Quantum Hot Standby + SCADA slow performance


Thread Starter


I'm testing the system with Hot-Standby on Quantum PLC and I'm experiencing a serious problem. When there is no hot-standby (only one PLC) SCADA has a good performance. Time between opening some control window or page and displaying values in it is short (less than 0.5 s). After starting hot-standby displaying takes about 3-4 seconds and is not acceptable. There is no change in SCADA in time between switching, I'm displaying the same page and the same window in both cases.

Was there such a problem ever reported? Has anyone experienced it? What is the solution? The problem is the same wheter if using CitectSCADA or InTouch.


Rafael N. Jacomino

PLC scan-time is affected in Hot-Standby systems. One large system that I worked on (140CPU43412A with 6 RIO drops and 1000 I/O points) ran at 14ms in single-CPU mode and at 73ms in Primary/Standby mode. This is really neglible when it comes to serivicing COMMs but it's a possible factor. The Quantum's Hot-Standby capabilities are tried-n-tested and amongst the best in the PLC arena. What programming environment are you using (984 or IEC)? If IEC, can you check the PLC Scan-time DIAG found in 'Online-->Memory Statistics...' menu? A large difference in PLC scan-time might point to a Hot-Standy misconfiguration. The SCADA link is through ModBus Plus or ModBus-TCP? If the latter, what NOE module are you using? What EXEC version of the CHS/NOE are you using? Can you ping the IP address offset in the Standby PLC?
PLC scan-time is biggest in Hot-Standby systems.
Example: 70ms without Hot-Standby, 200ms with Hot-Standby.
See `Quantum Hot Standby Planning and Installation Guide 840 USE 106 00`,`State Ram Transfer and Scan Time`.
You can use Non-Transfer Area.