CHAPTER 3: MODBUS FUNCTIONS CLOCK SYNCHRONIZATION OF MULTIPLE RELAYS
750/760 FEEDER MANAGEMENT RELAY – COMMUNICATIONS GUIDE 3 - 7
Clock Synchronization of Multiple Relays
The time and date of multiple relays on the same communication link can be synchronized
so that time stamping of events on different relays can be correlated. The following
procedure describes how to synchronize the clocks of multiple relays. Since the clock is
accurate to 1 minute per month, performing this procedure every 10 minutes will result in
synchronization accuracy of ±10 ms.
1. Store values for the date and time setpoints Set Date and Set Time at locations 1006h
and 1008h in the Memory Map respectively into each slave device.
2. Broadcast (via slave address 0) the Set Date operation code via function code 05h to
all connected slaves to synchronize the dates.
3. Broadcast (via slave address 0) the Set Time operation code via function code 04h to
all connected slaves to synchronize the times. If synchronizing to an external clock
source then this command should be sent when the time is equal to the value stored
in Set Time.
The following table shows the format of the master and slave packets for a master device
storing the date of June 10, 1994 and time of 2:15:30 PM to slave device 11 as required by
step one of the procedure.
05* Acknowledge
The addressed slave device has accepted and is processing a long duration
command. Poll for status.
06* Busy, Rejected Message
The message was received without error, but the slave device is engaged in
processing a long duration command. Retransmit later, when the slave
device may be free.
07*
Nak - Negative
Acknowledge
The message was received without error, but the request could not be
performed, because this version of the 750/760 does not have the
requested operation available.
Table 3: Exception Response Error Codes
Error
Code
Modbus Definition GE Multilin Implementation
* Some Modbus implementations may not support these exception responses
Master Transmission Bytes Example Description
Slave Address 1 11 message for slave 11
Function Code 1 10 store multiple setpoints
Data Starting Address 2 10 06 data at address 1100h
Number Of Setpoints 2 00 04 4 setpoints = 8 bytes total
Byte Count 1 08 8 bytes of data
Data #1 2 06 0A month = 06h (June), day = 0Ah (10)
Data #2 2 07 CA Year = 07CAh (1994)
Data #3 2 0E 0F Time (hr:min) = 0Eh:0Fh (14:15)
Data #4 2 75 30 Time (ms) = 7530h (30000 ms = 30 s)
CRC (Low, High) 2 37 0C computed CRC error code
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