Siemens SIMATIC TIWAY 1 - Manual (Page 91)

HOST COMMAND SET REFERENCE
UNILINK HOST ADAPTER 5–6
USER MANUAL
5.2 ERROR RESPONSE
An ERROR RESPONSE, rather than the normal command response, will be
returned when the host request is invalid.
Response: 00 dddd (aa)
Parameters:
dddd Error codes. All error codes and corrective actions are given in
Chapter 6.
aa The secondary address associated with the error code. This
parameter is only included if applicable.