Rtcm/voter ttl gps

Can the VOTER take gps input from a TTL GPS?

I tried but maybe I am missing a config.

-Michael N5ZR

I’m not sure. Maybe you could lookup the recommended Garmin puck to see if it is TTL. The RTCM accepts Garmin or Trimble sentences and needs 1PPS. Did you find the RTCM/Voter documentation?

It looks to me like a TTL one is what they want.
have a look http://voipvoter.org/voter.pdf
seems like they are knocking down the 5 volt to 3.3 volts for internal use.

In any case, the Garmin “LVS” model they specify in the manual is sourced at 4.0-5.5 VDC so probably is producing TTL. RTCM manual


I thought you were talking about serial comms TTL vs RS232. Isn’t above +3.3 volts a 1 and .7 or below a zero? RS232 on the other hand is +/-12 volts.

Hmmmm, guess I was not clear. I meant the serial comm and pps data from the GPS are reduced from 5 volts to 3.3 volts inside the RTCM.

But the main point is that since the recommended serial GPS puck is powered at 5 volts, it’s likely to be providing a TTL signal.

There is a USB version of the puck though you would not expect to use that for pps.


It expects RS232 GPS input as that is what garmin provides. I have TTL GPS units with active antennas and currently use a ttl to rs232 converter. I would like to just provide TTL. I wonder if there are some ports left over on the max232?

The output of the Garmin is “RS232 compatible” - actual voltages are 0-5 volts just like TTL. None of the pins on the PIC can stand more than about 3.6 volts - hence the resistors at the input. You would probably damage the RTCM is you put a real RS232 signal on it with a +/- 12-volt swing.

So the Garmin output is the same voltage range as a TTL signal. But I do not know if the polarity of 1’s and 0’s would be right.


Since it runs on a 5V source, i doubt it would produce more than 5V. When i traced the schematic I noticed its going straight to the PIC. Must be something in the code that makes it recognize rs232 over ttl. There is a voltage divider on the RX from the GPS. Maybe reduce that 10k would help.

Again though, I see the other side of that MAX3222 is actually tied to ground and could be utilized to convert the GPS to the RS232 the PIC programing wants.

The max3222 produces +/ 13.2 volts using a built in charge pump.