finder 7M Series - Appendices; MODBUS Communication Protocol

7M User’s Manual 28
5.1 MODBUS communication protocol
Modbus protocol enables operation of device on Modbus networks. For 7M.24 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.
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
Device address Function Code nx8 bit data bytes Error check
Slave to Master Response
Device address Function Code nx8 bit data bytes Error check
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.
5. APPENDICES
PLEASE NOTE
This chapter is a subject of change. Please download from www.ndernet.com the dedicated guide for Modbus and M-Bus
communication protocol