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For clarity, let’s use Equation 1 and Equation 2 in a calculation example. Say,
for instance, that we are going to read coil #34. Using Equation 1, we can
determine that coil #34 resides in register #3, as ⎣3.0625⎦ = ⎣3 r1⎦ = 3. Then,
using Equation 2, we can determine that the bit within register #3 that coil #34
targets is (34-1)%16 = 1, as 33%16 = mod(3 r1) = 1. Therefore, reading coil
#34 will return the value of register #3, bit #1.
Note that this discrete-to-register/bit relationship holds true regardless of
whether or not register #3 is assigned to a point. If register #3 is not assigned
to a point, then a Modbus exception will be returned. Either way, coil #34 will
always
access register #3, bit #1.
15.1.2 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)
15.1.3 Modbus RTU Master
• Supported Modbus master functions are indicated in Table 4. 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 4: 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)