User’s Manual 7M
34
APPENDICES
APPENDIX A: MODBUS - COMMUNICATION PROTOCOL
Modbus protocol enables operation of device on Modbus networks. For 7M.38.8.400.XXXX\ with serial communication the
Modbus protocol enables multi drop communication via RS485 communication. Modbus protocol is a widely supported
open interconnect originally designed by Modicon.
The memory reference for input and holding registers is 30000 and 40000 respectively.
PLEASE NOTE
The Modbus table is subject to change without notice.
For the latest and complete Modbus table please visit FINDER S.p.A web page.
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’.
The master could send the MODBUS request to the slaves in two modes:
•
Unicast mode
, where the master sends the request to an individual slave. It returns a replay to the master after the request
is received and processed. A MODBUS transaction consists of two messages. Each slave should have an unique address.
•
Broadcast mode
, where the master sends a request to all slaves and an answer is never followed. All devices should accept
the broadcast request function. The Modbus address 0 is reserved to identify the broadcast request.
MASTER TO SLAVE REQUEST
SLAVE TO MASTER RESPONSE
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 recognize)
•
Function code
e.g. 03 asks the slave to read its registers and respond with their contents
•
Data bytes
: tells the slave which register to start at and how many registers to read
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
Device address
Device address
Function Code
nx8 bit data bytes
Error check
Device address
Device address
Function Code
nx8 bit data bytes
Error check
