How to change the scan rate of the main driver worksheet?

What scan rate do you want to achieve? The default is 600ms, which should be adequate for most HMI style applications.

You can speed up this default scan rate by specifying a tagname in the project file as indicated ... but you then have to make that tag change at a rate faster than every 600ms. Is that what you want to do? Also, I believe taking that approach would probably change the default scan rate for ALL driver sheets ... which may not be a good idea. Perhaps someone else can confirm this. In general, HMI applications should avoid the need to scan inputs at high speed.

An alternative method to control input scan rate is to use a STANDARD DRIVER SHEET and make use of the 'Read Trigger:' field. Here you can specify a standard system tag such as 'Second' or 'Minute' to initiate scans, or a tag which you toggle only at particular times when a read scan is required. I expect you could also use a tag which toggles faster than 600ms. In fact, this might be achieved using the 'Clock' functionality of the scheduler which I think provides resolution to 0.1 second. Again ... perhaps someone else could confirm.

I hope what I've outlined is of some use. Most of all, I think you need to read the documentation more carefully and try some experimentation.

Regards,
Greg Shearer

Thanks for the reply Greg.
I was actually trying to slow down the scan rate. The project that I am working on has 20 PLCs and they are all communicating via cell modem or satellite. The scan rate seems too fast and is causing timeout errors. I am using standard driver sheets for the majority of the project and they are working fine but these are all existing PLCs that weren't created with SCADA in mind. So I am using the standard drivers for any tags that are reasonably grouped together and the main sheet for the remaining. The driver is MOTCP.

I'd recommend sticking with 'standard' driver sheets and use the 'Read Trigger:' to set a slower scan rate. I only ever use 'standard' driver sheets due the added control and feedback features they provide.

There are some other techniques you can apply as well. For example, you can implement handshake arrangements to ensure that no scans are ever missed for processing. This probably isn't a requirement for your application ... but it has been for mine.

If you need any further advice on possible scan control methods let me know ... as I have had to do a reasonable amount of work in this area.

Greg

Later.
On reflection, a self pacing scan arrangement might be just what you need to prevent 'over scanning'. I can go into more detail later, but the basis for this approach is to toggle the 'Read Trigger:' tag on completion of tag processing. You also use the 'Read Completed:' tag to initiate processing of the worksheet tags.

We need to be able to set the scan rate of each driver sheet. I may have PLC's that I only scan occasionally and others that I really care what is going on. It would be nice if we could set the scan time. I understand why they do it the way they do (block reads to reduce overhead) but we don't always need to know what happens every .6 seconds especially when dealing with 3000-4000 points at a time.