Applies To:
  • CitectSCADA 5.41, 5.41 Service Pack A, 5.41 Service Pack B, 5.42, 5.42 Service Pack A, 5.50, 5.50 Service Pack A, 5.50 Service Pack B, 6.00
  • CitectHMI 5.41, 5.41 Service Pack A, 5.41 Service Pack B, 5.42, 5.42 Service Pack A, 5.50, 5.50 Service Pack A, 5.50 Service Pack B, 6.00
Summary:
It takes a long time for my ABCLX device to come online. What are the factors that can decrease the time it takes for my device to come online?

Solution:
Essentially the things that contribute to a long connection time are as follows:



(a) System Overhead Timeslice

The lower this figure the worse connection speeds will be.



(b) Available memory in the PLC

The lower this figure the worse connection speeds will be.



(c) Large numbers of tags in the PLC

The more tags to download the longer it takes. Remember that is doesn't matter whether the tags are used in Citect, if they exist in the PLC we download *all* of them.



(d) Large numbers of tags in the Citect Project

The more tags to subscribe the longer it takes.



(e) Number of Forward Opens

This one is an interesting one. In some cases the lower this figure the worse connection speeds will be - however this is not always the case. This statement is true if the network is fast and the System Overhead Timeslice is high as ABCLX is able to open numerous connection sessions with the PLC. However, if the PLC does not allow ABCLX to open more and more connection sessions due to inadequate System Overhead Timeslice, or there is not sufficient available memory in the PLC, the connection process will take longer. Therefore reducing the number of Forward Opens may help in this case.



(f) Scanrate

The higher this figure the better the connection speed - but this should be set to double the value of the 'Group Ave Poll Time' or more.



(g) Bandwidth of the Ethernet network connecting the Citect PC to the PLC



(h) Multiple IO Devices in Citect to the same physical PLC

It is possible to configure multiple IO Devices in Citect that connect to the same physical PLC. This is recommended in cases where, for example, you wish to have a group of tags polled more slowly than the rest of the tags to free up network usage (at times other than at startup). This is often done for tags that appear on seldom used engineering pages and the like. This means that as Citect's ABCLX driver polls some tags more slowly, it frees up the network and PLCs CPU to more quickly process other tags configured in the PLC or the PLC program itself. This does however have a side effect. When establishing communications to the PLC, Citect's ABCLX driver is trying to bring two devices online. As a result it downloads the tags from the PLC for the first IO Device then subscribes these with the variable database in Citect. Then Citect's ABCLX driver downloads the tags from the PLC a second time for the second IO Device and subscribes those tags with the variable database. From this you can see that even though we are talking to the same physical IO device, ABCLX does not know this and as such will load up the CPU of the PLC to download the tags for each IO Device configured in Citect. This can add delays in Citect establishing initial communications.



With this information, it is possible to see what needs to be done to decrease the time it takes for ABCLX to bring the PLC online.

For further information please refer to the ABCLX driver help or contact Citect Support.
 
Keywords:
ABCLX, slow, long, time, connection, PLC  

Attachments