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Note that this coil-to-register/bit relationship holds true regardless of whether or
not holding register #3 is assigned to a point. If holding register #3 is not
assigned to a point, then a Modbus exception will be returned. Either way, coil
#34 will always
access holding register #3, bit #1.
13.1.2 Modbus RTU Slave
• Broadcast (for functions 5, 6, 15 and 16) is supported.
• Network characteristics selections
o Baud rate: 2400 / 4800 / 9600 / 19200 / 38400 bps
o Parity: odd / even / none (1 stop bit) / none (2 stop bits)
13.1.3 Modbus RTU Master
• Supported Modbus master functions are indicated in Table 2. These
functions are automatically invoked by the gateway in response to point
read or write requests. The specific read or write function code used
depends on the point’s assigned configuration.
Table 2: Supported Modbus Master Functions
Function Code Function
3 Read multiple registers
4 Read input registers
6 Write single register
16 Write multiple registers
• The slave response timeout (in seconds) is assigned via the designated
port’s “Timeout” selection. If “0” is chosen (an invalid timeout time), the
gateway will use a 2s timeout by default.
• Network characteristics selections
o Baud rate: 2400 / 4800 / 9600 / 19200 / 38400 bps
o Parity: odd / even / none (1 stop bit) / none (2 stop bits)
• Note that various manufacturers will document their Modbus slave
products in different ways. In particular, according to the Modbus
specification, registers have two different indices at which they can be
referenced: their “known as” value (which starts at number 1) and their
“addressed as” value (which is always 1 less than the “known as” value).
The “known as” value is typically the number that is presented for human
entry or consumption, while the “addressed as” value is the number that
appears “on the wire” when the Modbus packet is sent from master to