8. Modbus
8.1 Modbus overview
The transmitter supports the Modbus TCP/IP communication protocol (over Ethernet). The
transmitter and the connected probes are treated as separate Modbus slaves, each with their
own fixed unit identifier (unit ID) value.
Note the following limitations of the Modbus implementation:
• Use only one Modbus TCP/IP connection at a time. Design the system so that only one
Modbus TCP/IP client accesses the transmitter.
• Modbus TCP/IP can process reliably only one Modbus transaction at a time. Reduce the
polling rate of the client to avoid nested transactions.
The transmitter's Modbus response time is approximately 2000 ms. If the Modbus client gives
a timeout error, increase the request timeout parameter.
The transmitter's Modbus server port is 502.
More information
‣
Enabling Modbus TCP/IP on touchscreen (page 55)
‣
Enabling Modbus TCP/IP in web interface (page 60)
8.2
Unit identification of transmitter and probes
The transmitter and the connected probes are treated as separate Modbus slaves, each with
their own fixed unit identifier (unit ID) value. The unit ID in the Modbus request defines the
intended recipient of the request (transmitter or probe).
Table 10 Unit IDs of transmitter and probes
Device Unit ID
Transmitter 240
Probe connected to port 1 241
Probe connected to port 2 242
The unit IDs of the connected probes identify the probes based on the port that they are
connected to. The probes use these unit IDs (241 and 242) only when they are connected to
the transmitter. When the probes are accessed directly without the transmitter, the probes
have their own Modbus address settings instead of these unit IDs.
If the probe fails to respond to the request, the transmitter responds to the client with the
exception code Gateway Target Device Failed to Respond.
Chapter 8 – Modbus
61
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