PACiS GTW GTW/EN T/C40, Technical Manual

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GTW/EN CT/C40 Communication
Page 30/64 PACiS Gateway
4. MODBUS PROFILE
The purpose of this chapter is to describe the support of the MODBUS RTU protocol in the
PACiS system for Gateway PC.
The MODBUS RTU protocol defines a message structure that controllers will recognise and
use. It describes the process a controller uses to request access to another device, how it
will respond to requests from other devices, and how errors will be detected.
4.1 Introducing MODBUS
Controllers communicate using a master-slave scheme, in which only the master (called
SCADA) can initiate requests. The MODBUS gateway is a slave device that responds by
supplying the requested data to the master.
Transmission mode used is the MODBUS RTU. Queries and replies have the same format
on a master-slave query-response cycle:
Query message from Server Response message from Slave
Function code Function code
Eight-bit Data bytes Eight-bit data bytes
Error check Error check
4.1.1 Device address field
The device address field of a message frame contains eight bits. Valid slave device
addresses in queries are in the range of 0 – 247, but individual slave devices are assigned
addresses in the range 1- 247 and address 0
is reserved for broadcast operations.
The device address field of a message frame always identifies a slave (slave destination in
master to slave queries and slave source in slave to master responses).
If the gateway receives a frame with a broadcast device address, no reply is returned.
If the gateway receives a frame with a not expected device address, no reply is returned.
4.1.2 Function code field
The function code field of a message frame contains eight bits. Valid codes are in the range
of 1-255.
In master to slave queries, the function code identifies the operation that is to be executed by
the slave.
In slave to master responses, the function code is used to indicate either a normal (error-
free) response or a failure of the operation execution (called an exception response). For a
normal response, the slave simply echoes the original function code. For an exception
response, the slave returns a code that is equivalent to the original function code with its
MSB set to 1. In addition to the modification of the function code the slave places a unique
code into the data field of the response message, which identifies the reason for the
exception.
4.1.3 Data field
The data field of a message frame is constructed using sets of two hexadecimal digits, in the
range of 00h to FFh.
In master to slave queries, the data field contains additional information to be used by the
slave to execute the operation defined by the function code.
In slave to master responses, the data field contains the data requested by the master.
The data field can be non-existent in some kind of messages.