A newcomer wrote the one-button start/stop in 60 lines, causing the CPU scan cycle to spike to 12 ms. The boss slammed the table, asking if this program could be sold?
Today, I will break down the 9 most commonly used “one-button start/stop” instructions using three real ladder diagrams—same button, same motor, but some can do it in 1 line while others crash in 10 lines. After reading this, if you still write 60 lines, I concede!
1. What problem does the one-button start/stop solve?
There is only 1 green button (I0.0) and 1 contactor (Q0.0) on site. Requirement: press once to start, press again to stop, and repeat. Essentially, it is a “ping-pong switch”—turning each button’s “rising edge” into a “flip” of the coil state.
2. 9 methods, from Bronze to King
All examples below are based on S7-1200/1500, with one diagram corresponding to one idea, which can be directly copied for use.
| Rank | Name | Lines | Pros and Cons | Applicable Scenarios |
|---|---|---|---|---|
| Bronze | Set/Reset (S/R) | 3 | Simple, most intuitive | Student assignments |
| Silver | Self-locking position instruction | 2 | Compact logic | Small standalone machines |
| Gold | Addition instruction | 3 | Expandable counting | When counting is needed |
| Platinum | Increment instruction INC | 2 | Fastest single-byte flip | High-speed scanning |
| Diamond | Counter CTU | 2 | Built-in completion bit | When a limited number of counts is needed |
| Star | Logical negation INV | 2 | One line reversal | Minimalist |
| King | SR/RS Reset Priority | 1 | Prevents double coils | Must use on site |
| King+ | SR/RS Set Priority | 1 | Prevents double coils | Must use on site |
| Legend | Single Trigger on Rising Edge | 1 | No jitter | Button debouncing |
1️⃣ Set/Reset (S/R) — Newcomer’s favorite Ladder Diagram: Network 1: I0.0 rising edge → S Q0.0Network 2: I0.0 rising edge → R Q0.0Comment: Most intuitive, but may cause “set + reset” to occur simultaneously during button jitter, leading to motor twitching.
2️⃣ Self-locking position instruction — Classic 2 lines Ladder Diagram: Network 1: I0.0 rising edge → M0.0 negationNetwork 2: M0.0 → Q0.0Comment: Using M points as intermediate variables, the logic is clear, and the scan cycle is the shortest.
3️⃣ Addition instruction ADD — “Counting” Start/Stop Ladder Diagram: Network 1: I0.0 rising edge → VW0 = VW0 + 1Network 2: VW0 odd/even judgment → Q0.0 = VW0 & 1Comment: Quantifies button presses, can be expanded to “press 3 times to start, then 2 times to stop”.
4️⃣ Increment instruction INC_B — Byte-level flip Ladder Diagram: Network 1: I0.0 rising edge → INC VB0Network 2: VB0 lowest bit → Q0.0Comment: Single-byte 0-255 cycle, fastest only requires 0.1 μs.
5️⃣ Counter CTU — Officially comes with a “completion bit” Ladder Diagram: Network 1: I0.0 rising edge → CU CTU C0 PV=2Network 2: C0 completion bit → Q0.0Comment: The counter comes with a reset, can be used for “stop after pressing 5 times” to prevent mistakes.
6️⃣ Logical negation INV — One line reversal Ladder Diagram: Network 1: I0.0 rising edge → INV VB0Network 2: VB0 lowest bit → Q0.0Comment: Extremely simple code, but not friendly for newcomers, easy to misread.
7️⃣ SR Reset Priority — On-site King Ladder Diagram: Network 1: SR Q0.0, S1=I0.0 rising edge, R=I0.0 rising edgeComment: Official library instruction, prevents double coils, reset priority, fault-safe.
8️⃣ RS Set Priority — Brother version Ladder Diagram: Network 1: RS Q0.0, S=I0.0 rising edge, R1=I0.0 rising edgeComment: Logic is opposite to SR, set priority, suitable for “start immediately upon power” scenarios.
9️⃣ Single Trigger on Rising Edge — Ultimate no-jitter solution Ladder Diagram: Network 1: I0.0 rising edge → Q0.0 negation(written in hardware interrupt OB40)Comment: Interrupt-level scanning, no fear of button jitter.
3. 5 Major Pitfalls on Site, Help You Avoid Them in Advance
| Pitfall | Phenomenon | Solution |
|---|---|---|
| Button jitter | Motor starts and stops erratically | Add 20 ms delay filtering or use interrupts |
| Double coils | Compilation error | Use SR/RS uniformly |
| Scan cycle doubles | 60 lines of code | Use position instructions or INC to reduce to 2 lines |
| Power-on misoperation | RS defaults to set | Switch to SR and add initial scan reset |
| Counter overflow | CTU up to 32767 | Set PV to 2, reset immediately after use |
4. 3 Real Cases of Playing with “One-Button Start/Stop”
Case 1: Remote Start/StopUsing SR instruction + Modbus TCP, a mobile app taps once to start, taps again to stop, with 0 wiring on site.
Case 2: FootswitchFootswitch signal jitter of 50 ms, using interrupt OB40 + rising edge negation, no faults for half a year.
Case 3: Foolproof CountingCTU set PV=5, the 5th button press stops and alarms, preventing misoperation.
With the same button, some can do it in 1 line, while others crash in 60 lines.It’s not that the function can’t be achieved, but the approach was not chosen correctly.
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