Build a compact null modem connector with 9 pin serial (DB9) RS232 interface

OK - so this isn't rocket science but when I made this null modem they were selling in the shops for something like £12 or so. You can get them now for around £3 or £4 but if you fancy making your own they are quite easy.

What is a null modem connector?

The serial interface is quite an old method of connecting two electronic devices to enable them to communicate with each other. These days you are more likely to hear about Universal Serial Bus - USB but some devices do still use the old serial device. The connections on these devices are fairly standard but some variations exist. The connectors come in 25 pin and 9 pin varieties. Depending on the complexity and purpose of the interface, it will require a variable number of the pins to be connected. At a minimum, 3 pins have to be connected - that is the transmit line, the receive line and the ground line. Beyond that there are flow controls and handshakes which require more connections but they are rather beyond me!

Why all the fuss - isn't it just a connector?

Well yes it is but the problem is that both devices will often be fitted with female (socket) connectors (pin numbering below) and wired according to the details below.

So if you just get 2 male (plug) connectors (pin numbering below) and wire them together with the same pin connections you will end up with the the ground lines (pin 5) correctly connecting but you will get the transmit line (pin 3) of one device connecting to the trasnmit line of the other device and the receive line (pin 2) of one device connecting to the receive line of the other device. Clearly this is not going to work.

What is required therefore is to cross the transmit and receive lines so that transmit on one device goes to receive on the other device and vice versa.

When I needed to connect my GPS to my PDA (palmtop computer) or my dive computer to my PC I required a null modem connector. You can buy them as cables (known as a crossover cable) but I wanted something that was compact and robust as I would be taking it out and about and the cables from my GPS unit were already long enough. The following unit was constructed with relative ease:

Parts (with Maplin part numbers and prices from 2000)

• 2x DB9 male connectors (RK60Q D-Range 9 Way Plug @ £0.59)
• 3x short lengths of wire
• 1x DB9 double headed casing (JM07H 9 Way Plastic D Box @ £0.99)
• soldering equipment
  
  
  
  Total cost (2000) approx £2.17

Instructions

• Open up the housing (needs prising appart with 2 small screwdrivers
• Place the two serial connector plugs at the ends of the housing
• Measure and cut the 3 lengths of wire so there is sufficient room for a bit of bending within the casing and so that soldering is easy
• Solder the wires as follows:
  • pin 5 to pin 5
  • pin 2 to pin 3
  • pin 3 to pin 2
• Snap housing back in to place and use!

Finished device in use

iPAQ H2210 PDA

via proprietary connector with female socket DB9
to

home made null modem connector with double ended male plug DB9
to

female socket on
proprietary Garmin connector to

Garmin etrex Summit GPS Unit