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SIA-D_
Rev. 07
78 / 117
ISO 9001:2008
9. PROTOCOL
This document describes the steps to follow to read and write data on the SIA-D relay, as per
the ModBUS/RTU 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/RTU protocol is used, so any program or PC can communicate easily
with the equipment.
The SIA-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/RTU functions is implemented:
•
Reading function 3.
•
Writing function 16.
The ModBUS/RTU protocol is independent from the hardware. Therefore, the physical layer can
exist in different hardware configurations: RS232, RS485, fibre optic or Ethernet.
Specifically, the relay has a front RS232 port and, as an option, a rear RS485 port. The data
stream in any of the configurations is “half-duplex”.
Each byte of data is transmitted asynchronously and is made up of: 1 start bit, 8 data bits, 1
stop bit and 1 parity bit, if this is how it is programmed. Therefore, the data has 10 or 11 bits,
depending on whether or not it includes parity.
When the equipment has a single front port, the address can be configured but the rest of the
parameters are fixed: the speed is 19200, without parity and with 1 stop bit.
If the equipment is fitted with two switched ports, one front and one rear, the following features
can be configured: speed (1200, 2400, 4800, 9600 or 19200), parity (even, odd or no parity),
the stop bits (1 or 2) and the address (1 to 247).
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 case of “broadcast” messages).
The end of the frame is marked by a dead time or silence time in the communication medium.
The length of this time of silence varies depending on the transmission speed, as it is equivalent
to 3 characters.
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.