	Secondary transmitted data
15		TC	DB	 114	  <---	Transmitter clock
16		SRD	SBB	 119	  <---	Secondary receiver clock
17		RC	DD	 115	  --->	Receiver clock
18		DCR	--	  -	  <---	Divided clock receiver
19		SRTS	SCA	 120	  --->	Secondary request to send
20	4	DTR	CD	 108.2	  --->	Data terminal ready
21		SQ	CG	 110	  <---	Signal quality
22	9	RI	CE	 125	  <---	Ring indicator
23		--	CH	 111	  --->	Data rate selector
24		--	CI	 112	  <---	Data rate selector
25		TC	DA	 113	  <---	Transmitted clock

.-.-.- from another posting in c.s.i.p.h .-.-.-
.-.-.- sorry, don't know the author .-.-.-.-.-.
Connecting devices
------------------
 
  Normally, a 7 wire connection is used. Connect:
        GND1    to    GND2
	RxD1    to    TxD2
	TxD1    to    RxD2
	DTR1    to    DSR2
	DSR1    to    DTR2
	RTS1    to    CTS2
	CTS1    to    RTS2
  If a modem is connected, add lines for the following:
        RI, DCD
  If software wants it, connect DCD1 to CTS1 and DCD2 to CTS2.
  BEWARE! While PCs use pin 2 for RxD and pin 3 for TxD, modems normally
have those pins reversed! This allows to easily connect pin1 to pin1, pin2
to pin 2 etc. If you connect two PCs, cross RxD and TxD.
 
  If hardware handshaking is not needed, a so-called null-modem connection
can be used. Connect:
        GND1    to    GND2
	RxD1    to    TxD2
	TxD1    to    RxD2
Additionally, connect (if software needs it):
        RTS1    to    CTS1 & DCD1
	RTS2    to    CTS2 & DCD2
	DTR1    to    DSR1
	DTR2    to    DSR2
You won't need long wires for these!
  The null-modem connection is used to establish an XON/XOFF-transmission
between two PCs (see software section for details).
  Remember: the names DTR, DSR, CTS & RTS refer to the lines as seen from
the PC. This means that for your data set DTR & RTS are incoming signals
and DSR & CTS are outputs!
.-.-.-.- end .-.-.- 

Michael
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