Meggitt VM600 User Manual

VM600 networking manual MAVM600-NET/E 8 - 3
Edition 9 - February 2018
Description of Modbus RTU protocol
SETTING UP A MODBUS CONNECTION (CPUM SW VERSION 067 OR EARLIER)
NOTE: When the CPUM card contains an optional serial communications module
(allowing RS-485/RS-422 communication over the “A” and “B” pairs of connectors
on its associated IOCN card), only full duplex transmission is possible.
8.3.2 Modbus TCP
The Modbus TCP protocol has exactly the same layout as the Modbus RTU protocol, with the
exception of the framing sequence check pattern and the address interpretation.
Again, communications are based on the master-slave principle and the VM600 rack is the
slave in the system. The master equipment requests data from the addressed slave, which
can only respond. The slave cannot initiate a transaction. The master can address individual
slaves to request or send data. It can also request an action to be taken by one or all the
slaves in the network.
8.4 Description of Modbus RTU protocol
8.4.1 Frame and timing
NOTE: The following definitions of the Modbus protocol are adapted to the VM600 MPS.
The Modbus RTU protocol is a serial data transmission format widely used in
communications with programmable controllers. It is easily adaptable to other types of
remote units thanks to its particular message structure (that is, it doesn't operate with
variables but with memory addresses).
As shown in Figure 8-2, the first character of a frame is the slave address, followed by the
Modbus function code and the information field. Finally, two bytes are reserved for an error
check code based on cyclic redundancy checking (CRC).
Each character is composed of 10 bits: 1x start bit, 8x data bits and 1x stop bit.
The Modbus RTU protocol uses no delimiter characters at the beginning or at the end of a
message. Each frame must be preceded and followed by a silent interval of at least 3.5
character times (CTs). See Figure 8-3.
Table 8-1: RTU communication parameters supported by the VM600
Baud rate Parity Slave address
600, 1200, 4800, 9600,
19200, 38400, 57600, 115200
Even, odd or none 1 to 247
Slave
address
Modbus
function code
Data
CRC
(2 bytes)
Figure 8-2: Constitution of a frame

Meggitt VM600 Specifications

General

Brand	Meggitt
Model	VM600
Category	Measuring Instruments
Language	English