Page C-2
MPS Motor Protection System Rev. 6-F-022117
Appendix C, MPS Modbus Protocol
The addressed slave responds with its address and
Function Code 3, followed by the information field. The
information field contains an 8-bit byte count and the 16-bit
data from the slave. The byte count specifies the number of
bytes of data in the information field. The data in the
information field consists of 16-bit data arranged so that the
MSB is followed by the LSB.
The maximum number of 16-bit registers that can be read
is 120.
C.4.3 WRITE TO REGISTER
Function Code 6 or 16 is used to make set-point changes.
C.4.3.1 WRITE SINGLE REGISTER (CODE 6)
The function code format for writing a single register is
shown in Table C.2.
The message consists of the MPS address followed by the
Function Code 6 and two 16-bit values. The first 16-bit
value specifies the register to be modified and the second
value is the 16-bit data.
Provided no errors occurred, the slave will re-send the
original message to the master. The response message is
returned only after the command has been executed by the
MPS.
The following message will set register 3 to 300 in
slave 5:
0x05 | 0x06 | 0x00 | 0x03 | 0x01 | 0x2C | 0x78 | 0x03
TABLE C.2 WRITE SINGLE REGISTER (CODE 6)
Byte 1
Byte 2
Byte 3
Byte 4
Byte 5
Byte 6
Byte 7
Byte 8
Slave Address
Function Code
MSB Register Address
LSB Register Address
MSB of Data
LSB of Data
LSB of CRC
MSB of CRC
C.4.3.2 WRITE MULTIPLE REGISTERS (CODE 16)
The function-code format in Table C.3 can be used for
writing single or multiple registers.
TABLE C.3 WRITE MULTIPLE REGISTERS (CODE 16)
Byte 1
Byte 2
Byte 3
Byte 4
Byte 5
Byte 6
Byte 7
.
.
.
Byte n
Slave Address
Function Code
MSB Register Address
LSB Register Address
MSB of Quantity
LSB of Quantity
Byte Count
MSB of Data
LSB of Data
LSB of CRC
MSB of CRC
The MPS will reply with the slave address, function code,
register address, and the quantity followed by the CRC code
for a total of 8 bytes.
C.4.4 COMMAND INSTRUCTION (CODE 5)
Modbus Function Code 5 (Force Single Coil) is used to
issue commands to the MPS. The format for the message
is listed in Table C.4 and the command code actions and
corresponding coil number are listed in Table C.5.
TABLE C.4 COMMAND FORMAT CODE 5
Byte 1
Byte 2
Byte 3
Byte 4
Byte 5
Byte 6
Byte 7
Byte 8
Slave Address
Function Code
MSB of Command Code
LSB of Command Code
Fixed at 0xff
Fixed at 00
LSB of CRC
MSB of CRC
TABLE C.5 SUPPORTED COMMANDS
0x0000
0x0001
0x0002
0x0003
0x0004
0x0005
0x0006
0x0007
0x0008
0x0009
0x000A
0x000B
0x000C
0x000D
1
2
3
4
5
6
7
8
9
10
11
12
13
14
STOP
START1
START2
Reset Trips
Set Real-Time Clock
Clear Data-Logging Records
Clear Trip Counters
Clear Energy Totals
Clear Running Hours
Emergency I
2
t and Trip Reset
Select Local Control
De-select Local Control
Re-enable Temperature Protection
Start Trace
Except for a broadcast address, the slave will return the
original packet to the master.
C.4.5 COMMAND INSTRUCTIONS USING WRITE
COMMANDS
For PLC's not supporting Function Code 5, MPS
commands can be issued using Write Single Register (Code
6) and Write Multiple Register (Code 16).
Commands are written to MPS register 6 (Modbus
register 40007). Supported commands are listed in the
COMMAND CODE column in Table C.5.
When using the Write Multiple Registers function code,
the write should be to the single MPS Register 6. If
multiple registers are written starting at MPS Register 6, the
first data element will be interpreted as the command code
but no other registers will be written. If the command is
successful, the MPS will return a valid response message.
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