Making a Null modem cable

This page also contains details on converting a Psion Dacom Surfer modem for use with your 5 (there are probably only 3 people in the known universe who wish to do this, but what the heck !)

(Please see my Disclaimer)

When you connect your Psion 5 to a modem, whatever type it is you will need to use an adapter between the two, known as a Null Modem Adapter.

The reason for this is that in modem terminology there are two types of equipment that are connected together :

• DTE - This is the PC end
• DCE - This is typically a modem

Now if you wish to connect a 5 to a modem, a null modem adapter converts the wiring such that instead of connecting two DCE devices together, you are connecting DTE to DCE.

You can buy null modem adapters from most Computer stores, but if you fancy knocking on up yourself, because of a connector gender, or a cable length requirement ...

OK here's the wiring plan : (Note the third is a straight thru connector, NOT to be used with a modem, without a null modem adapter.)

If you can find a source for the Honda connector used on the Series 3c and 5 serial cable (Let me know !!), you can try making up a Series 5 cable yourself ! The following data was provided on a faxed data sheet from Psion when I enquired about pin outs on the Series 5. As the fax held no copyright or disclaimer I have made the data available in the following tables. Note : I have not tried any of these cablings myself.

And finally ...
If you have a Psion Dacom Surfer modem like me, you will find that you have a mess of cables and converters between the modem and your 5. (The modem has a 4 foot cable, you will need a null modem adapter, possibly a gender bender, then you've got the 5 serial cable as well.) I decided that as I wouldn't be using the modem with a PC I would adapt the Surfer cable. These instructions will leave you with a Surfer having a small cable with a 9 way connector which goes straight into the 5 serial cable. This connector incorporates both a gender bender and null modem adapter.

Here goes :

• First chop off the existing Surfer 9 way D, leaving as short a cable still plumbed into the Surfer as you like. Leave yourself room for error though in case you cock up. Mine has about 5" of cable left.
• get a Male 9 way D type connector from Maplins/Radio Shack etc, and solder up the now bare modem cables to the D type using the following guide :
  Surfer Modem cable
  to 9 Way Male D

  Brown	3
  Green	2
  Purple	6
  Grey	5
  White	4
  Blue	8
  Red	7

• You can now plug your Series 5's serial cable straight into your modem. This minimizes messy cables, gender benders and null modem adapters !
• If you wish to use the Surfer with your PC again, you can solder a female 9 way D connector to the end of the now severed original cable, using exactly the wiring plan above. You now have a female terminated cable to adapt your Surfer back to connection with a PC.

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©1998 Phil Whiles

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Last revised: 13th May '98