Control
PLC scan cycle and data sampling error
Before reading a PLC tag for AI, know where in the scan cycle the value was produced and which timestamp describes it.
What is the difference between a PLC and SCADA?
Short answer
A PLC is a real-time controller that reads inputs, runs logic and drives outputs. SCADA is a supervisory layer for many controllers, with alarms, trends, logging and remote commands. A PLC runs without SCADA, and SCADA does not replace control logic. An HMI is the operator screen for one machine or area.
What each one does
A PLC (programmable logic controller) reads sensor inputs, executes logic written in a language such as those defined in IEC 61131-3, and writes outputs to actuators. It does this in a scan cycle, and the control decision itself lives in the PLC.
SCADA collects data from many PLCs and RTUs into one supervisory system. Trend charts, alarm lists, event logs and remote setpoint changes are typical SCADA functions. The data is often also written to a historian for later analysis.
An HMI is the operator screen for one machine or one area. A SCADA system usually contains HMI screens, but an HMI on its own is not SCADA.
Worked example: a tank level
Hypothetical tank, 2.0 m high. The PLC runs a high-level interlock that closes the inlet valve at 1.8 m (90 %, since 1.8 / 2.0 = 0.90). An operator in SCADA lowers the level setpoint to 1.5 m, which is 75 % (1.5 / 2.0 = 0.75). SCADA sends that request, but the PLC is what applies it.
Now assume the SCADA network is down for 10 minutes. The PLC keeps running, and the interlock still closes the valve at 1.8 m, because it does not depend on SCADA. What is lost is visibility: trends and alarms stop updating on screen.
The example shows where protection belongs. A critical interlock belongs in the controller or a separate safety system, not on a screen.
Common mistakes
The first mistake is hanging a safety function on a SCADA alarm. An alarm needs an operator to look at it. An interlock has to act automatically in the controller.
The second mistake is assuming SCADA will run fast control loops. SCADA polling is usually slower than the PLC scan cycle, so fast loops belong in the PLC.
The third mistake is reading every delay on a trend as controller delay. Network polling and historian compression can add delay too. Check the PLC scan time and the SCADA polling interval before blaming the controller.
Follow-up questions
Can I run a PLC without SCADA?
Yes. The PLC executes its own program and I/O, and SCADA is added for monitoring and remote access. Without SCADA, however, you lose central alarm logging, trends and command management, so plan how those will be covered.
Are HMI and SCADA the same thing?
No. An HMI is the operator screen for one machine or area. SCADA is the supervisory system that covers many controllers. A SCADA system often includes many HMI screens, but a single HMI is not a SCADA system.
Sources
Example numbers are hypothetical, not measured field results.