Cooper SMP 4 - Using the SMP Gateway as a Time Source

200 • SMP Gateway User Manual
21.3.5 Using a SCADA Protocol’s Time Adjustment Feature
When no external time source is available to the SMP Gateway, the SCADA or control center can
itself serves as a time source, if the protocol it uses to communicates with the gateway supports
time adjustment (synchronization). DNP3 and IEC 60870-5-104 are such protocols.
As each protocol has its own distinctive characteristics, refer to the protocol’s documentation
found on the SMP Gateway Software and Tools CD-ROM to learn how to enable this protocol-
specific feature.
21.4 Using the SMP Gateway as a Time Source
Following the August 2003 blackout, NERC has recommended that substation devices be
synchronized using a GPS clock. When the SMP Gateway is synchronized by an accurate time
source such as an external IRIG-B source or an internal GPS clock, it can distribute accurate time
to substation devices in one of the following ways:
 Through a modulated or demodulated IRIG-B signal.
 By acting as an SNTP server.
 Using a master protocol, such as DNP3, IEC 60870-5-101 or IEC 60870-5-104. The accuracy
may be limited by the protocol, but the overall synchronization of all the devices is typically
accurate to within one second.
A slight delay may be introduced in the time information transmitted to devices. Output time
accuracy is function of the type of time input used to adjust the SMP Gateway’s internal clock,
and of the type of protocol or signal used to synchronize the devices. The resulting delays are
presented in the following table.
Output Delay Accuracy (± ms)
Protocol
output
SNTP
output
Modulated
IRIG-B
output
Demodulated
IRIG-B
output
Protocol
input
500 500 - -
SNTP input
500 100 - -
Modulated
IRIG-B input
500 100 10 10
Demodulated
IRIG-B input
500 100 10 1
GPS input
500 100 10 1
For example, if the SMP Gateway is time-adjusted using demodulated IRIG-B, a device’s clock
adjusted by the gateway using demodulated IRIG-B may show a delay of ± 1 milliseconds. Note
that the SMP Gateway can be more accurate: consider these values as “worst case scenarios”.