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12.
MODBUS TCP PROTOCOL
The communications parameters are as follows:
•
IP Address (By default 192.168.0.121)
•
Subnet (By default 255.255.255.0)
•
Gateway (By default (192.168.0.1)
This document describes the steps to follow to read and write data on the SIL-D relay, as
per the ModBUS TCP protocol. This memory map is only valid for one piece of equipment
and one version of the memory. The positions of existing objects in the memory remain
fixed from one version to the next, but new objects will naturally have new addresses
which will, in turn, remain fixed in future versions. The memory map is described further
on.
The standard ModBUS TCP protocol is used, so any program or PC can communicate
easily with the equipment.
The SIL-D always acts as a slave, which means that it never initiates communications.
The master is always responsible for initiating communications.
Only a subset of the ModBUS TCP functions is implemented:
•
Reading function 1 y 3.
•
Writing function 5.
The master must know the address of the slave that it is going to communicate with. No
unit will act on requests from the master if the message is not addressed to them. The
exception is when the 0 address, or “broadcast” address, is used, in which case the relay
will act but will not send an answer of any type.
Communications are made in packages or frames, which are groups of data that are sent
asynchronously. The master transmits a frame to the slave, and the slave then replies with
another frame (except in the ca
se of “broadcast” messages).
The following table shows the generic package format that is valid for transmission and
reception. However, each function has its own peculiarities, as will be described further on.
12.1. ModBus package format
MBAP
HEADER 7 byte Contains the Transaction Identifier (2 bytes), initizalized by the client. Protocol
Identifier (2 bytes), 0 = MODBUS protocol. Length (2 bytes). Number of following bytes
(2 bytes). Relay identifier (1 byte).
FUNCTION
CODE
1 byte
This is one of the function codes supported by the equipment. In this case, the only
function codes supported are 1 and 3 to read and 5 to write. When the slave has to
reply with an exception one of these frames, it is indicated by putting 1 in the most
important bit of the correspondent function. Thus, an exception for the function 3, will
be indicated with a 83 as a function code; and an exception for the function code 16 or
0x10 in hexadecimal, will be indicated with an 0x90.
DATA N bytes This part consists of a variable number of bytes, depending on the function code. It
may include: addresses, data lengths, settings, commands or exception codes sent by
the user.