West Control Solutions ProVU 4 - Manual (Page 147)

ProVU4 Controller, Profiler & Recorder/Controller - Product Manual
Page 146 Glossary 59407, Issue 1 – March 2008
Modbus RTU
Modbus RTU is the serial communications protocol used on instruments fitted with the
RS485 Communications module into Option Slot A. Alternatively, the Modbus TCP protocol
is available if the Ethernet Communications Module is fitted.
Modbus RTU is a Master/Slave protocol. Only the Master may initiate communications. Each
slave is given a unique address, and the message contains the Modbus address of the
intended slave. Only this slave will act on the command, even though other devices might
receive it (an exception is specific broadcast commands sent to address 0 which are acted
upon by all slaves but not acknowledged).
The commands can instruct the slave to change a value in one of its registers, or ask it to
send back one or more values contained in its registers. The Modbus RTU format follows the
messages with a cyclic redundancy check (CRC) checksum to ensure that the message
arrives undamaged.
This instrument can act as a Slave or Setpoint Master over RS485. In this mode the unit
continuously sends it’s setpoint value using Modbus broadcast messages.
Refer to the Serial Communications and Modbus Parameter sections of this manual for more
information.
Also refer to: Modbus TCP, RS485 and Serial Communications.
Modbus TCP
Modbus TCP is a version of the Modbus protocol for networks that support the Internet
Protocol, such as Ethernet. It is available if the Ethernet Communications Module is fitted into
Option Slot A.
This instrument can only act as a Slave when using Modbus TCP. The master device
initiates the communications, and the instrument only acts on the command if it has been
sent to its IP address. The data model and function calls used by Modbus TCP and RTU are
identical; only the encapsulation is different. Modbus/TCP does not require a checksum to
ensure that the message arrives undamaged.
Refer to the Serial Communications and Modbus Parameter sections of this manual for more
information.
Also refer to: Ethernet, Modbus RTU and Serial Communications.
Modulating Valve
A valve that can be positioned anywhere between fully closed and fully open by means of an
incorporated motor. A typical application would be controlling temperature in a furnace
heated by gas burners. This instrument can control modulating valves that have a positioning
circuit. These require proportional (mA or VDC) control signal from a linear output, relative to
the desired valve position. PI control is used for valve control.
To directly control the valves ‘open’ and ‘close’ motor windings, a special Valve Motor Drive
(VMD) controller algorithm is required. This instrument does not currently support this type of
algorithm.
Also refer to: Linear Output, PI Control, Proportional Control and Valve Motor Drive Control.