Instruction manual

50
16. Protocol-Specific Information
This section will discuss topics that are specific to each of the available network
selections.
16.1 Modbus
The gateway supports Modbus slave functionality via both Modbus RTU and
Modbus TCP/IP, and Modbus 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” register assignment. Other notes of interest are:
• Points are addressed by their assigned holding register (4X reference) or
input register (3X 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 2.
Table 2: Supported Modbus Slave Functions
Function
Code
Function
Modbus TCP/IP
Class
1 Read coils 1
2 Read input status 1
3 Read multiple registers 0
4 Read input registers 1
5 Write coil 1
6 Write single register 1
8 Diagnostics (RTU & subfunction 0 only) -
15 Force multiple coils 2
16 Write multiple registers 0
• Register number entry radix is decimal (e.g. 10 = 10
10
)
• Configuration tip: Improved network utilization may be obtained by
appropriately grouping points into blocks having contiguous 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