Distributed outputs must have a defined state before the PLC establishes control and when communication is interrupted. Leaving the behaviour implicit can cause unexpected energisation, process interruption or a prolonged unsafe condition.
Power-up values
Set each output to the state required during module start-up. The correct value depends on the circuit design: de-energised is not automatically safe, and some valves or actuators may require an energised holding state until coordinated control is available.
Communications-loss values
A watchdog should detect the absence of valid host communication within a defined period and transfer outputs to approved values. The timeout must be longer than normal network jitter but short enough to meet the process response requirement.
Validation
- Test cold start, warm reboot and field-power interruption.
- Disconnect RS-485 and verify the watchdog response for every output.
- Confirm recovery behaviour when communication returns.
- Document which functions remain autonomous and which require the host controller.
- Do not use general-purpose I/O logic as a substitute for a certified safety system.
Technical basis: Titania Modbus DI-8 DO-8 datasheet TTN-DS-2026-001, issued 19 July 2026. Confirm the current hardware, firmware and controlled document revision before implementation.
