Michell Instruments Liquidew EExd User Manual page 73

Liquidew 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 sample temperature (register 8). The message
shows a master addressing a Liquidew EExd with a slave address of 01H and a Modbus
function of 03H that informs the Liquidew 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 Liquidew EExd may transmit the following
message:
Byte No
Read Response Message
This response repeats the address of the Liquidew 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
Meaning
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 CRC Lo Byte
8 CRC Hi Byte
Meaning
1 Slave Address
2 Modbus function code
3 Byte Count
4 Data MSB
5 Data LSB
6 Data MSB
7 Data LSB
8 CRC Lo Byte
9 CRC Hi Byte
APPENDIX F
Value
01H
03H
00H
07H
00H
02H
??H
??H
Value
01H
03H
04H
05H
26H
00H
FAH
??H
??H
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