This & That: Converting a Polstar PGM-248 GPS for use with a Kenwood APRS radio

This & That

Converting a Polstar PGM-248 GPS for use with a Kenwood APRS radio

A couple of years ago I bought a GPS for use with an ETEN M600 PDA. Shortly thereafter I left my job and had to return the M600 (bad timing!) but I kept the GPS since it was bought with my own cash. The GPS has sat unused ever since, but today I decided to convert it for use with one of my Kenwood APRS-capable 2-way radios. For more information about APRS see this link.

This tiny (41x41x18mm) Polstar PGM-248 'GPS mouse' is powered via a car cigarette lighter socket adapter. In its original configuration the adapter passes 5v DC 'upstream' to power the GPS and 5v DC 'downstream' to charge the PDA. GPS data is passed 'downstream' from the GPS receiver, through the power adapter and on down to the PDA.

Output is NMEA via TTL or RS-232, with a default baud rate of 4800 (optionally 9K6, 19K2 or 38K4).

Quite simply, if I could replace the specialised multi-pin ETEN connector (originally intended to connect to the M600) with a stereo 2.5mm jack plug (to connect to the Kenwood radio) I would have a compact GPS receiver feeding the radio with positional data. First task was to find wiring data for the Polstar GPS, which came from the PGM-248's data sheet on the Polstar website.

The following information is provided for interest only. No responsibility can be accepted for damage caused by implementing this conversion.

Between the GPS receiver and the power adapter is a PS/2 connector, the pin-out of the male (GPS) side of which is:

Cutting off the specialised ETEN connector revealed just 4 of the 6 wires; to my relief the wire colours (which were labelled inside the connector) matched the diagram above.

Black - ground
Red - 5v DC
White - serial GPS data RX
Green - serial GPS data TX

Interestingly this would seem to be the same four connections found in a standard USB plug (even down to the wire colours).

I only wanted three of the available wires: serial data RX & TX and ground.

The hardest part of the conversion was locating a stereo 2.5mm plug among my accumulated ~~rubbish~~ spare parts! The white cable was soldered to the tag leading to the tip of the plug. The green cable was soldered to the tag leading to the centre connector of the plug. The black wire was soldered to the ground/earth tag. The red (5vDC) cable was cut short and bound with insulating tape.

Don't tell me that the white cable is the worst soldering job of all time - I know! I've already mended it!

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Thanks! Nigel GJ7LJJ