Worked example: Motor start / stop with seal-in

The classic latching circuit you learn in week one of any PLC course. Pressing Start turns the motor on. Pressing Stop turns it off. The motor stays on between presses because it seals itself in through one of its own contacts.

Surfaces exercised: Ladder Diagram editor, variables table, parallel branches in LD, normally-closed contacts, the Simulator + debugger.

Expected time: about 5 minutes.

The circuit

Reading the rung left to right:

• A parallel pair of contacts: start_btn and motor_on. Either can pass current.
• A normally-closed contact for stop_btn. Closed by default; opens (blocks) when stop_btn is TRUE.
• The motor_on coil.

When start_btn is pressed, the top branch of the parallel passes current through the closed stop_btn contact to the coil. The coil sets motor_on = TRUE. From the next cycle onwards, the bottom branch (the motor_on contact) is also passing, so even when start_btn returns to FALSE, current still flows through motor_on's own contact, keeping the coil energised. That's the seal-in.

Pressing stop_btn opens the normally-closed contact, breaking the path to the coil. motor_on goes FALSE, the seal-in contact opens, and the motor stays off until the next start_btn press.

Step 1: Declare the variables

In a new LD program, add three variables:

All three are local for this demo. In a real installation, start_btn and stop_btn would be Input class with Location mapped to physical discrete inputs (%IX0.0, %IX0.1); motor_on would be Output mapped to %QX0.0. See Variables editor.

Step 2: Draw the rung

You need a parallel branch in the middle of a regular series. The LD toolbox handles this through the branch action on a selected element:

1. Drag a Contact into the empty rung. Name it start_btn, variant default.
2. Right-click on the start_btn contact and pick Add parallel branch. A second lane appears below.
3. In the new lane, drop another contact: motor_on, variant default.
4. Click after the parallel block to position the cursor on the main wire.
5. Drag a Contact in: stop_btn, variant negated (the normally-closed style with a slash).
6. Drag a Coil at the end of the rung: motor_on, variant default.

Save.

Step 3: Run on the Simulator

Click Start Simulator. After the compile, the rung shows live colouring, initially everything is grey because all three variables are FALSE and the normally-closed stop_btn is the only thing passing current.

Step 4: Toggle the buttons

The Simulator has no physical buttons, so you toggle the variables manually from the debugger.

1. Open the Debugger panel at the bottom (it auto-opens when the Simulator starts).
2. The variables list shows start_btn, stop_btn, motor_on. Right-click start_btn and pick Force value → TRUE. The rung lights up, power flows through the top branch, through the stop_btn NC contact, into the coil. motor_on flips to TRUE.
3. Right-click start_btn again and force it back to FALSE. Notice the bottom branch is now green (the seal-in is holding the coil energised through motor_on's own contact). motor_on stays TRUE.
4. Force stop_btn to TRUE. The NC contact opens, the rung goes grey, motor_on flips to FALSE.
5. Force stop_btn back to FALSE. Nothing happens, the motor stays off until you press start again.

What you've shown

• The seal-in pattern is two contacts in parallel, both reading inputs that can break the latch.
• Forcing values in the debugger is how you simulate physical inputs when there are none.
• The live colouring reveals exactly which branch is carrying current at any moment, which is the easiest way to convince yourself a circuit is doing what you intended.

Where to next

• Wire it to real hardware: change start_btn / stop_btn to Input class with %IX addresses, motor_on to Output with %QX0.0, deploy to a vPLC, and connect the pins.
• Expose motor_on over Modbus so an HMI can see it. See Modbus slave: expose digital outputs.
• Try a reset-priority variant: if both buttons are held, which wins? Move stop_btn to before the parallel branch.