Creating an Alive Signal

An alive signal, also called a heartbeat, repeatedly changes between TRUE and FALSE while a scenario is running. An external PLC or device can monitor those changes to confirm that the scenario is still alive.

1. Create the pulse

1. Add a PWM (Pulse Width Modulation) block.

2. Set Time mode to Seconds.

3. Set Interval to the complete cycle and Up Duration to the time the output remains TRUE.

For example, an interval of 0.3 seconds and an up duration of 0.1 seconds produces TRUE for 0.1 seconds and FALSE for 0.2 seconds, repeatedly.

2. Send the signal to the external device

Connect the PWM block's Boolean output to the Data or Value input of the communication writer used by the device. Connect the writer to its communication client or connection as usual.

• If the writer has an Enable input, connect a Logic Input set to TRUE.

• For a TCP writer such as TCPIPWrite, set Send Policy to Always so both signal states are sent. The example below uses this configuration.

• For Siemens S7 Write, connect Siemens S7 Connect to S7 Client, set DB Data Type to Boolean, and configure the required DB number, byte address, and bit position.

• For Modbus Write, connect Modbus Connect to Modbus Client, choose Coil, and set the target address.

Example heartbeat sent through TCPIPWrite

3. Configure the receiver

The external device should monitor for a change of state, rather than the current value alone. With the example above, a timeout of about one second allows for normal scheduling and network jitter while still detecting a stopped scenario quickly.

• Use a dedicated PLC bit or tag; do not share it with process-control logic.

• Choose a slower pulse if the device, PLC scan time, or network cannot reliably observe short transitions.

• If the writer provides Data on Stop, set it to FALSE when the receiver needs a known final state.

The external device raises its timeout alarm when it no longer receives state changes. The signal stops when the scenario stops evaluating the PWM and communication blocks.

4. Test the signal

Before connecting to production equipment, use Scope to confirm the expected TRUE/FALSE cycle. Then verify at the receiver that it sees both states and raises an alarm after the configured timeout.