PLC Scan, I/O & Control Architecture

What a PLC is

A programmable logic controller is an industrial computer built to replace racks of relays. The first, the Modicon, came from Bedford Associates in the late 1960s; the idea was a solid-state box with a stored program that behaves like the relay logic electricians already knew ¹. That’s why the most common PLC language still looks like a relay schematic.

A PLC has three parts that matter:

• Input modules. Each terminal reads a field device: a pushbutton, limit switch, photoeye, pressure switch. Inside, an opto-isolator turns the field voltage (often 24 V DC or 120 V AC) into a clean logic bit, with an LED on the front showing its state ¹.
• The program. Contacts and coils on the screen aren’t real; they’re bits in memory. An input bit can be used in as many rungs as needed, which is one reason a PLC beats hardwired relays ¹.
• Output modules. Transistors, TRIACs, or small relays switch real loads: contactor coils, solenoid valves, stack lights ¹.

IEC 61131-3 defines the standard languages: ladder diagram, function block diagram, structured text, and sequential function charts for organizing steps ².

Through this course, picture one small cell: an infeed conveyor brings steel brackets to a clamp-and-drill station, and a robot unloads finished brackets to an outfeed conveyor.

The scan

A PLC doesn’t watch its inputs continuously. It runs a loop, the scan:

1. Read inputs: copy every input’s state into an input image table.
2. Solve the program, rung by rung, top to bottom, using that snapshot.
3. Write outputs: copy the results to the output modules.
4. Housekeeping: communications, diagnostics. Then repeat.

A small program scans in a few milliseconds. Because the program works from a snapshot, an input that changes in the middle of step 2 isn’t seen until the next scan, and an output set late in the program doesn’t reach the field until step 3.

Why short pulses get missed

At the drill station, a photoeye watches for the leading edge of each bracket. Suppose the infeed belt runs at 1 m/s:

• A bracket 40 mm long blocks the beam for 40 ms. A PLC with a worst-case 12 ms scan plus input filter will see it every time.
• The gap between two touching-but-not-quite brackets might be 6 mm: 6 ms of light. That can fall entirely between two input reads, and the PLC never knows the parts were separate.

Fixes, roughly in order of cost: slow the belt or space the parts; use the input module’s high-speed counter or interrupt input; put the counting in a faster periodic task; or use a sensor with built-in pulse stretching. “The PLC is fast” is not a timing analysis; the numbers are.

The I/O list

Before anyone writes code, the electrical designer and the programmer agree on an I/O list, one row per point. For the drill station:

The last column is the point. A broken wire looks like an input turning off, so each input is wired so that “off” is the safe interpretation. That’s why stop buttons use normally closed contacts: if the stop button were wired normally open, a broken wire would mean the button could never stop the machine; wired normally closed, a broken wire stops it ¹.

Command is not proof

Conveyor_RunCmd is what the PLC asked for. Conveyor_RunningFb is what happened: an auxiliary contact on the motor starter, a drive’s “running” status, or a speed sensor on the tail pulley. Keep them as separate tags and compare them. If the command has been on for 2 seconds and there’s no feedback, something is wrong (a tripped overload, a broken belt) and the program should drop the command and raise a specific alarm.

Where the PLC sits

ISA-95 describes the layers of a plant ³:

• Field devices sense and act.
• PLCs and motion controllers run deterministic control.
• HMI and SCADA let people supervise and record.
• MES schedules and tracks production.
• ERP handles orders, inventory, and money.

Design so a slow or failed upper layer never stops the lower one: if the MES server goes down, the drill station should finish its part and wait, not crash. NIST’s guide to operational technology security makes the same point from the security side: OT controls physical equipment, so availability and safety come first, and PLCs need managed accounts, backups, change control, and network segmentation rather than being treated like office PCs ⁴.

Try it

Write the I/O list for the infeed conveyor: a run command, running feedback, a photoeye at the stop, a jam photoeye, and a downstream-ready signal from the drill station. For each, write what a broken wire reads as and whether that’s safe. Then work out the shortest part gap your belt speed allows for a 10 ms scan.