Another one with faulty GPS module. Tracking gps problems can be really difficult, especially without schematics so I decided to give it a try with home made GPS module for 10 euros. The only condition is that chartplotter needs to have NMEA 0183 input. I have few NEO 6M gps modules lying around, you can buy them for 8 euros more or less. It can track up to 22 satellites on 50 channels and achieves the industry’s highest level of sensitivity i.e. -161 dB tracking, while consuming only 45mA supply current. Unlike other GPS modules, it can do up to 5 location updates a second with 2.5m Horizontal position accuracy. The u-blox 6 positioning engine also boasts a Time-To-First-Fix (TTFF) of under 1 second. It also outputs standard NMEA sentences.
NEO 6M serial output is working on TTL logic, so for chartplotter to be able to read serial communication, we need to level shift from TTL to RS422. One of the most farmilliar TTL to RS422 driver is MAX 490 integrated circuit. The next problem is that serial TTL output from NEO 6M is low at idle, while the NMEA0183 is high on idle. So we need to invert the signal coming from MAX 490. Luckily, in the IC's datasheet we can see that inverted signal is already available on pin 6, so no other hardware modification in needed, although it would be easy to do with NPN transistor or with opto coupler if you want to isolate RX/TX devices.
The last thing to do is set the baud rade. By default NEO 6M is working on 9600, while NMEA0183 devices work on 4800. So we can change the baud rate on the chartplotter, or we can change it on GPS module. Either by software or hardware. I decided to go with hardware, all you need to do is pull pin 14 high, and pin 15 low. And it is up and running for 10 euros. Next step will be to create PCB so that everything looks nice, and mount everything in a good looking box.
