When tags update late in Ignition, check the PLC driver diagnostics before splitting the default Scan Class: a large Scan Class alone does not prove a communications bottleneck. If the driver is overloaded, reduce unnecessary polling with leased Scan Classes or address the driver and PLC connection capacity; dividing the same tags into more Scan Classes is not the first fix.
Check the PLC driver diagnostics first
- Open the diagnostics for the driver connection serving the delayed tags. Record the available request, response, error, and timing indicators, along with the affected PLC connection. Compare those readings with a connection that updates normally, if one is available.
- If the diagnostics show delayed or failing communication, investigate the driver-to-PLC path before reorganizing Scan Classes. The delay can result from the amount of requested data, PLC connection capacity, network conditions, or driver configuration; the driver diagnostics help narrow the cause.
- If the diagnostics do not point to communication delay, check whether the delayed tags share a Scan Class and whether that class is being asked to poll more data than necessary. Then check query tags separately.
Do not infer overload from tag count alone. PLC protocol, data layout, driver settings, and the controller’s available communication capacity all affect how much polling a connection can handle. Read the specific driver’s diagnostics and advanced settings rather than applying settings intended for another PLC or driver.
Compare the likely causes with the observed symptom
| Observation | Likely direction | Next check |
|---|---|---|
| Many tags on one PLC connection update late | Connection throughput or driver configuration may be limiting polling. | Review that driver’s diagnostics and advanced settings. |
| Only tags that operators are not viewing need frequent updates | Those tags may be polled more often than their use requires. | Consider a leased Scan Class for tags that can update less often when not actively viewed. |
| Query tags are delayed along with other tags | Query-tag polling may be competing with other work. | Check whether query tags are already assigned to a separate Scan Class. |
| Tags remain delayed after Scan Class groups are split | Grouping alone may not have changed the communication load. | Return to driver diagnostics and connection capacity. |
Check whether the Scan Class changes actual demand
A Scan Class organizes polling; splitting one large class into several smaller groups does not, by itself, establish that the PLC or driver has less work to do. If the same tags are still polled at the same rates through the same connection, the underlying demand remains. Changing group boundaries without evidence of a scheduling or optimization issue can waste time and may make performance worse in unusual optimization cases.
Instead, identify which tags need frequent updates and which can tolerate slower updates. Leased Scan Classes can reduce polling for tags when operators are not actively looking at them, according to the system configuration. Use them where the operating requirement permits less frequent updates; do not slow tags whose current data is needed for control or operator response.
Isolate query-tag polling
Check query tags as a separate workload. Assigning them to separate Scan Classes can make their polling easier to manage independently from PLC tags. This organization does not prove that query execution is the root cause; compare the update behavior before and after changing the assignment.
- List the delayed tags and identify which are query tags.
- Confirm whether query tags already use a separate Scan Class. If they do, leave that division in place while you continue diagnosing the PLC driver path.
- If query tags share the heavily loaded class, move them into a separate class, then observe whether their update behavior or the PLC-tag delays change.
Change driver architecture only after measuring
If driver diagnostics indicate that one PLC connection is the bottleneck, review the driver’s advanced settings for that PLC and protocol. Some PLC drivers expose settings that materially affect performance, but the correct choices depend on the specific driver and controller. Use its documented diagnostics and configuration guidance; do not copy settings blindly.
Another option is to create multiple simultaneous driver connections to the PLC and divide tags between them. This can distribute polling across connections, but it is not a default remedy: verify that the PLC and driver support the arrangement and that connection limits or controller load do not become the new constraint. Change one factor at a time so the result can be attributed to the change.
Apply the change and verify tag updates
- Record baseline driver diagnostics and tag update behavior for the affected PLC.
- Choose the branch indicated by the checks: reduce polling with leased classes for tags that do not need constant updates; separate query-tag polling if it is mixed with PLC polling; or evaluate driver settings and parallel connections when diagnostics point to a connection limit.
- Make one change at a time. Keep query tags in their own class if they are already separated, and avoid splitting a Scan Class solely because it contains many tags.
- Recheck the same driver diagnostics and the same delayed tags. Confirm that updates improve and that the change has not shifted delays or errors to another connection or tag group.
If delay persists, capture the driver diagnostics, connection details, affected tag groups, and the changes tested. Stop changing Scan Class groupings when they do not alter measured behavior; escalate with those diagnostics to the official support channel for the software or PLC driver.
Frequently asked questions
What happens if I split one large Scan Class into several smaller ones?
Splitting alone may not improve update performance if the same tags still poll at the same rates through the same PLC connection. Check driver diagnostics first and verify any grouping change against actual tag updates.
What happens if I use a leased Scan Class?
It can reduce how often tags are polled when operators are not actively viewing them. Use it only for tags whose operating requirements allow less frequent updates.
What happens if the PLC driver diagnostics show delays?
Investigate the driver-to-PLC connection, the driver’s documented advanced settings, and connection capacity. Record the diagnostic readings before changing configuration.
What happens if query tags are in the default Scan Class?
Move them to a separate Scan Class so their polling can be managed independently, then compare update behavior. If they are already separated, continue checking the PLC driver path.
What happens if multiple drivers connect to one PLC?
Splitting tags across simultaneous driver connections may help when one connection is the bottleneck, but first check driver and PLC support and connection limits. Stop and escalate to official software or driver support if diagnostics remain unclear or delays persist after measured changes.