13. Protocol-Specific Information
This section will discuss topics that are specific to each of the available network selections.
13.1 Modbus
The gateway supports Modbus slave and master functionality via Modbus RTU. The slave implementations share common access methods, which is to say they support the same functions and reference the internal points via a common “Modbus Slave” holding register assignment. Other notes of interest are:
•Points are addressed by their assigned holding register (4X reference) via Modbus slave protocols.
•Points can access both holding registers (4X references) and input registers (3X references) via Modbus master protocols.
•Supported Modbus slave functions are indicated in Table 1.
Table 1: Supported Modbus Slave Functions
Function Code | Function |
|
|
1 | Read coils |
3 | Read multiple registers |
5 | Write coil |
6 | Write single register |
15 | Force multiple coils |
16Write multiple registers
•Register number entry radix is decimal (e.g. 10 = 1010)
•Configuration tip: Improved network utilization may be obtained by appropriately grouping points into blocks having contiguous holding register assignments. In this way, the “read multiple registers” and “write multiple registers” functions can be used to perform transfers of larger blocks of registers using fewer Modbus transactions compared to a situation where the read/write registers were arranged in an alternating or scattered fashion.
•Because the transaction is handled locally within the gateway, write data checking is not available. For example, if a write is performed to a register with a data value that is
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