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Serial Cable Specs (8/9/94)

Q: I want to make my own serial cable. Which wires and which connector pins do I use?

A: To create a hardware flow control capable cable for Mac-to-Newton or Newton-to-Newton communications (also called a "null-modem" cable) all you need are two mini-din-8 connectors and seven wires connected as follows:

Ground (4) -> Ground (4)  (also connect to connectors' shrouds)
Transmit+ (6) -> Receive+ (8)
Transmit- (3) -> Receive- (5)
Receive+ (8) -> Transmit+ (6)
Receive- (5) -> Transmit- (3)
Data Term Ready (1) -> Clear To Send (2)
Clear To Send (2) -> Data Term Ready (1)

You should use twisted pairs for 6/3, 8/5, and 1/2, to improve signal quality and reduce attenuation, especially in long cables. You can use side-by-side pairs, as in telephone hookup cable, for short cable runs.

Remember that because RS-422 uses a differential signal for transmit and receive, you always need two transmit and two receive pairs, and a break of either wire will cause communications in that direction to fail. The advantage, however, is significantly longer and more reliable cable runs than RS-232.

If you don't use hardware flow control, you can eliminate the 1/2 pair, but that's not recommended unless you know this cable will be used only in software flow control situations.

Q: What's the pin mapping on the Newton-to-PC (DIN-to-DB9) cable?

A: Here it is:

Note that the pin numbers shown are as defined above.

PC (DB9)    Newton (DIN)    
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1           1    
2           3    
3           5    
4           7,2    
5           4,8    
6           1    
7           N/C    
8           N/C    
9           N/C    

N/C=not connected.