This Tech Note gives guide lines and some parameters which affect time taken to refresh tags per InTouch window if one of DAServers acquires about thousands of tags from a preium PLC, and each InTouch window displays hundred of tags.
DASBMTCP acquires thousands of tags from a premium PLC. If we have per each InTouch window hundred of tags are being displayed , we want to know how long will it take to refresh all data when switching to the window.
There really is no way to calculate hard numbers there are just too many variables. But with explaining possible bottlenecks and what happens when a window is shown, you can estimate from there. The only true way to find out is to actually test the system.
Major variables that can impact update intervals (there are many other minor variables as well):
Item data types: 450 booleans could update incredibly fast, where 450 strings would have a update rate of many seconds. Obviously a customer system is somewhere in between, but the ratio of data types can have a significant impact.
Item location in PLC memory: If items are contiguous, you might have 120 items per message (4-5 total messages for 450 items), but if items are all over the place, you could have dozens and dozens of messages for the same number of items.
PLC response time: Every message sent to the PLC must be processed and returned. Some PLCs are fast, and some are slow depending on their PLC CPU power and % of CPU available.• Network latency: Every message must travel the network, a local LAN might only add 1ms to each message turnaround, but a WAN, etc. could add tens or hundreds of milliseconds.
PLC support and associated DAS configuration for Maximum Outstanding Messages: How many concurrent messages can the PLC handle? Similar to a 1 lane road vs 4 lane highway.
Assuming they are setup of Advise Only Active in their Access Name here is what happens:
When the window is closed, the items are already subscribed, but not activated (except alarmed items are always activated). Deactivated items are not included in messages sent to the PLC for updates.
The window opens and the displayed items are activated
The DAServer adds the items to messages and the messages are queued for update
The DAServer starts sending out messages according to the Maximum Outstanding Message count, so if this setting is 1, then one message is sent
The PLC processes the message and sends a reply
The DAServer sends the next message and the cycle repeats until all the messages are up-to-date
Then the DAServer waits for the Device Group Update Interval time and re-queues all messages for update again and the message update cycle repeats.