Hi.
I have an EM-406a module. and i am trying to interface it with a Rabbit RCM5700 microcontroller kit.
I connected the TX of the GPS to the RX of one of the serial ports of my microcontroller . And i was able to get a fix on my location and get the coordinates.
However, when I connect the RX of the GPS to the TX of the uC, it (almost immediately) loses the fix. it still gives NMEA messages, but with no position.
I have tried to disable all messages and poll only the ones that i need. but it still cannot acquire it’s position.
For some reason, after i try to send any message to it, the LED becomes very dim and it stays like that for a day.
I do not care about the LED being dim as it still responds to my polling messages (with invalid points).
Why does it lose the fix when i connect RX ??
I would appreciate it very much if anyone could help me with this.
Thank you!
You might be getting interference with the GPS signal from the Rabbit board, the Rx connection could be acting as an antenna and radiating an oscillator harmonic.
Thank you for your quick response, Leon.
I don’t think the problem is from radio interference. because even if I do not try to send anything on the RX. it still loses it’s position. (just by plugging it in, without any programming).
Is there any way to check for this type of interference?
other ideas would also be appreciated.
If it is just plugged it in it can cause an EMC problem, it’s nothing to do with whether it is transmitting data. Where I used to work we got a spurious signal from an RS-232 lead on a system I designed, we had to add RF filters to the connections inside the enclosure to get CE conformance.
How are you powering the GPS?
I was using a PC power supply to give a steady 5V to it.
(all grounds of uC and GPS are joined together, if that’s what you are asking).
I recently started using 4 (D size) batteries in series to provide 6V ( it can handle 4.5-6.5).
I dont think power is the issue here. As i said before, it works fine (gets the position) when i dont plug in the RX line (of the gps) .it even responds to my NMEA commands when i plug the RX line in but with no position lock!
After i issue any command and switch off the power, then turn the power back on, the LED becomes very dim. I have read that this means Binary mode (it is not a sign of bad power) . it still responds to NMEA commands, though…
just leave it unplugged for a couple of hours and plug it back and it is back to normal.(bright LED, flashing when locked)
I honestly dont know what to think…
I don’t know anything about the Rabbit boards but from what I can see the Rabbit board uses 3.3v logic levels and the EM-406a uses 2.85v logic levels so unless you’re using a level translator you’re pumping to much voltage into the GPS. Try dropping the signal across a resistor divider.
Thank you very much wayneft.
the problem was with the logic levels. I just tested it with a voltage divider today and it worked perfectly…
the LED is still dim… so it still goes into binary mode out of it’s own will… but that is not an issue for me…
thanks again everyone! I appreciate everyone’s help. 