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2.9) How do I make a Null modem cable?




Description

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

2.9) How do I make a Null modem cable?

For starters you will need soldering skills and the necessary components. Namely cable, connectors (9 pin female D-type), a soldering iron, solder and the will to use them. All of these items, bar the will, can be found down at the local electronic components store. Assuming you have them all then you will need to decide what kind of machines you are hooking together.

There are three cases and I need to define a few terms.

'Archimedes' is defined to be A300 series, A400 series (including the /I machines), R140, A540, A3000 (but not the A30x0 machines) and R260 machines. 'RiscPC' is defined to be both the RiscPC series but also the A5000, A4000, A30x0 & A4 machines. All of these machines have a 'PC Style' serial port that conforms closely to RS232 specifications.

This means that if you are connecting your Acorn machine to a non Acorn machine then generally treating the foreign machine as a RiscPC, in terms of serial handling, will work. There are exceptions, Macintoshes in particular have had non-standard serial ports and may require further research before you can create a cable for them.

The cases are :-

Archimedes to Archimedes

 Arc (9 pin)                      Arc (9 pin)
 -----------                      -----------
 
 +---1---DCD                        DCD----1---+
 |                                             |
 |   2---RxD------------------------TxD----3   |
 |                                             |
 |   3---TxD------------------------RxD----2   |
 |                                             |
 +---4---DTR------------------------DTR----4---+
 |                                             |
 |   5---0v-------------------------0v-----5   |
 |                                             |
 |   6---DSR------------------------RTS----7   |
 |                                             |
 |   7---RTS------------------------DSR----6   |
 |                                             |
 +---8---CTS                        CTS----8---+
 |                                             |
 +---9---RI                         RI-----9---+

Archimedes to RiscPC

 Arc (9 pin)                      RiscPC (9 pin)
 -----------                      --------------
 
 +---1---DCD                        RI-----9---+
 |                                             |
 |   2---RxD------------------------TxD----3   |
 |                                             |
 |   3---TxD------------------------RxD----2   |
 |                                             |
 +---4---DTR------------------------DTR----4   |
 |                                             |
 |   5---0v-------------------------0v-----5   |
 |                                             |
 |   6---DSR------------------------RTS----7   |
 |                                             |
 |   7---RTS------------------------CTS----8   |
 |                                             |
 +---8---CTS                        DSR----6---+
                                               |
     9---RI-------------------------DCD----1---+

RiscPC to RiscPC

  RiscPC (9 pin)                   RiscPC (9 pin)
  --------------                   --------------
  
     2---RxD------------------------TxD----3
 
     3---TxD------------------------RxD----2
 
     4---DTR---------------------+--DSR----6
                                 |
     6---DSR--+                  +--RI-----9
              |                  |
     9---RI---+                  +--DCD----1
              |
     1---DCD--+---------------------DTR----4
 
     5---0v-------------------------0v-----5
                                 
     8---CTS------------------------RTS----7
   
     7---RTS------------------------CTS----8

Note that most PC compatible machines have 25 pin D type male ports for their second COM port. You have two options in this case - either re-wire the cable for the 25 pin port or you can buy a 9 to 25 pin converter plug. Either solution works well. Here are the relevant pins for the 25 pin port :-

  Pin No.  Function
  -------  --------
     8       DCD
     3       RX
     2       TX
     20      DTR
     7     GND (0v)
     4       RTS
     5       CTS

 

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