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8.1. ModBus package format
CLIENT
ADDRESS 1 byte Each device on a communications bus must have a unique address, as otherwise
two devices could answer the same request at the same time. All of the ports of
the relay will use this address, which can be programmed with a value between 1
and 247. When the master device transmits a packet to the slave address 0, this
is a broadcast packet. All of the slave devices on the bus will execute the requested
action, but none will reply to the master. Broadcast will only be accepted for write
commands, as there is no meaning in transmitting a read request on broadcast,
given that no device will reply to it.
FUNCTION
CODE 1 byte This is one of the function codes supported by the device. In this case, the only
supported function codes are 3 for read and 16 for write. When the slave has to
reply to one of these packets with an exception, this is indicated by setting the Most
Significant Bit of the corresponding function to 1. Thus, an exception for function 3
will be indicated with 0x83 as function code, and an exception to code 16, or 0x10
in hexadecimal, is indicated with 0x90.
DATA N bytes This part of the packet consists of a variable number of bytes, depending on the
function code. It can include: addresses, data strings, settings, commands, or
exception codes sent by the client.
CRC 2 bytes Two byte control code. ModBUS/RTU includes a 16 byte CRC in each packet, for
error detection. If the slave detects an erroneous packet, through the inclusion of
an incorrect CRC, it will take no action, nor will it reply to the master. The CRC
ordering is LSB-MSB.
DEAD TIME Time
necessary to
transmit
3.5
bytes.
A packet is taken to have ended when nothing is received for a period of 3.5 bytes.
That is:
•
15 ms at 2400 bps
•
2 ms at 19200 bps
•
...etc.
8.2. Function codes
CODE
HEX
DEC
MODBUS NAME DEFINITION COMMENT
0x03
3 Read Holding Logs Read any value This function allows the master to read 1 or more
consecutive addresses in a relay. The logs are
always 16 bits, with the most significant byte first.
The maximum number of logs that can be read in
one packet is 60.
0x10
16 Preset Multiple Logs Write
This function allows the writing of one or more logs,
representing one or more settings. The logs are
values 2 bytes in length, transmitted with the most
significant byte first. The maximum number of logs
that can be written in one packet is 60.