COMMERCIAL IN CONFIDENCE
Page 11
2.4 ModBus Message Construction.
The following message function codes are implemented in the instrument. Where a value of xx is
shown, substitute the correct value for your installation and the data item required. All CRCs are shown
as ?? since must be automatically generated according to the data contained in the message.
2.4.1 Read Coil Status (single bit read)
Byte No Meaning Value (hex)
1 Slave address xx
2 ModBus Function code (read coil status) 01
3 Starting Address MSB, always 00
4 Starting Address LSB xx
5 No of points MSB 00
6 No of points LSB 01
7 CRC lo byte ??
8 CRC hi byte ??
Note that in this implementation, only one bit may be read per message, so the number of
points should always be set to 1, but in fact this value is ignored anyway. If the address is not a
valid readable bit, an error response code 2 (invalid address) is returned, otherwise the
following response is sent:
Byte No Meaning Value (hex)
1 Slave address xx
2 ModBus Function code (read coil status) 01
3 Byte count 01
4 Bit value (01 if bit is set, 00 if not) 00 or 01
5 CRC lo byte ??
6 CRC hi byte ??
2.4.2 Read Holding Registers
Byte No Meaning Value (hex)
1 Slave address xx
2 ModBus Function code (read holding register) 03
3 Starting Address MSB xx
4 Starting Address LSB xx
5 No of registers MSB 00
6 No of registers LSB 01
7 CRC lo byte ??
8 CRC hi byte ??
Note that the only one register may be read per message, so the number of registers should be 1
(although this value is ignored in this implementation). The normal response will be:
Byte No Meaning Value (hex)
1 Slave address xx
2 ModBus Function code (read register) 03
3 Byte count (always 2, even though the register may be only 1
byte wide)
02
4 Data MSB (will be 0 if a single byte register) xx
5 Data LSB xx
6 CRC lo byte ??
7 CRC hi byte ??
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