HMI and SCADA
An HMI (human-machine interface) is the screen an operator uses to see and run a process. It can be a touch panel on one machine or a PC in a control room.
SCADA (supervisory control and data acquisition) is the system behind those screens across a plant. It collects data from many PLCs, decides which alarms are active, stores history and passes the operator's commands back down. The PLC still does the control, every few milliseconds. SCADA supervises: if the network drops, the tank keeps being controlled.
- Field. Sensors and actuators: LT-101, FT-101, FV-101, P-101.
- Control. The PLC reads them, runs the loops and interlocks.
- Supervisory. SCADA servers, historian and operator stations.
Tags and polling
Every value is a tag: a name, a value, a unit, a quality and a time stamp. The server asks each PLC for its tags on a fixed cycle (here once a second) and keeps the latest copy. Screens show that copy, so what you see is always a little old.
Quality matters as much as the value. A broken sensor or a lost link does not make a value zero; it makes it untrustworthy. Good systems mark it BAD and freeze it, so nobody acts on a number that is no longer true.
Alarms
An alarm tells the operator that something needs a response, and its priority says how soon. Acknowledging means "I have seen it". It changes nothing in the plant. An alarm clears when its condition goes away, and stays on the list until it is both cleared and acknowledged.
Shelving hides a nuisance alarm for a set time, on purpose and on record, so it cannot be forgotten. In a flood, read the times: the first alarm is often the cause and the rest are consequences.
Trends and the historian
The historian stores tag values with time stamps, often for years. A trend shows how a value moved, which says far more than one number. A level that falls steadily points to lost inflow; a line that goes perfectly flat often means a dead sensor, not a steady process.
Protocols in plain words
Modbus is old and simple. The client asks for numbered registers ("send me 40001 to 40010") and the device answers with raw numbers. There are no names, units, quality or security built in; both sides must agree what each register means.
OPC UA is newer. Devices offer named tags with types, units, quality and time stamps, clients can subscribe to changes instead of asking every time, and connections can be signed and encrypted. Many plants run both.
Why screens need to be calm
Operators watch the same screens for twelve hours, and the moment that matters may come once a month. Good screens show equipment in grey and save colour for abnormal states, put trends next to numbers, and raise few alarms that each need an action. A bright, busy screen hides the one thing that is going wrong.
