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.