Give the machine a face, then open it to the world. An NA-series panel shows operators what's happening — and the same controller talks upward to SCADA and the cloud. The key idea: how a value gets from the PLC to the screen.
The tank level changes in the PLC. 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 — drag fast and you'll see it lag. Switch to subscription and it tracks instantly.
Pick a channel
💡 NX/NJ controllers expose your global variables as an OPC UA address space and can publish to MQTT brokers natively — no gateway box. The HMI, the plant historian and the cloud all read the same named tags. The host-comms wiring is 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?