Applies To:
  • CitectSCADA 3.00

When reading data from a disk PLC (DiskDrv) on a networked system you may encounter network loading problems. This may give you out of buffers in the lan.write.pool, See Q1388 and Q1131. The problem is that Citect is optimised to read the data as fast as the PLC can respond, this is fine for real PLC's. However when using a disk PLC, all the data may be cashed and the response is instantaneous, this can cause a lot of LAN traffic. 

To solve the problem the disk PLC needs to be slowed down. The new DiskDrv.DLL in has parameters as follows.

MaxPending = 4
Delay = 1

The above parameters are defaults, MaxPending can be reduced to 1 to slow down the response and Delay can be increased to 50 or so to also slow the response. You can monitor the effect by running Probe on a Display Node while adjusting the parameters on the IOServer. To make it easy make sure that you are displaying a page with disk PLC variables only. See the design Guide on how to use probe in the Citect Kernel.