How do I detect RT errors from a BC application?

Check the 32-bit status field in the BC message buffer (API_BC_MBUF structure) after the message has completed. This is not the 1553 status word sent by the RT; it is in the mess_status1 or mess_status2 (for RTRT messages) fields. This 32-bit message status field contains many status/error flag bits, but the key one for detecting RT errors is the BT1553_INT_NO_RESP (0x04000000) bit. If this is set, then the RT did not respond with status. If the RT detects any error in a message, it does not respond.