Vaisala DMT340 User Manual download pdf (Page 143)

Chapter 5 ___________________________________________________________________ Modbus

VAISALA _______________________________________________________________________ 141

CHAPTER 5

MODBUS

This chapter contains information that is needed when operating the

transmitter using the Modbus protocol.

Overview of Modbus Protocol Support

The DMT340 transmitter can be accessed using the Modbus serial

communication protocol. Support for Modbus protocol is available on all

DMT340 transmitters as a standard feature from software version 5.10

onward. The supported Modbus variants and the connections they use are

listed in Table 31 below.

Table 31 Supported Modbus Variants

Supported Modbus Variant

Connections

Modbus RTU (Serial Modbus)

RS-232 (standard User Port)

RS-422/485 interface (optional module)

Modbus TCP (Ethernet Modbus)

LAN interface (optional module)

WLAN interface (optional module)

The supported Modbus functions, registers, configuration options, and

diagnostics are described in Appendix B, Modbus Reference, on page

181.

Note the following limitations of the Modbus implementation:

- Modbus TCP mode accepts only one TCP connection at a time.

Design the system so that only one Modbus TCP client accesses the

transmitter.

- Modbus TCP can process reliably only one Modbus transaction at a

time. Reduce the polling rate of the client to avoid nested transactions.

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Vaisala DMT340 User Manual Page 143