What an Assembly Object Is
An Assembly Object is a CIP data container used in EtherNet/IP to group I/O points for exchange between devices. Each Assembly has an instance number, a fixed byte size, and a direction.
PLC scanner configurations reference Assembly Objects by instance number to set up I/O connections with EtherNet/IP devices.
Naming Convention
| Name | Direction | Meaning |
|---|---|---|
| DataFromDevice | Device → PLC | Values read from the EtherNet/IP device (inputs) |
| DataToDevice | PLC → Device | Values written to the EtherNet/IP device (outputs) |
[!NOTE] The naming can be confusing — “DataFromDevice” is data coming from the EtherNet/IP device, which appears as inputs in the PLC scanner.
Configuration Requirements
Each Assembly Object mapping requires:
- Instance number — identifies which Assembly to access
- Size — byte count (must match between device and PLC scanner)
- Access mode — DATA_TABLE_READ or DATA_TABLE_WRITE
Common Issues
- Size mismatch — PLC scanner and device disagree on Assembly byte count → connection failure
- Wrong instance number — points map to the wrong Assembly → data offset errors
- Missing EDS file — PLC can’t auto-populate Assembly details → manual configuration required