What Implicit Messaging Is
Implicit messaging is EtherNet/IP Class 1 communication over UDP (port 2222). It provides cyclic I/O exchange at a fixed interval — the PLC scanner and device exchange Assembly Object data continuously without per-request overhead.
Implicit messaging is used when high-speed, deterministic I/O exchange is required.
Implicit vs Explicit
| Aspect | Implicit (Class 1) | Explicit (Class 3) |
|---|---|---|
| Transport | UDP multicast | TCP |
| Timing | Fixed cyclic interval | On-demand |
| PLC scanner required | Yes | No |
| Latency | Low, deterministic | Variable |
| Setup complexity | Higher | Lower |
Requirements
- PLC must be configured as an I/O scanner
- EDS file should be imported for auto-configuration
- Assembly Object instance numbers and sizes must match between PLC and device
- Requested Packet Interval (RPI) must be configured (typically 10–500 ms)
[!CAUTION] Implicit messaging requires tighter setup than explicit. Mismatched Assembly sizes or RPI values cause immediate connection faults — not silent failures.
When to Use Implicit Messaging
- High-speed process control (update rates < 100 ms)
- Applications requiring deterministic timing
- PLC-to-PLC I/O exchange
- When the PLC is already configured as a scanner
Most building automation gateway integrations use explicit messaging instead, due to simpler configuration.