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Chapter 4
Functional Overview
Return Data Buffer
Note
Refer to the FlexMotion Software Reference Manual for more information on RDB
buffering and readback.
Error Stack
7344 Hardware User Manual
Data or status requested by the host with a command is buffered in the
return data FIFO buffer (RDB). The RDB is 26 words deep and is large
enough to hold the biggest return data packet or many smaller return data
packets.
When data exists in the RDB, the Ready-to-Send bit in the CSR is set. The
host can then perform a bus read to get the return data from the controller.
You can use the RDB in two ways—as a temporary buffer holding a single
data return packet, or as a small FIFO buffer. Typically, once the requested
data is available in the RDB, it is read back by the host. It is possible
however, to request a number of pieces of data and leave them in the buffer
(RDB) for retrieval at a later time. The FlexMotion software supports both
ways of using the RDB.
If the RDB fills up and there is no place to put requested return data, the
7344 controller generates an error and sets the error message bit in the CSR.
To handle run-time or modal errors, the 7344 controller maintains an error
stack. If an error is detected during command execution, the Command ID,
Resource ID, and Error Code are placed on a last-in-first-out (LIFO) stack
and the error message bit in the CSR is set.
Since commands can be buffered in the command FIFO, errors detected at
command execution time are delayed and are not reported as packet errors.
Errors are pushed on the error stack to be read back by the host at a later
time. This error-handling structure also correctly detects errors generated
by incorrectly sequenced programs and other modal errors.
The error stack is organized as a LIFO buffer so that old errors can be
ignored and the most recent error is readily available to be read with the
Read Error Message function, just like any other status or data value. Older
errors can then be read back in the inverse order to which they were
generated, and reviewed for their relevance.
4-12
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