Westermo U200 Operator manal User Manual
Page 34
V4.5
www.westermo.com
U/R/T200 series
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The CPU handles the time sync protocol, T200 configuration via e.g. SNMP, serial interface
versus an external clock source (if this available) and the interface versus the FPGA. The
NMEA protocol over RS232 or RS422 versus an external GPS is often relevant in order to
have reference to absolute time. RS422 is the preferred interface for both serial data and the
PPS signal in order to meet various installation requirements (distance between GPS receiver
and PTP clock).
NMEA 0183 is an interface protocol created by the National Marine Electronics Association.
NMEA is a simple, yet comprehensive ASCII protocol which defines both the communication
interface and the data format. For those applications requiring output only from the GPS
receiver, NMEA is in most cases the preferred choice. The GGA and ZDA are the two NMEA
reports used by the T200. The format of these two reports is as follows:
$GPGGA,
M,
UTC time
hhmmss format
Latitude
ddmm.mmmmm format
Latitude N/S
Latitude hemisphere North or South
Longitude
dddmm.mmmmm format
Longitude E/W
Longitude hemisphere East or West
GPS quality
0=fix not available,
1=Non-diff GPS available,
2=diff. GPS fix available
Nr of satellites in use
00 to 12 satellites
Horizontal dilution precision
0.5 to 99.5
Antenna height above/below mean -9999.9 to 99999.9 meters
Geoidal height
-999.9 to 9999.9 meters
Diff. GPS data age
nr of seconds since last valid RTCM transmission
Diff. ref. station ID
0000 to 1023.
GPZDA - Time and Date
$GPZDA,
UTC
hhmmss.s
Day
dd (01 to 31)
Month
mm (01 to 12)
Year
yyyy
Unused
-
Unused
-
The most relevant time synchronization protocols are based on SNTP/NTP
(RFC2030/RFC1305) or P1588 (IEEE Std 1588™-2002). These protocols are all based on
UDP/IP.
Timing accuracies in the order of one millisecond (ms) can be achieved on a time client by
using the built-in SNTP client software available on newer versions of Windows or other
operating systems when time updates are performed versus the T200 time server. 5-25
microseconds (µs) timing accuracies is possible by performing time stamping of incoming an
outgoing time packets on the client interrupt service routine. 1 µs accuracy or better can be