User Manual
M3x5/EN M/B
iSTAT M3x5
Page 115
16.
APPENDIX B1: MODBUS COMMUNICATIONS
The Modbus protocol is a widely supported open interconnect protocol originally designed by
Modicon that can be enabled on the
M3x5
via RS232, RS485, USB or Ethernet
communication.
There are two Modbus protocol types: Modbus RTU for serial communication (and USB) and
Modbus TCP for Ethernet communication. It is also possible to pass Modbus RTU
communications over Ethernet.
On Serial Communication, Modbus is a single master multiple slave protocol suitable for a
multi-drop configuration as provided by the RS485 connection. Up to 32 devices can be
connected in this way. Single - drop RS232 or USB connection is also possible.
16.1 Transactions
Communication operates on a master-slave basis where only one device (the master) can
initiate transactions called 'Requests'. The other devices (slaves) respond by supplying the
requested data to the master. This is called the 'Request - Response Cycle'.
Master to slave request:
Device address
Function Code
nx8 bit data bytes
Error check
Slave to master response:
Device address
Function Code
nx8 bit data bytes
Error check
16.2 Request
This Master to Slave transaction takes the form:
Device address:
Master addressing a slave (Address 0 is used for the broadcast
address, which all slave devices recognise.).
Function code:
E.g. 04 ask the slave to read its Input Registers and respond with
their contents.
Data bytes:
Tells the slave which register to start at and how many registers
to read.
16.3 Response
This Slave to Master transaction takes the form:
Device address:
To let the master know which slave is responding.
Function code:
This is an echo of the request function code.
Data bytes:
Contains the data collected from the slave.
16.4 Request - response cycle example
Ia
160.00 A = 16000* 10-2 A
Data type “T3” 32 bit
unsigned
FE 00 3E 80(16)
Data held in Modbus
addresses
30036(10) & 30037(10)
30036(10) - 30000(10) = 36(10) 00 24(16)
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