An NA-series panel gives the machine a face: it shows operators what's happening on the line. The same controller also talks upward to SCADA and the cloud. This lesson is about how a value gets from the PLC to the screen.
The tank level changes in the PLC, and the HMI has to find out. Polling asks "what's the value?" on a fixed timer. Subscription says "tell me when it changes." Drag the real level and watch which display keeps up.
In polling mode the panel only refreshes on its timer, so if you drag fast you'll see it lag. Switch to subscription and it tracks instantly.
Pick a channel
💡 Many NX/NJ controllers expose your global variables as an OPC UA address space, and the NX1P and NX102 can publish to MQTT brokers with Omron's MQTT Communications Library (SYSMAC-XR020), with no gateway box in between. The HMI, the plant historian and the cloud all read the same named tags. The host-comms wiring is covered in the FINS & CIP deep dive (D1).
A value changes 5 times within one polling interval. How many of those changes does a polling client see?