Ladder logic is the language most electricians learn first. It's also only one of five programming languages a modern PLC actually supports. The five languages compared, and why Structured Text handles math ladder logic struggles with.
Ladder logic is the language most electricians learn
first because it visually resembles the relay circuits it replaced. It
is also only one of five programming languages a modern PLC actually
supports — and for some kinds of logic, one of the other four is
genuinely the better tool.
IEC 61131-3 is the international standard that defines five PLC
programming languages, giving programmers across different PLC brands a
shared vocabulary of language types even though specific software
implementations still vary by manufacturer. See our related guide on
Siemens PLC and ladder logic
for how ladder diagram, the most common of the five, is actually
structured.
| Language | Best suited for |
|---|---|
| Ladder Diagram (LD) | Relay-style discrete logic, familiar to electricians |
| Function Block Diagram (FBD) | Process control with reusable, interconnected function blocks |
| Structured Text (ST) | Complex math, loops, and conditional logic, similar to Pascal |
| Instruction List (IL) | Low-level, assembly-style coding (largely deprecated in modern tools) |
| Sequential Function Chart (SFC) | Step-by-step sequential processes with clear states |
Ladder logic can technically perform arithmetic, but nested
calculations and conditional branches get visually cluttered fast,
spread across many rungs that are hard to follow. Structured Text
handles the same logic in a few lines of readable, C-like code, which
is why recipe calculations, complex alarms, and data processing
routines are increasingly written in ST even inside otherwise
ladder-based programs.
SFC represents a process as a series of connected steps and
transitions, which maps naturally onto batch operations with distinct
phases — fill, heat, mix, drain — where the current state matters as
much as the individual logic inside each step. Many modern PLC
programs mix languages, using SFC for the overall sequence and calling
ST or FBD blocks within individual steps.
IEC 61131-3 languages fit inside the hands-on automation work
covered in SCMEP’s Automation and
Robotics training catalog. As a NIST
Manufacturing Extension Partnership affiliate serving South Carolina
manufacturers since 1989, our focus is helping teams pick the right
language for the actual logic they’re writing, not defaulting to
ladder for everything.
If your team wants to expand beyond ladder logic into Structured
Text or SFC, you can browse the
Automation and Robotics training catalog or email
the training
team.
IEC 61131-3 is the international standard that defines five PLC programming languages, giving programmers across different PLC brands a shared vocabulary of language types even though specific software implementations vary.
The five languages are Ladder Diagram, Function Block Diagram, Structured Text, Instruction List (largely deprecated), and Sequential Function Chart, each suited to different kinds of logic.
Structured Text handles complex math, loops, and conditional logic in a few lines of readable, C-like code, while the same logic in ladder logic gets visually cluttered across many rungs.
SFC works well for batch processes with distinct phases, like fill, heat, mix, and drain, since it represents a process as connected steps and transitions where the current state matters as much as the logic inside each step.
