11.4 Application Note #4 - Use of the Protective Buffer

The ADIs contain a data latch which may be used as a Protective Buffer enabling the recovery of data under certain error conditions. The Protective Buffer is always loaded with the data transferred during an operation.

If an error occurs during a Write operation, recovery is by re-attempting the Write Operation, preceded by resetting the NTA if necessary. For diagnostic purposes the Protective Buffer may be read after the Write operation to determine the data received by the Slave. This is accomplished using an MS=5 operation, available in Advanced Mode (see Section 4.7).

If an error occurs during a Read operation, and this operation results in the destruction on the data at the original location (as in a Read-And-Clear device) then the Data can be recovered by reading the Protective Buffer. This is not accessible as described in the FASTBUS Specification Appendix J.5.2. Instead, the Protective Buffer may be read using an MS=5 Operation, available in the Advanced Mode. All operations to FIFO-like registers are as requested by any sequence of Single, Block or Pipelined Operations.

If protective buffer operation conforming to the FASTBUS specification is required, external logic may be added which uses OUST, ORD, OMS and IAC to modify single word reads from a FIFO or Read-and-Clear device into reads of an external buffer.

When operating in Advanced Mode (IADV=1), the Protective Buffer may be used to perform benign diagnostic checks of the Slave by writing to the Buffer, then reading the contents. These operations will not affect other parts of the Slave.