Vaisala HMT330 - Manual (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 HMT330 transmitter can be accessed using the Modbus serial
communication protocol. Support for Modbus protocol is available on all
HMT330 transmitters as a standard feature from software version 5.10
onward. The supported Modbus variants and the connections they use are
listed in Table 32 below.
Table 32 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 C, Modbus Reference, on page
199.
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.