Use ladder logic for interlocks, permissives and anything a maintenance technician will troubleshoot online, because live contact states make faults visible. Use SCL for calculations, loops, array handling, string work and complex state machines, where ladder becomes unreadable. Both compile to the same machine code, so there is no performance argument either way.
- Choose per block, not per project; a single machine can have ladder interlocks and SCL sequence logic.
- Ladder wins on online troubleshooting; SCL wins on maths, loops and anything repeated over an array.
- The person maintaining the machine at 2 a.m. is part of the decision.
What Each Language Is Good At
Ladder logic descends from relay diagrams. Rungs of contacts and coils map directly onto the electrical drawing, which is why an electrician can read it without training in software.
Structured Control Language is Siemens' implementation of Structured Text, one of the IEC 61131-3 languages. It is a text language with IF, CASE, FOR and WHILE, and it handles anything you would write as an equation.
| Task | Better in ladder | Better in SCL |
|---|---|---|
| Motor start/stop with seal-in | Yes | Workable but less clear |
| Interlock and permissive chain | Yes | Harder to trace online |
| Analog scaling and maths | Cumbersome | Yes |
| Looping over 50 devices | Impractical | Yes |
| Step sequence with many steps | Grows very large | Yes, with CASE |
| String and recipe handling | Very awkward | Yes |
| Timers and counters | Yes | Equally fine |
| Fault word packing | Verbose | Yes |
Both compile to the same underlying code on an S7-1200 or S7-1500, so arguments about one being faster are misplaced. What differs is how quickly a human understands it.
Side-by-Side Comparison
Take one common job: check whether any of thirty sensors has faulted, and set a summary bit.
In ladder, this is thirty contacts in parallel across a rung, or thirty rungs, and adding a thirty-first sensor means editing the network. In SCL it is a four-line FOR loop over an array, and adding a sensor means changing the array size.
Now take a different job: prove why a conveyor will not start. In ladder, you go online and the permissive chain shows you the open contact immediately, in colour. In SCL you read the IF condition and then check each variable in a watch table, which takes longer and needs someone comfortable with code.
| Criterion | Ladder | SCL |
|---|---|---|
| Online fault finding | Excellent, visual | Moderate, needs watch tables |
| Maths and scaling | Poor | Excellent |
| Repetition over many devices | Poor | Excellent |
| Readable by electricians | Yes | Usually not |
| Version comparison and review | Awkward | Good, it is text |
| Reuse across projects | Moderate | Strong with generic blocks |
Learn both properly
Softwell covers ladder and SCL side by side, with dedicated SCL project development sessions.
Choosing Per Block on a Real Machine
A practical split for a typical packaging line looks like this.
- Ladder: safety-adjacent permissives, motor control blocks, manual and jog logic, hardwired fault handling, anything the maintenance team will open first.
- SCL: the step sequence, analog scaling and filtering, recipe parameter handling, production counting and OEE calculation, communication data packing.
- Either: timers and counters, which read perfectly well in both.
Write the sequence in SCL with a CASE on the step number, but publish the step number and blocking condition as plain integers so the HMI can show them as text. That gives you SCL's compactness without losing visibility on the shop floor; see PLC step sequence programming and the HMI sequence status screen.
Where an SCL block is doing something non-obvious, comment the intent, not the syntax. "Ramp the setpoint to avoid a pressure spike" helps; "increment counter" does not.
Troubleshooting, Standards and Team Skills
The deciding factor on most projects is not technical. It is who maintains the machine.
If the plant's maintenance team reads ladder and nothing else, an all-SCL program means every fault becomes a phone call to the OEM. If the team includes engineers comfortable with code, SCL blocks reduce the program size dramatically and make modifications safer.
A workable house standard
- Interlocks, permissives and manual control in ladder, always.
- Sequences, maths and data handling in SCL.
- Every SCL block publishes status as integers and bits the HMI can display.
- Block headers state purpose, author, date and revision.
- No absolute addressing into data blocks in either language.
Document the standard once and apply it across projects. Consistency matters more than which side of the argument you land on, because the real cost is a plant where every machine is written differently.
For the SCL side in depth, see the SCL programming manual and SCL programming training.
Hands-On Lab: Write the Same Function Twice
Hands-on- TIA Portal with an S7-1200 or S7-1500 CPU, or PLCSIM
- An empty project for experimentation
- Estimated time: 40 minutes
Build a ladder permissive
Write a five-condition permissive chain in ladder driving a Ready bit.
Rewrite it in SCL
Write the same permissive as an IF statement in an SCL block.
Compare online
Break one condition and find the cause in each version.
Build an array scan in SCL
Write a FOR loop that checks 30 sensor bits and sets a summary fault bit.
Attempt the same in ladder
Build the equivalent for just five sensors in ladder and extrapolate the effort for thirty.
You have working versions of the same logic in both languages and can justify, in one sentence per block, which language each part of a machine should use.
Frequently asked questions
Is SCL faster than ladder logic on Siemens PLCs?
No meaningful difference. Both compile to the same machine code on S7-1200 and S7-1500. Choose on readability and maintainability, not speed.
Can I mix ladder and SCL in one project?
Yes, and you should. The language is chosen per block, so a single machine can have ladder interlocks calling SCL function blocks.
Which should a beginner learn first?
Ladder, because it maps onto the electrical drawing and makes online troubleshooting visual. Add SCL once you meet maths, loops or long sequences.
Why do OEMs often insist on ladder?
Because the end customer's maintenance team has to fault-find at night without the OEM present, and ladder shows the open contact on screen without any code reading.