Guralp Systems CMG-DCM build <10,000 - Manual (Page 95)

Operator's Guide
Set the Port function to “NMEA out. NMEA (time + fixed position)
output” and the Port speed to 4800. Click “Submit” to save these
changes.
Go back the configuration of the serial port and click on “NMEA
output settings”. You will see this screen:
November 2010 95
Note: For Authenticated Digitisers, Port C should be used in
order to provide NMEA output to the internal digitiser module.
If a GPS receiver is used, the “Data Out” Port (exposed as the
DATA connector) can be used to provide timing to additional,
external digitisers.