I was struggling all day with some tags not reading. Changing the tag group to polled worked, but then I rebooted and the default "subscribed" mode started working again...so instead of this being a plea to help, I'm turning it in to more of an architecture question:
Are there downfalls to changing the default tag group to "polled"? Or is "subscribed" the better option?
Our gateway is probably under 10,000 OPC tags spread across maybe 30 devices. Is it best practice to have a whole bunch of tag groups or just one? The manual describes what all the settings do but doesn't describe the situations that apply or the best design guidelines.
Subscribed mode saves network traffic with external OPC servers, and reduces optimization CPU cycles for Ignition's native drivers. Use it wherever possible.
However, many external OPC servers, particularly anything layered on top of old OPC DA technologies, have scattered subscription bugs.
These bugs fall into two categories:
Unusually low subscription item count limits. For these, split off one or more tag groups, as Ignition will make separate subscriptions for separate tag groups.
Pseudo-random failures in any subscriptions. For these, split off one or more tag groups set to Polled.