Worked example: Blink

The simplest possible PLC program. A single BOOL variable, blink, toggles between FALSE and TRUE once per second using two timers.

This is what the EDF Demo project ships with, and it's a great smoke test that your editor connection, the Simulator, and the debugger all work end-to-end.

Surfaces exercised: Ladder Diagram editor, variables table, TON + TOF function blocks from iec-standard-fb, the Simulator, the live execution colouring, the debugger.

Expected time: about 3 minutes.

The circuit

A normally-closed contact reading blink feeds a 500 ms on-delay timer (TON), which feeds a 500 ms off-delay timer (TOF), which drives the blink coil. The coil's value is read by the normally-closed contact at the start of the same rung, a self-feeding loop.

When blink is FALSE, the normally-closed contact passes current, the TON starts counting, and after 500 ms its output goes high. The TOF then starts its own count. After another 500 ms the coil sets blink to TRUE. The contact now blocks current and the cycle reverses. Net effect: blink toggles every second.

Step 1: Create the project

If the EDF Demo project is already open, skip ahead. Otherwise:

1. From the Autonomy Edge dashboard, create a new project. Pick Ladder Diagram (LD) as the main language, leave the task interval at the default T#20ms.
2. Click Open in editor. The editor loads on the project's empty main program.

Step 2: Declare the variable

In the variables table above the body, add one row:

Tick the Debug column so you can watch it in the debugger later.

Step 3: Draw the rung

Drag elements from the activity-bar toolbox (Block, Coil, Contact) into the empty rung.

The element sequence, left to right:

1. Contact named blink, variant negated (normally-closed).
2. Block named TON. Set its PT input to T#500ms.
3. Block named TOF. Set its PT input to T#500ms.
4. Coil named blink, variant default.

Wire the contact's output to TON's IN, TON's Q to TOF's IN, and TOF's Q to the coil.

Save with Cmd / Ctrl + S.

Step 4: Run it on the Simulator

The Simulator is the default target. Click the Start Simulator button in the activity bar (the Play triangle).

After a few seconds of compile, the bottom panel switches from Console to Debugger and the LD rung lights up with green wires showing live power flow. The TON's ET value updates inline as it counts.

Step 5: Watch the variable

In the Debugger panel at the bottom, click main.blink to add it to the chart. The trace square-waves between 0 and 1 every second.

If main.blink doesn't appear in the list, you didn't tick the Debug column in step 2. Tick it, save, and the variable appears on the next poll.

What you've shown

• IEC bodies are wired top-to-bottom in source files and compile through STruC++ to AVR-targeted C++.
• A single variable can be both written (by the coil) and read (by the contact) within the same rung. The execution order is left-to-right per rung, top-to-bottom across rungs.
• The Simulator's polling latency is well under a second: the live colouring keeps up with a 1 Hz blink with no noticeable lag.

Where to next

• Add a second rung. A counter (CTU) that counts every rising edge of blink shows you 1-per-second on its CV output.
• Try the same circuit on a real vPLC. See Connecting to a vPLC, the rung's behaviour is identical.
• Move on to Motor start / stop with seal-in: the next-simplest pattern after blink.