Prevac MG15 - Modbus Protocol Implementation

MULTI GAUGE CONTROLLER COMMUNICATION
6.4 MODBUS PROTOCOL
6.4.1 INTRODUCTION
This implementation is used to provide a popular format od data exchange between the devices in
the network. Modbus enables communication between up to 248 devices connected to the same
network. Each SLAVE device has its unique address set via the switch on the rear panel (details in
chapter 2.5.6). It identiies unequivocally the device during communication over the RTU Modbus
protocol. During communication with the device through the Ethernet (Modbus TCP) network its
IP address is the unique address.
The data register map is unique for the MG15 device and its deinition is given in the tables starting
from ??.
The data transmission protocol describes the rules and structure of the messages used by all the
devices in the data exchange network. This protocol determines also the sequence of data exchange,
error detection and deines that in serial networks only the Master device can initiate transmission,
and in Ethernet networks any device may send commands, but this is usually done by the Master
device. It enables creating a single- or multi-branch network structure. These two types of network
are presented in ig. 6.6.
Figure 6.6: Modbus RTU network structures model
• Each Slave device in the network has its unique address.
• The MG15 device supports subset of Modbus codes.
• Refer through “parameter address” to the MG15 parameters.
• Sending queries with the device unique address results in response only of the device from
this address. Then the Slave device will check errors, execute the required task, and send a
response with its own address, data and correct checksum.
• Address ”0” of the device is a special case. It used for messages send to all the Slave devices
and is used only for writing.
• Sending a query with the device address = “0” activates broadcast communication. This means
that the information will be received by all the devices in the network. Each of them will ex-
ecute the requested action, but not send a response.
User manual rev. 03 Page 87