ProSoft Technology PTQ-PDPMV1 - Manual (Page 200)

Hot Standby Support PTQ-PDPMV1 ♦ Quantum Platform
User Manual PROFIBUS DP Master Network Interface Module for Quantum
Page 200 of 306 ProSoft Technology, Inc.
August 12, 2014
7.2.10 Conditions for Switchover
Each PTQ situated in the Primary or the Standby local rack must provide both
CPUs with its own diagnostics in order to request and perform a switchover.
Diagnostics must be crossed between Primary and Standby PTQs and the
associated CPU. Both CPUs (Primary and Standby) must be informed anytime of
the status of all PTQs. The PTQ module will post Local and Remote status
information in the Input Status/Control Data Area block word offset 60 to 63.
Note: For the backplane driver, the 100 ms to 300 ms spec depends on scan time. The backplane
driver cannot detect a switchover in 300 ms if the scan time is 500 ms. The backplane drivers can
communicate with the PLC only at the end-of-scan.
In other words:
 The Primary must be informed of the status of its own PTQ(s)
 The Primary must inform the Standby of the status of its own PTQ(s)
 The Standby must be informed of the status of its own PTQ(s)
 The Standby must inform the Primary of the status of its own PTQ(s)
Conditions for user to consider switchover are as follows:
 PTQ Master module failure
 Bus not connected or all devices not responding
 PTQ Master module not configured
 Bus cable break. Status information provided to user to determine
appropriate Master with most slaves will be available in the Input data block.
Based on this crossed information updated anytime and simultaneously, the user
application code can perform a switchover using command register and status
bits. A switchover can be performed only if the Primary PTQ fails and the
Standby PTQ is able to take control. We cannot allow the system to switch over if
the Standby PTQ status is not known with precision. In case of several PTQs per
rack, the application code will have to diagnose the health status of all Standby
PTQs before performing a switchover. These diagnostics have to be taken into
consideration in order to avoid leaving a bad situation on one side (Primary) and
getting a worse one on the other side.