Michell Instruments Condumax II User Manual page 77

Condumax II User's Manual
The table below is an example of a message sent by the master, to read the signal
level (register 6) and the phase time left (register 7). The message shows a master
addressing a Condumax with a slave address of 01H and a Modbus function of 03H,
that informs the Condumax that it wishes to read two registers starting from address
06H, where bytes 3 & 4 hold the starting address and bytes 5 & 6 hold the number of
registers to be read. Bytes 7 & 8 contain the CRC code that is calculated using bytes 1
to 6 as represented below.
Byte No
1
2
3
4
5
6
7
8
In response to the above message, the Condumax will transmit with the following
message.
Byte No
1
2
3
4
5
6
7
8
9
This response repeats the address of the Condumax and the function code, along with
the byte count, the data and the CRC. In this example, the request asked for the values
from two registers therefore, the number of bytes returned is four, the value of register
6 is contained in bytes 4 & 5 and the value of register 7 in bytes 6 & 7. Register 6 =
50.15% and register 7 = 5m 26s in this example.
Any errors within the data of Read Request Message, will result in an exception being
raised.
Michell Instruments
Read Request Message
Meaning
Slave address
MODBUS function code
Starting address MSB
Starting address LSB
No of points MSB
No of points LSB
CRC Lo Byte
CRC Hi Byte
Read Response Message
Meaning
Slave address
MODBUS function code
Byte count
Data MSB
Data LSB
Data MSB
Data LSB
CRC Lo Byte
CRC Hi Byte
APPENDIX E
Value
01H
03H
00H
06H
00H
02H
??H
??H
Value
01H
03H
04H
13H
97H
05H
26H
??H
??H
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