Lessons · Industrial control · HMI tags
What the screen shows
An HMI reads bits from the PLC. Indicator tags are coils that mirror state, including 'commanded but not running'.
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What it is for
Operators act on the screen. A lamp that shows the command instead of the actual state hides failures.
How to think about it
Does the lamp show the command or the actual state? Mirror actual outputs into indicator coils, and add a fault lamp for command without feedback.
Worked example
[Motor] (RunLamp)Shows the actual state.
[Run] [/Motor] (FaultLamp)Commanded but not running.
Your turn
Mirror the pump to its lamp.
[] (PumpLamp)
Run a rung and watch the scan
The trap
Wiring the lamp to the button. It lights even when the motor did not start.
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