---
name: evolv-ot-edge-plc-integration
description: Engineer OT edge connectivity and PLC interoperability for EVOLV. Use when implementing or reviewing OPC UA/Modbus and similar integrations, namespace/tag mapping, quality/timestamp semantics, retry/reconnect behavior, and deterministic command/feedback contracts at the edge.
---

# EVOLV OT Edge PLC Integration

## Mission
Deliver reliable, deterministic edge protocol integration between EVOLV Node-RED nodes and PLC/SCADA systems.

## Harness Execution Contract
- Start from current integration topology, topic contracts, and protocol endpoints.
- Define invariants before edits:
  - command/feedback contracts remain deterministic
  - reconnect/retry behavior is bounded and observable
  - quality/timestamp semantics are preserved end-to-end
- Validate with connection-loss and recovery evidence.

## Scope
- Edge/connector nodes (existing and new)
- Topic mapping code in `nodes/*/src/`
- Admin endpoints/config for connector behavior and credentials

## Workflow
1. Map PLC tags/NodeIds/registers to EVOLV message contracts.
2. Define write acknowledgement and feedback confirmation rules.
3. Implement reconnect/backoff/session handling.
4. Enforce quality, timestamp, and stale-value semantics.
5. Verify failover behavior and command idempotency.

## Standards
- Never assume connection continuity; model transient faults explicitly.
- Keep protocol mappings versioned and auditable.
- Separate transport errors from process-state errors.
- Ensure secure defaults align with OT/IT security skill.

## Test Expectations
Cover:
- disconnect/reconnect and session re-establish paths
- duplicate/late/out-of-order message handling
- read/write mapping correctness and unit conversion
- safe behavior under degraded quality or timeout

## Deliverables
Return:
- integration contract map (protocol <-> topic/payload)
- retry/recovery strategy and limits
- changed files/tests with failure-injection evidence
- operational rollout risks and mitigations

Decision interview triggers:
- command authority or handshake behavior changes
- protocol mapping breaks requiring migration
- timeout/retry strategy changes affecting availability/safety