Chapter 5 ________________________________________________________________ MODBUS
VAISALA ______________________________________________________________________ 143
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 30 below.
Table 30
Supported MODBUS Variants
Supported MODBUS Variant
Connections
MODBUS RTU (Serial MODBUS)
RS-232 (standard User Port)
RS-422/428 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
185.
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.
