Motorola MC68824 User Manual page 89

II
this parameter. Note that an underrun also counts as a retry. If a valid response arrives, the TBC
goes through the following steps:
• Loads the frame into the appropriate priority receive queue.
• Links the transmitted RWR frame to the response frame by placing the pointer to the re-
sponse's FD in the transmitted RWR frame's immediate response FD pointer in the transmitted
frame's FD.
• Gives confirmation in the RWR frame's FD.
• Sets TXRWR bit (transmitted request with response) in interrupt status word 0 and interrupts
the host if enabled by the interrupt status mask.
If no response is received after the RWR retry limit is reached, the TBC gives negative confirmation
on this frame in the RWR's FD's confirmation/indication word, updates the interrupt status word
to indicate TXRWR, and interrupts the host if enabled.
A valid response is defined as an LLC data frame with a MAC action field set to response which
is received by the requester within the RWR max retry limit and before any control frames or
other data frames are received.
If the TBC hears an unexpected frame while waiting for a response, the TBC will:
• Store that frame into the appropriate queue.
• Give a negative confirmation as well as set the unexpected frame received during RWR bit
in the RWR's FD.
• Set the TXRWR bit in interrupt status word 0 as well as the unexpected frame 6 bit in interrupt
status word 1.
4.5 RECEPTION OF RWR FRAMES AND TRANSMISSION OF RESPONSE
Upon receiving an RWR frame addressed to this station, the TBC will store it into the appropriate
receive queue and perform the following steps:
• The source address of the received frame is stored into the destination address field of the
FD pointed to by the response destination address pointer. This pointer resides in the ini-
tialization table at offset 84 and must have been previously initialized by the host to point to
a valid FD.
• If the frame is not a retry, then the TBC loads the pointer to the received RWR frame's FD
into the RWR pointer field located in the initialization table at offset 8C. The received RWR
frame bit in interrupt status word
o
will then be set and an interrupt will be generated if
enabled.
A retry is defined as an RWR frame addressed to this TBC that arrives immediately after another
RWR frame which was addressed to it. If another valid frame such as a protocol frame, a non-
RWR data frame, or a data frame not addressed to this station arrives in between two RWR frames,
then the second RWR frame is considered to be a new frame and not a retry.
The response is sent by the TBC according to the steps defined below:
• Ifthe TBC is in predefined response mode, set by the host through the SET MODE 1 command,
then the pointer stored by the host in the response pointer field in the initialization table
(offset 88) is considered valid, and the response is transmitted immediately after the token
bus goes to silence. The TXRDF bit (transmitted response data frame) in interrupt status word
o
is also set and an interrupt is generated if enabled.
MOTOROLA
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MC68824 USER'S MANUAL