Guralp Systems CMG-DCM build <10,000 - Manual (Page 122)

Acquisition Modules and Platinum Firmware Digitiser Synchronisation
This is also the recommended configuration for CMG-DAS units, where GPS
reception is available.
9.3 Using NTP with CMG-NAM units
Where GPS reception is not practical but an internet connection is available,
NTP can be used to synchronise the Platinum clock, which can then generate
NMEA output. This NMEA data-stream can be fed to the GPS input of the
digitiser module using an external cable from a serial port.
Note: This technique is only applicable to CMG-NAM and
CMG-NAM64s, where the Linux kernel can correctly handle the
incoming PPS from the GPS receiver.
The data flow when Internet NTP is used is illustrated below:
To configure the NTP subsystem, see section 7.4 on page 84.
To configure NMEA output, see section 9.7 on page 125.
9.4 Using GPS with Cylindrical Digitisers
Güralp Systems Ltd's cylindrical digitisers provide a CMG-DM24 and a
CMG-EAM in a single package. An internal, bi-directional connection is
available between Port C of the EAM and the digitiser module. This
connection can operate in one of two modes:
• The EAM's NTP subsystem can provide NMEA to the digitiser module's
clock circuitry. In this case, the external GPS socket is automatically
disconnected. This is the recommended configuration when GPS
synchronisation is impossible but internet NTP is available; or
• An external GPS receiver can provide input to both the digitiser
module's clock circuitry and the EAM's NTP subsystem. This
configuration is possible but not recommended.
MAN-EAM-0003 122 Issue E - February 2014