Modbus Rtu Communications; Introduction; Modbus Rtu Basics; Physical Layer - Michell Instruments Promet EExd User Manual

Promet EExd User's Manual
Appendix D

Modbus RTU Communications

D.1

Introduction

Implemented within the Promet EExd are Modbus RTU communications that enable
remote access to the instrument's configuration and data logging facilities. This protocol
offers two-way communication between a PC or PLC known as the master, to one or
more instruments known as slaves. Communication is achieved by the master reading
or writing to registers within the slave. The slave acts upon information contained
within the registers that can be written, and the master obtains measured values and
status information from the register that can be read. Appendix E lists these registers
and Appendix G specifies the number or data formats that apply to each register.
D.2

Modbus RTU Basics

Modbus RTU operates on a Query-Response Cycle (see the diagram below). The function
code in the query tells the addressed slave device which actions to perform using the
information contained in the data bytes. The error check field provides a method for the
slave to validate the integrity of the message contents.
If the slave makes a normal response, the function code in the response is an echo of
the function code in the query and the data bytes will contain data collected by the
slave, such as register values or status information. If an error occurs, the function code
is incremented by 80H. This indicates that the response is an error response, known as
an exception, and the data bytes contain a code to describe the error. The error check
field allows the master to confirm that the message contents are valid.
MASTER
(P.C.)
Device Address
D.3

Physical Layer

The physical connection from the master to the Promet EExd uses 2-wire RS485 plus
a ground connection. Data lines A, B and ground are connected to communication
connectors within the instrument. The serial port protocol is as follows:
Baud Rate:
Start Bit:
Data bits:
Parity:
Stop bit:
Michell Instruments
Query Message from Master
Function Code
Data Bytes
CRC
9600
1
8
None
2
Response From Slave
APPENDIX D
SLAVE
(Condumax)
Device Address
Function Code
Data Bytes
CRC
49