West Control Solutions Pro-EC44 - Manual (Page 226)

Pro-EC44 2-Loop Graphical Profile Controller & Recorder
Pro-EC44 Product Manual - 59540-1 October 2013 Page 219
Cascade Control is another type of Master & Slave application where the slaves setpoint is
set using the master controllers PID power output.
Also refer to: Cascade Control; Linear Output; Retransmit Output; Remote Setpoint; Auxiliary
Input Offset; Serial Communications and Setpoint.
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
“broadcast commands” sent to address 0, which are acted upon by
all slaves). The commands can instruct the slave to change values in its memory registers, or
ask it to send back values contained in the registers. Each query or response message
includes a cyclic redundancy check (CRC) checksum to ensure that it arrives uncorrupted.
This instrument can act as a slave, or it can be a
“setpoint master” over RS485. In this mode
the unit continuously sends its setpoint value using broadcast messages.
Refer to the Serial Communications and Modbus Parameter sections for more information.
Also refer to: Modbus TCP; RS485; Serial Communications and Setpoint.
Modbus TCP
Modbus TCP is a version of the Modbus protocol for networks such as Ethernet, which
support the Internet Protocol. It is available if an Ethernet communications module is fitted
into option slot A.
This instrument can only act as a Slave when using Modbus TCP. A master device initiates
the communications, and the instrument only acts on the command if it has been sent to its
own IP address. Modbus/TCP does not require a checksum to ensure that the message
arrives intact. Apart from this, the data model and function calls used by Modbus TCP and
RTU are identical; only the message encapsulation is different.
Refer to the Serial Communications and Modbus Parameter sections for more information.
Also refer to: Ethernet; Modbus RTU and Serial Communications.
Minimum Motor On Time
This defines the minimum drive effort needed to initiate valve movement if the valve was
previously stationary. It ensures that frictional and inertial effects are taken into account when
driving the valve, and reduces the actuator switching operations when close to setpoint.
If the pulse required to position the valve would be less than the minimum on time, the output
is suppressed. Each of these short pulse times is accumulated until their value exceeds the
minimum on time, and the output is turned on for this time.
When the control deviation error is inside a “neutral zone”, the PID algorithm inhibits
integration in order to avoid oscillation. The neutral zone (symmetrical to setpoint) is:
2 * PropBand * (MinOnTime / MotorTravelTime)
Also refer to Motor Travel Time; Self-Tune and Valve Motor Drive Control.