GPS-500 Setup

Looking for a cheap GPS receiver? Look no further than the GPS-500 Microsoft Streets and Trips GPS by Pharos. This tiny unit features a SiRF Star III chipset and most are selling on ebay for just under 10 dollars! This GPS receiver can be interfaced via a RS232 serial link using a MAX232 chip from Maxim IC and a low cost USB to serial converter.

Difficulty : Medium
Time: 1 Hour

GPS-500 Side

GPS-500 Top

After purchasing this GPS receiver, I thought that it would be ready to go, very wrong. In reality the unit needs a special converter cable to interface with a serial port, who has time to buy and wait for that?

Assuming that most devices use 5 volts, I started the very dangerous method of applying power and ground to various pins on the GPS receiver. Once the power was applied correctly a blue led turned on.

Now, I just needed to get the NMEA output data from the GPS receiver. Using a RS232 converter like the MAX232ACPE shown in the schematic below, I connected pin 14 of it to the RX pin of a serial port and listened for data via hyperterminal. Next, I attached a wire to pin 11 of the MAX232ACPE and tried the remaining pins on the GPS receiver.

GPS Test Schematic

Upon finding the TX pin of the GPS receiver, hyperterminal was receiving garbage text because of an incorrectly set baud rate. After some trial and error, the baud rate was found to be 4800bps.

Based on what I have found, the pinouts are as follows below. The unused pins could be RX or some sort of special functionality that the SiRF Star III GPS chipset has to offer.

GPS-500 pinout

Lastly, I added a switch on the output of the GPS’s TX line, a usb to serial converter, and LOTS of hot glue to secure the wires in order to interface with my laptop. The switch was added to shut off the GPS’s TX output from sending data to the computer during the first 5 seconds of powering up. Otherwise, hyperterminal would cease to read from the device.

Finalized GPS Schematic

The SiRF Star III technology is quite remarkable, I was able to acquire 8 GPS satellites on the first floor of a two story house!

GPS Teardown

GPS Laptop Setup