# Type 3: Communication error

Communication errors happen when messages cannot be delivered or timeouts occur. The recipient of the message is either unknown or busy.

These error messages contain the following additional diagnostic data:

Data format | Element | Contains
UNS16 | Medium | Communication medium
UNS16 | Typ | PDU code or function block type
UNS32 | Partner | Sender

Practical examples:

- Invalid PDU code received
- Channel block marked as 'not processed'
- Incorrect content of a PDU block