West Control Solutions N8100 - Manual (Page 36)

3.4 ERROR RESPONSE
The circumstances under which a message received from the master device is
ignored are:
Parity error detected
Syntax error detected
Timeout elapsed
Receipt of a Type 4 message without a preceding Type 3 command
message.
Negative acknowledgements will be returned if, in spite of the received message
being notionally correct, the Controller cannot supply the requested information
or perform the requested operation. The {DATA} element of a negative
acknowledgement will be indeterminate.
3-10 O054-3
59125
Figure 3-1 Controller Status Byte