KROHNE ALTOSONIC V12 EN - Manual (Page 111)

MODBUS PROTOCOL DESCRIPTION AND SET-UP
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ALTOSONIC V12
www.krohne.com 04/2013 - 4002643502 - MA ALTOSONIC V12 R02 en
10.1 Introduction
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 flowmeter 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 the flowmeter. 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 that a controller uses to request access to other devices.
• how to respond to requests from the other devices.
• 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).
• an error check for testing the integrity of the message.
The response of the slave contains:
• the slave address.
• data conform the request type.
• an error check.
If the data integrity test fails, no response is sent back.
If a request can not be processed an exception message is returned.
10.2 Physical Communication Layer
The Modbus over serial line protocol is a master-slave protocol. The physical layer can be half-
duplex or full-duplex. In case of the ALTOSONIC V12 the physical layer is a half-duplex (two-
wire) connection according to RS 485 specifications.
The end of a RS 485 line has to be terminated by means of a resistor. This terminating resistor is
included in the line driving circuit in the flowmeter.