Motorola MC68824 User Manual page 90

• If the TBC is not in predefined response mode, then the TBC must wait for the command
RESPONSE READY from the host before sending out the response. Before issuing the RE-
SPONSE READY command, the host must prepare a response and update the response
pointer field located in the initialization table (offset 88) if necessary. If this command is issued
within two slot times, then the TBC will transmit the response prepared by the host.
However, if the host issues the command later than two slot times but before the TBC receives
the next RWR retry frame, the TBC will still transmit the previously prepared response as a
response to the current retry of the RWR frame. The TXRDF bit (transmitted response data
frame) in !nterrupt status word 0 is also set and an interrupt is generated if enabled.
Note that if the host still owes a previous response to the TBC, the TBC will neither update the
RWR pointer field nor change the status of RXRWR in interrupt'status word O.
The following four paragraphs describe the relationship between two of the pointers residing in
II
the initialization table at offset 84 and 88 which are used to implement the RWR mechanism.
Case 1:
The TBC is in predefined response mode and the DA of the response frame must be identical
to the SA of the received RWR frame.
• The response destination address pointer (offset 84) should have been previously set by
the host to the same value as the response pointer (offset 88).
• The TBC will set DA equal to SA of received frame in the FD pointed to by the response
destination address pointer (offset 84).
Case 2:
The TBC is in predefined response mode and the DA of the response frame must be different
from the SA of the received RWR frame.
• The response destination address pointer (offset 84) should have been previously set by
the host to point to a dummy FD.
• The response pointer (offset 88) should have been previously set by the host to point
to the response frame's FD where the host has set DA as required.
Case 3:
The TBC is in non-predefined mode and the DA of the response frame must be identical to
the SA of the received RWR frame.
• The response destination address pointer (offset 84) should have been previously set by
the host to the same value as the response pointer (offset 88).
• The response pointer (offset 88) must be a constant, i.e., always point to the same FD.
• The TBC will set DA equal to SA of received frame in the FD pointed to by the response
destination address pointer (offset 84).
Case 4:
The TBC is in non-predefined response mode and the DA of the response frame must be
different from the SA of the received RWR frame.
• The response destination address pointer (offset 84) should be set by the host to point
to a dummy FD.
• The response pointer (offset 88) is set by the host to point to the response frame's FD
where the host has set DA as required upon getting the RXRWR interrupt (this bit could
also be polled).
NOTE
Cases 2 and 4 describe operations which are inappropriate for IEEE 802.4.
MC68824 USER'S MANUAL
MOTOROLA