What Explicit Messaging Is
Explicit messaging is EtherNet/IP Class 3 communication over TCP (port 44818). It provides on-demand, request/response data exchange — the client sends a request and waits for a reply.
Most QuickServer and FieldServer gateway integrations use explicit messaging because it is simpler to configure and does not require PLC scanner setup.
Explicit vs Implicit
| Aspect | Explicit (Class 3) | Implicit (Class 1) |
|---|---|---|
| Transport | TCP | UDP multicast |
| Timing | On-demand (request/reply) | Cyclic at fixed interval |
| PLC scanner required | No | Yes |
| Setup complexity | Lower | Higher |
| Use case | Configuration, diagnostics, polling | High-speed I/O exchange |
How It Works
- Client opens a TCP connection to port 44818
- Client sends a CIP service request (e.g., Get Attribute Single)
- Device processes the request and returns a response
- Connection can be reused for subsequent requests
When to Use Explicit Messaging
- Gateway reading/writing PLC data tables (DATA_TABLE_READ/WRITE)
- Diagnostic queries and parameter reads
- Integrations where poll rate requirements are moderate (>100 ms)
- When PLC scanner configuration is not available or not desired