The next step after our PLC beginner's guide: ladder logic fundamentals, why Siemens dominates search demand for this topic, a worked motor start/stop example, and Siemens-specific TIA Portal terminology.
If you’ve read our beginner’s guide to PLC
training and understand what a PLC actually is, this is the next
step: ladder logic, the programming language most PLCs actually run, and
why so much of the search traffic for this topic says “Siemens”
specifically.
Siemens’ S7 platform, programmed through TIA Portal, is a dominant
industrial PLC standard in Europe, in process industries like chemical
and water/wastewater treatment, and among manufacturers with European
supply-chain or ownership ties — including plants in the US that were
built or equipped by European-headquartered automotive and process
companies. Allen-Bradley/Rockwell tends to hold a larger share of North
American discrete manufacturing overall. Neither brand is “the” industry
standard everywhere — which platform you’ll actually work on depends
heavily on your sector and your plant’s history, and it’s worth checking
what your target employers actually run before committing deeply to one
ecosystem’s terminology.
Ladder logic represents control logic as a series of horizontal
“rungs” between two vertical power rails, visually resembling an
electrical relay diagram — which is exactly the point, since it was
designed to be readable by electricians already familiar with relay
circuits. Each rung evaluates left to right, and a program runs rung by
rung, top to bottom.
| Element | What it does |
|---|---|
| Normally Open (NO) contact | Passes power only when its associated bit is true |
| Normally Closed (NC) contact | Passes power when its associated bit is false, blocks when true |
| Coil (output) | Energizes when the contact logic ahead of it on the rung completes the circuit |
| Counter (CTU/CTD) | Increments or decrements a value on each qualifying input transition, comparing against a preset |
This is the standard first ladder logic example taught almost
everywhere, because it demonstrates the core trick of holding a signal
“on” after a momentary button is released. The rung: a Normally Open Start
contact in series with a Normally Closed Stop contact, feeding a “Motor
Run” output coil. Pressing Start energizes the coil — but since Start is
a momentary button, it releases immediately. To keep the motor running,
a second Normally Open contact tied to the same “Motor Run” output is
wired in parallel with the Start contact, so once the coil energizes, that
parallel contact “seals in” the circuit and holds the motor on until
someone presses Stop, which breaks the circuit regardless of the seal-in
contact’s state.
Counters build on the same logic with an added twist: instead of a
contact passing or blocking power directly, a counter instruction
increments each time its input contact transitions (commonly on a rising
edge), and a separate “done” bit turns on once the count reaches a preset
value — the basis for part counting, batch tracking, and cycle counting
on a production line.
TIA Portal, Siemens’ engineering software, isn’t limited to ladder
logic (LAD) — it also supports function block diagram (FBD) and
structured text (SCL) as programming languages for the same hardware,
and larger Siemens programs often mix all three depending on which style
fits a given piece of logic best. On counters specifically, Siemens’
current IEC 61131-3 standard blocks use the same CTU/CTD/CTUD naming
found across most modern PLC platforms, though older S7 documentation
and legacy programs may reference S7-specific counter instructions
(S_CU/S_CD) with slightly different behavior — worth checking which
convention a specific plant’s existing programs use before assuming.
Most PLC work on the floor isn’t writing new programs from scratch —
it’s troubleshooting logic someone else wrote, sometimes years ago. The
seal-in pattern above is worth memorizing precisely because it’s
everywhere: once you can spot it, you can quickly identify which contact
actually keeps a motor, pump, or conveyor running after its start button
is released, which is usually the first thing worth checking when
something won’t stay on, or won’t turn off.
Most programming software, including TIA Portal, shows the live status
of each contact and coil directly on the rung while the program runs —
energized elements highlight, so you can watch logic execute in real time
against the actual machine instead of just reading static code. That live
view is usually faster for diagnosing a fault than reading the program
cold, since it shows you exactly which condition in the rung isn’t being
met.
Whatever the brand, the physical hardware breaks down the same way: a
CPU that executes the program, input modules that read sensors and
switches, output modules that drive motors, valves, and indicator lamps,
a power supply, and typically an HMI (human-machine interface) as a
separate screen for operators to monitor and adjust the process. Brand
differences show up in the software and instruction naming far more than
in this basic hardware architecture.
Siemens-specific PLC certification isn’t a standalone course in
SCMEP’s current catalog — the concepts here build on our
PLC training beginner’s guide as part of
broader Automation and Robotics
training. As a NIST Manufacturing Extension
Partnership affiliate serving South Carolina manufacturers since
1989, our focus is making sure you understand the underlying logic
well enough to work across brands, not locking you into one vendor’s
terminology before you know which platform your target employer
actually runs.
If you’re trying to figure out which PLC platform to focus on, you can
browse the Automation and Robotics
training catalog or email the training team.
Ladder logic controls the same relays and contactors a motor control
circuit uses at the hardware level. See our related guide on
relays and motor control
symbols for the relay vs. contactor distinction and NEMA vs. IEC
symbol differences.
PLC programming is the device-level logic layer; two related layers sit
above and beside it. See our related guides on
SCADA systems for the
plant-wide supervisory layer, and
industrial Ethernet
protocols for how that PLC’s data actually gets where it’s going.
Siemens’ S7/TIA Portal platform is dominant in Europe, in process industries like chemical and water treatment, and among manufacturers with European ownership or supply-chain ties. Allen-Bradley/Rockwell tends to hold more share in North American discrete manufacturing overall — which platform matters most depends heavily on your specific sector.
Ladder logic is a graphical PLC programming language that represents control logic as horizontal “rungs” between two vertical power rails, resembling an electrical relay diagram. It was designed to be readable by electricians already familiar with relay control circuits.
A Normally Open (NO) contact passes power only when its associated bit is true. A Normally Closed (NC) contact passes power when its bit is false and blocks power when the bit is true — the opposite behavior.
No. TIA Portal supports ladder logic (LAD), function block diagram (FBD), and structured text (SCL) as programming languages for the same hardware, and larger programs often combine multiple languages depending on what fits each piece of logic.
