FANOX SIA-C, User Manual

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8.
MODBUS RTU PROTOCOL
This document describes the steps to follow to read and write data on the SIA-C 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-C 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.
The address and Baudrate (only in RS485 port) can be configured but the rest of the parameters are
fixed: without parity and with 1 stop bit.
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.