🚀 go-pugleaf

gkaiway@direct.ca (gary kaiway) wrote:



>Does anyone happen to know how to make a null-modem cable?  (25pin to 
>25pin.)  I'm in the process of trying to fix one, and need to know 
>which pins to swap.

Always swap pins 2 and 3. Always run wire 7.

In addition you may need to do some other things. There is difference
of opinion on the best way to do these. 

One way is to jumper 4-5, 6-8-20  within each connector. This lets pin
6 (data set ready) , pin 8 (carrier detect) and pin 20 (data terminal
ready)  be pulled up -- generally by the data terminal ready from the
computer. A "data set" is the modem and it drives pins 8 and 6
normally. This allows a 3-wire cable.

The other way is to let pin 20 of each connector drive pins pins 6 and
8 at the other end.

Extract of U.S.Robotics file SPHSASCII.TXT:

			Serial Interface Pin Definitions 
				
						Signal Source 
	
	DB-25   DB-9    Circuit Function        Computer _ Modem 
	1       -       AA      Chassis Ground          Both    
	2       3       BA      Transmitted Data        Computer        
	3       2       BB      Received Data           Modem   
	4       7       CA      Request to Send         Computer        
	5       8       CB      Clear to Send           Modem   
	6       6       CC      Data Set Ready          Modem   
	7       5       AB      Signal Ground           Both
	8       1       CF      Carrier Detect          Modem   
	12      -       SCF     Speed Indicate          Modem   
	20      4       CD      Data Terminal Ready     Computer        
	22      9       CE      Ring Indicate           Modem