KROHNE OPTISONIC V6 Modbus - Manual (Page 3)

2011-07-19
OPTISONIC_V6_Modbus_6.doc 3/29
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ts 1 INTRODUCTION
Scope
This manual describes how to use the Modbus protocol with the OPTISONIC V6 gas flow meter.
Introduction to Modbus
Modbus communication is based on the master-slave principle. Only the master can initiate transactions
(requests), and only the addressed device (slave) responds. The master can also send a broadcast
message (“message to all”); none of the slaves will respond to such a message.
The OPTISONIC V6 flow meter always acts as a Modbus compatible slave when communicating with host
systems. Slaves are identified by means of a “device address”. Check the documentation to find the preset
device address of your OPTISONIC V6 flow meter. In case necessary, the address can be reprogrammed.
Contact the manufacturer for information about the procedures and tools that are needed for reprogramming
the device address.
The Modbus protocol defines a message structure that Modbus enabled controllers will recognise and use,
regardless of the type of network over which they communicate. It describes:
• the process a controller uses to request access to other devices,
•
how to respond to requests from the other devices, and
•
how errors will be detected and reported.
The Modbus request consists of:
• an address,
• a function code defining the requested action,
• data (if necessary for the requested function), and
•
an error check for testing the integrity of the message.
The slave’s response contains:
• the slave address,
• data conform the request type, and
•
an error check.
If the data integrity test fails, no response is sent back.
If a request cannot be processed an exception message is returned.