How to wire a null-modem cable

I swiped this source from, it was so damn useful I was afraid it might disappear. Finding this document was one of the things that made me consider that the web might actually be useful. It saved me the trouble of walking across the street to my office to pull out a paper copy from the back of some hideous manual. The ambivalence I have towards this event exactly sums up my feelings for the net in general.

25pin 9pin 9p 25p FG 1 – X – 1 FG
TD 2 3 – 2 3 RD
RD 3 2 – 3 2 TD
RTS 4 7 – 8 5 CTS
CTS 5 8 – 7 4 RTS
SG 7 5 – 5 7 SG
DSR 6 6 – 4 20 DTR
DTR 20 4 – 6 6 DSR

Frame Ground
Transmit Data
Recieve Data
Request To Send
Clear To Send
Signal Ground
Data Set Ready
Data Terminal Ready

You assemble one connector from each side of the hyphens. The frame ground on the 25 pin plug is optional and should I have been told only be connected on one side of the cable. If you need Data Carrier Detect on your cable (BBS programs, Unix Getty and so on) then you should apply this patch. On a 25 pin plug you can connect pin 6 and 8 On a 9 pin plug you can connect pin 1 and 6 These connections should be made on the same side of the cable.

The standard for RS232 sets these signals on the various pins.

   Common name   RS232C Name          Description

25pin         9 pin        DCE Direction
1			AA	x	Protective Ground
2	TXD	3	BA	I	Transmitted Data
3	RXD	2	BB	O	Received Data
4	RTS	7	CA	I	Request to Send
5	CTS	8	CB	O	Clear to Send
6	DSR	6	CC	O	Data Set Ready
7	GND	5	AB	x	Signal Ground
8	CD	1	CF	O	Received Line Signal Detector
9			--	x	Reserved for Data set Testing
10			--	x	Ditto
11				x	Unassigned
12	SCF			O	Secondary Received Line Signal 						Detector
13	SCB			O	Secondary Clear to send
14	SBA			I	Secondary Transmitted Data
15	DB			O	Transmission Signal Element 						Timing	(DTE Source)
16	SBB			O	Secondary Received Data
17	DD			O	Receiver Signal Element Timing 						(DCE Source)
18				x	Unassigned
19	SCA			I	Secondary Request to Send
20	DTR	4	CD	I	Data Terminal Ready
21	CG			O	Signal Quality Detector
22		9	CE	O	Ring Indicator
23	CH/CI			I/O	Data Signal Rate Selector 						(DTE/DCE Source)
24	DA			I	Transmit Signal Element Timing (DTE Source)
25				x	Unassigned

I will not accept any responsibility for the correctness of this information nor any liability for damages that have been inflicted on you through the use of this information

Posted on June 17th, 2009 in | No Comments »