Appendix
A - 2
A.2 Outline of Modbus protocol
The communication system of Modbus is a single master / multi slave system. Only the master can transmit a
demand (query) to designated slave or the bloadcasting query to the all slaves. The slave performs the function
according to query and returns a response message. In case of bloadcasting query, the slave performs the demand of
query and does not return a response message. The slave returns a response message to query for own device address.
Message configuration of query consists of device address of slave, function code which designate contents of
command, data, error check field. Message configuration of respons is consists of check filed of command, response
data, error check field.
The message configuration of query and response message are shown in the following figure.
Query message from master
Device address*
Function code
Query data
Error check
Response message from slave
Device address*
Function code
Response data
Error check
Figure A.1 The query - response cycle
There are two kinds of serial transmission modes, ASCII mode and RTU mode. However, either of modes can run on
one network. In ASCII mode, 1 byte data is converted to ASCII code (two bytes) and transmitted. In RTU mode, 1
byte data is transmitted as it is.
The check algorithm of the error check field depends on transmission modes. In case of ASCII mode, the LRC
(Longitudinal Redundancy Check) method is used. In case of RTU mode, the CRC (Cyclical Redundancy Check)
method is used.
* Device address : In this book, it is written as "Device No.".
Summary of Contents for EH-150 Series
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Page 156: ...Chapter 7 System setup 7 12 M E M O ...
Page 164: ...Chapter 8 Daily and Periodic Inspection 8 8 M E M O ...
