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THE IRIG-B TIME CODE STANDARD
IRIG-B wiring considerations
Network Time Protocol / Simple Network Time Protocol
SNTP
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DC Level Shift IRIG time code in the MultiSync 100 is developed at a level of approximately
5 volts peak. This signal is normally distributed using copper wiring, however, in the case of
the MultiSync 100 the TTL outputs are coaxial with BNC connectors. It is usually possible to
connect an unmodulated IRIG driver to numerous IEDs using coaxial cable or preferably
shielded, twisted pair cable. The general limitation on length between clock and IEDs is
100m, and the number of IEDs is dependent on voltage drop calculations. An accuracy of
one millisecond should be possible for any reasonable configuration of coaxial cable and
IEDs; better accuracies require a careful design.
Installation of IRIG-B cables must follow best practices for installation of copper cables in
noise-inducing environments. Cables should be grounded and ground loops should be
avoided, therefore, ground at one point only. This ground point must be at the clock itself if
multiple devices will be connected, so it is an industry best practice to ground time-code
outputs at the clocks. A termination resistor can be added to the end of the coaxial run to
achieve good impedance matching, particularly for short or lightly loaded lines.
Designing an application for IRIG-B requires specific information about the clock output
port and device clock input ports, and a known cable resistance. Clock ratings are the
output voltage and output current; device ratings are the minimum and maximum voltage
rating and the impedance of the clock port. Devices are normally connected in parallel to
the clock output port. Solving the resulting equivalent circuit will determine if the clock has
enough capacity to drive the connected devices, and if the voltage level at each device is
sufficient for operation. The MultiSync 100 clock output ports are rated for 0-5V and
150mA sink/source.
Network Time Protocol (NTP) is a networking protocol for clock synchronization between
devices operating over packet-switched, variable-latency data networks, and is intended
to synchronize all participating devices to within a few milliseconds of UTC time. NTP can
achieve better than one millisecond accuracy in local area networks under ideal
conditions.
NTP functionally relies on a statistical average of the calculated round-trip delay and offset
between the device to be synchronized and multiple time servers on diverse networks. NTP
does not, therefore, directly account for switching time delays, asymmetry in network
paths, or reconfiguration of networks. Routine switching, network reconfiguration, and
traffic load reduce the accuracy of NTP time synchronization.
The 64-bit timestamps used by NTP consist of a 32-bit part for seconds and a 32-bit part
for fractional second, giving a time scale that rolls over every 232 seconds. NTP uses an
epoch of January 1, 1900, so the first rollover occurs in 2036. NTP can adjust for leap
seconds, but does not transmit information about local time zones or daylight saving time.
Simple Network Time Protocol (SNTP) is a less complex implementation of NTP. SNTP
disregards drift values and uses simplified system clock adjustment methods. As a result,
SNTP achieves only a low quality time synchronization when compared with a full NTP
implementation. It is typically used in applications where high accuracy timing is not
required.
CHAPTER 2: THEORY OF OPERATION
MULTISYNC 100 GPS CLOCK – INSTRUCTION MANUAL
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