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DCP200 Profile Controller & Recorder - Product Manual
51-52-25-150, Issue 1 – April 2009 Glossary Page 147
Cascade Control is another type of Master & Slave application where the slaves setpoint is
set using the master controllers PID power output.
The terms Master and Slave are also used in a different context in relation to serial
communications.
Also refer to: Cascade Control, Linear Output, Retransmit Output, Remote Setpoint, Auxiliary
Input Offset, Serial Communications and Setpoint.
Minimum Duration Of Change
A form of alarm hysteresis unique to the Rate Of Change Alarm. It is the minimum time that
the rate of change in the process variable must be above the alarm threshold, before the
alarm will change state (from on to off, or off to on).
Settings = 1 to 9999 secs. Default value = 1sec.
Caution: If the duration is less than this time, the alarm will not activate no matter how fast
the rate of rise.
Also refer to: Alarm Hysteresis, Alarm Types and Rate Of Change Alarm.
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 as a 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. A 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 intact.
Refer to the Serial Communications and Modbus Parameter sections of this manual for more
information.
Also refer to: Ethernet, Modbus RTU and Serial Communications.