Ep. 13 | Ladder Logic vs C++: Why PLC Programmers Still Choose Ladder and Why They Are Right & P&G Stories

Ladder logic is not a legacy habit. It is the right visual representation for the Boolean, state based logic that actually runs machines, and this week explains why that matters at 3 AM with a line down.

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The anchor question came from a computer science background: why do PLC programmers use ladder? The usual answer is cynical. Technicians cannot read C++, so write ladder or spend your nights driving to the plant. That is not the real reason, and I have met technicians who program better than most engineers. The real reason is what control logic actually is. IEC 61131-3 defines five languages for programmable controllers, and ladder is one of them because base machine control is overwhelmingly Boolean and state driven. A single valve permissive in a regulated plant can carry ten conditions gated through AND, OR and NAND relationships, and that reads far faster as rungs than as nested if and else if statements. Ladder came out of relay logic and reads like an electrical schematic, which is the document the people troubleshooting that machine were already trained on.

Performance is not the argument people think it is. Ladder, structured text and C++ all end up as machine instructions on a controller, and if you need a for loop or reusable code, ladder gives you both through Add On Instructions and function blocks. The choice was never about capability. It is about which representation survives contact with the person diagnosing a fault two years from now, at night, with production stopped. MES layers, data applications and historian integrations belong in C++, Python or Rust. Base control does not.

The other long segment answers someone one month into their first controls role who feels like they know nothing. I started at Procter and Gamble in April 2014 at a site in Maine, and my first week was commissioning a Schubert case packer shipped from Germany. It was the only significant Siemens asset on a floor otherwise running ControlLogix, so an S7-1500 machine had to exchange handshakes with the line, join the safety system, and get added to DeviceNet and ControlNet. The work was verifying voltages and grounding faults, powering up only after those checks passed, dry running motors, then validating component by component until product moved through. It was overwhelming and that was the point. The lesson that took me too long: use the OEM, the machine builder, the integrator and the vendor support contract you already pay for.

Three real builds get reviewed. A commissioning panel running a Siemens S7-1500 with Danfoss FC 302 and IC7 drives daisy chained off a switch. A home test rig combining an S7-1500, an ET 200SP remote I/O drop, a Schneider Electric controller and a WAGO Compact Controller 100, with a Pilz safety relay sitting unwired. And a first design build: a pump station on an AutomationDirect CLICK controller, which runs about $120, driving three phase pumps through contactors. The wiring is good. The screen is where the work is. Start and stop are the most used controls on that station and the smallest targets on the display, while navigation buttons are large, and colour is doing work that labels should be doing.

The episode closes on Allen-Bradley history: PLC-2 racks, a PLC-5 and CRT programming terminals shown at Vintage Computer Festival West. PLC-5 and SLC families still run production today, and their programming software goes end of life alongside the hardware. I am also building an Opto 22 groov RIO demo for the Ignition Community Conference in Sacramento in late September.

Timestamps
0:00 Ignition Community Conference and the Opto 22 groov RIO build
1:20 A Siemens S7-1500 and Danfoss FC 302 commissioning panel
5:25 First month as a control engineer and knowing nothing
6:10 Procter and Gamble, 2014, and a Schubert case packer
9:00 Commissioning sequence: voltages, grounding, dry run
12:25 Vendor support, OEMs and integrators as real resources
13:40 A home test rig with Siemens, Schneider Electric and WAGO
17:45 Why PLC programmers use ladder logic instead of C++
20:15 Boolean logic and state conditions, the real answer
26:20 An AutomationDirect CLICK pump station reviewed
30:30 HMI critique: button sizing, colour and label hierarchy
33:35 Allen-Bradley PLC-2, PLC-5 and CRT programming terminals

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