ProSoft Technology ProTalk PTQ-PDPMV1 User Manual page 188

Hot Standby Support
User Manual
7.1.4 Modicon Quantum Hot Standby with Unity and IEC Logic
Overview
A Modicon Quantum Hot Standby with Unity system requires two backplanes
configured with identical hardware, software, and firmware. One of the controllers
(PLC) functions as the Primary controller and the other as a Standby controller.

The Primary updates the Standby after every scan.

The Primary and Standby communicate constantly monitoring the health of
the system.

If the Primary fails, the Standby takes control within one scan.
7.1.5 Understanding System Scan Time in Modicon Quantum Hot
Standby with Unity Systems
Effect on System Scan Time
The scan time of any Modicon Quantum Hot Standby with Unity system depends
on the amount of data transferred. Because data must be transferred from
Primary to Standby, any Modicon Quantum Hot Standby with Unity system
always has a higher scan time than a comparable stand-alone system.
Performance Considerations
A Modicon Quantum Hot Standby with Unity system increases the length of a
MAST scan, creating system overhead.
Note: System overhead is the time required to copy the application data to the communication link
layer.
The network scan (communication between Primary and Standby "copros")
1 Exchanges data between both controllers
2 Runs in parallel with the application program.
Page 188 of 306
PROFIBUS DP Master Network Interface Module for Quantum
PTQ-PDPMV1 ♦ Quantum Platform
ProSoft Technology, Inc.
August 12, 2014