Michell Instruments Liquidew EExd, Руководство пользователя

Страница: 61 / 90

Liquidew EExd User’s Manual
Michell Instruments 53
APPENDIX E
Appendix E Modbus RTU Communications
E.1 Introduction
Implemented within the Liquidew 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.
E.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. 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.
Device Address
Function Code
Data Bytes
CRC
Device Address
Function Code
Data Bytes
CRC
Query Message from Master
Response From Slave
MASTER
(P.C.)
SLAVE
(Condumax)
E.3 Physical Layer
The physical connection from the master to the Liquidew 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: 9600
Start Bit: 1
Data bits: 8
Parity: None
Stop bit: 2