Michell Instruments Promet EExd User Manual page 65

Promet EExd User's Manual
The table below is an example of a message sent by the master, to read the time to the
next log (register 7) and Channel 2 gas temperature (register 8). The message shows
a master addressing a Promet EExd with a slave address of 01H and a Modbus function
of 03H that informs the Promet EExd that it wishes to read two registers starting from
address 07H. 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
Read Request Message
In response to the above message, the Promet EExd may transmit the following message.
Byte No
Read Response Message
This response repeats the address of the Promet EExd 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 7 is contained in bytes 4 & 5 and the value of register 8 in bytes 6 & 7. Register
7 = 5m 26s and register 7 = 00FA = 252 = 25.2°C in this example. Any errors within
the data of Read Request Message will result in an exception being raised.
Michell Instruments
1 Slave Address
2 MODBUS function code
3 Starting Address MSB
4 Starting Address LSB
5 No of points MSB
6 No of points LSB
7
8
1 Slave Address
2 MODBUS function code
3
4
5
6
7
8
9
Meaning
CRC Lo Byte
CRC Hi Byte
Meaning
Byte Count
Data MSB
Data LSB
Data MSB
Data LSB
CRC Lo Byte
CRC Hi Byte
APPENDIX E
Value
01H
03H
00H
07H
00H
02H
??H
??H
Value
01H
03H
04H
05H
26H
00H
FAH
??H
??H
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