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

oBaud rate: 2400 / 4800 / 9600 / 19200 / 38400 bps

oParity: odd / even / none (1 stop bit) / none (2 stop bits)

13.1.3Modbus 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

3Read multiple registers

4Read input registers

6

Write single register

16Write 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

oBaud rate: 2400 / 4800 / 9600 / 19200 / 38400 bps

oParity: 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

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