Read Holding Registers - CAL Controls 3300 Communications Manual

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
Byte No
1
2
3
4
5
6
7
8
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
1
2
3
4
5
6
2.4.2
Byte No
1
2
3
4
5
6
7
8
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
1
2
3
4
5
6
7
COMMERCIAL IN CONFIDENCE
Read Coil Status (single bit read)
Meaning
Slave address
ModBus Function code (read coil status)
Starting Address MSB, always
Starting Address LSB
No of points MSB
No of points LSB
CRC lo byte
CRC hi byte
Meaning
Slave address
ModBus Function code (read coil status)
Byte count
Bit value (01 if bit is set, 00 if not)
CRC lo byte
CRC hi byte

Read Holding Registers

Meaning
Slave address
ModBus Function code (read holding register)
Starting Address MSB
Starting Address LSB
No of registers MSB
No of registers LSB
CRC lo byte
CRC hi byte
Meaning
Slave address
ModBus Function code (read register)
Byte count (always 2, even though the register may be only 1
byte wide)
Data MSB (will be 0 if a single byte register)
Data LSB
CRC lo byte
CRC hi byte
Page 11
Value (hex)
xx
01
00
xx
00
01
??
??
Value (hex)
xx
01
01
00 or 01
??
??
Value (hex)
xx
03
xx
xx
00
01
??
??
Value (hex)
xx
03
02
xx
xx
??
??