The Rule
When connecting to an M580 PLC via Ethernet, use the service port — not the two network ports. The two NOC (Network Option Card) ports run RSTP (Rapid Spanning Tree Protocol), which may not conform to your sites network security policy.
Why This Matters
RSTP is a Layer 2 protocol that manages network redundancy by detecting loops and disabling redundant paths. While useful in properly designed network architectures, it can cause issues when:
- The site network policy prohibits devices from participating in spanning tree — introducing an RSTP-enabled device can disrupt the existing network topology.
- The NOC ports are connected to managed switches that have their own STP/RSTP configuration — conflicting spanning tree instances can cause link flapping or ports being blocked unexpectedly.
- Security policies require all devices on the network to be explicitly configured and authorized for Layer 2 protocols.
Service Port
The service port is a simple Ethernet connection with no spanning tree involvement. It provides direct point-to-point communication for:
- Programming and configuration via Control Expert.
- Firmware updates.
- Diagnostics and troubleshooting.
When Using the NOC Ports
If the application requires the NOC ports (for Modbus TCP communication to SCADA, I/O scanning, or redundant network paths), coordinate with the site network administrator to ensure RSTP compatibility. Document the spanning tree configuration on the NOC ports and verify it aligns with the switch infrastructure.