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2.6) What configuration of serial cable should I use for modem work?




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This article is from the comp.sys.acorn FAQ, by Paul Vigay with numerous contributions by others.

2.6) What configuration of serial cable should I use for modem work?

Here follows a diagram of the necessary connections for common terminal programs to work properly. They are as far as I know the informal standard agreed upon by commercial comms software developers for the Arc.

Pins 1, 4, and 8 must be connected together inside the 9 pin plug. This is to avoid the well known serial port chip bugs. The modem's DCD (Data Carrier Detect) signal has been re-routed to the Arc's RI (Ring Indicator) most modems broadcast a software RING signal anyway, and even then it's not really necessary to detect it for the modem to answer the call.

 Arc (9 pin)                      Modem (25 pin)
 -----------                      --------------
  
 +---1---DCD
 |
 |   2---RxD------------------------RxD----3
 |
 |   3---TxD------------------------TxD----2
 |
 +---4---DTR------------------------DTR---20
 |
 |   5---0v-------------------------SG-----7
 |
 |   6---DSR------------------------CTS----5
 |
 |   7---RTS------------------------RTS----4
 |
 +---8---CTS
  
     9---RI-------------------------DCD----8
Of course you can connect the Modem pin 20 to any one of pins 1, 4, or 8 on the Archimedes plug, as they are all connected together anyway.

Chocks Away Extra Missions (the flight simulator from 4th Dimension) suggests that the serial cable be wired as above except that pins 1-4-6 are connected together and the modem's CTS (pin 5) be connected to the Arc's pin 8 (ie the connections at pins 6 and 8 be swapped over at the Arc's end). This has been tried and it also seems to work fine.

However newer Arc's like the A5000 have come out (and indeed the occasional A310) with a `corrected` serial port. This newer serial port operates as it should and is directly compatible with standard PC cables. Older comms software about do not take this in account and assume that you have a cable patched in the manner described above. If you do not use such a patched cable on these `fixed` serial ports this software will generally fail to work completely. (Usually hardware flow control fails.)

With the advent of the Risc PC a standard PC cable is advised.

 

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