Meggitt VM600 User Manual

8 - 2 VM600 networking manual MAVM600-NET/E
Edition 9 - February 2018
Modbus communication
SETTING UP A MODBUS CONNECTION (CPUM SW VERSION 067 OR EARLIER)
In this client-server model of Modbus communication, the CPUM of the VM600 is the Modbus
server that responds to external transaction requests from client equipment, such as a
computer or DCS.
Modbus is a request/response protocol and offers services specified by function codes.
Modbus function codes are elements of Modbus request/reply PDUs and are discussed
further in 8.4 Description of Modbus RTU protocol.
Versions of the Modbus protocol exist for serial communication interfaces (the Modbus RTU
protocol) and for Ethernet (the Modbus TCP protocol).
Visit the Modbus Organization website for the most up-to-date information:
http://www.modbus.org/specs.php
8.3 Modbus communication
Communication is performed using either:
• A serial line using the Modicon standard Modbus RTU protocol
and/or
• An Ethernet line using the Modicon standard Modbus TCP protocol.
8.3.1 Modbus RTU
The Modbus RTU protocol can be used with RS-232 and RS-485 serial interfaces. The
address field of a Modbus RTU message frame contains eight bits so valid slave device
addresses are in the range from 1 to 247 (decimal). (Address 00 is reserved for broadcast
transactions, to all server devices in the network, but is not supported by the Modbus
implementation from Meggitt Sensing Systems.)
In practice, however, the maximum number of instruments or devices that can be addressed
is limited by the RS-485 specification. This states that the bus can support up to 32
“unit loads”, so the maximum number of devices that can be connected depends on how
much each connected device loads the system down.
The serial link can be half-duplex or full-duplex with selectable transmission rates from 1200
to 19200 bps. The range of supported communication parameters are given in Table 8-1.
The default VM600 communication parameter settings can be seen in 8.4.2 Communications
parameters for the VM600.
Client
(master)
1: Request
Server
(slave)
Figure 8-1: A Modbus transaction
2: Response
VM600 CPUMComputer or DCS

Meggitt VM600 Specifications

General

Brand	Meggitt
Model	VM600
Category	Measuring Instruments
Language	English