2-4 SR3 SERIES PROTECTIVE RELAY PLATFORM – COMMUNICATIONS GUIDE
MODBUS PROTOCOL CHAPTER 2: RS485 INTERFACE
4. Di (+) AL —> AL
5. j+1 —> j
6. shr(A)
7. is there a carry? No: go to 8. Yes: G (+) A —> A
8. is j = 8? No: go to 5. Yes: go to 9.
9. i+1 —> i
10. is i = N? No: go to 3. Yes: go to 11.
11. A —> CRC
Timing
Data packet synchronization is maintained by timing constraints. The receiving device
must measure the time between the reception of characters. If 3.5 character times elapse
without a new character or completion of the packet, then the communication link must
be reset (i.e. all slaves start listening for a new transmission from the master). Thus at
115200 baud a delay of greater than 3.5 x 1 / 115200 x 10 x = 0.3 ms will cause the
communication link to be reset.
3 Series supported functions
The following functions are supported by the 3 Series relay :
• FUNCTION CODE 03H - Read Setpoints
• FUNCTION CODE 04H - Read Actual Values
• FUNCTION CODE 05H - Execute Operation
• FUNCTION CODE 06H - Store Single Setpoint
• FUNCTION CODE 07H - Read Device Status
• FUNCTION CODE 08H - Loopback Test
• FUNCTION CODE 10H - Store Multiple Setpoints
Refer to section 6 of this guide for more details on MODBUS function codes.
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