Mac Packet Transmission - Motorola MC68838 User Manual

Media access controller
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If this protocol sequence is violated, the MAC treats either a FILLER or a TX_START
immediately appearing after a TX_DATA as an abort request and does not send a
closing T-symbol. TX_DATA and TX_END are ignored if they are received before a
TX_START. The MAC also considers a TX_END transfer occurring before the MAC has
finished sending out the FC, DA, and SA fields as an abort request, and sends a
TABORT is asserted (high) for one BYTCLK cycle when any of the following events
1. Every cycle that the MAC detects XMIT_PATH parity error on TPATHx
(if TX_PARITY_ON is one), or
2. When an FSI protocol error in TXCTLx is detected, or
3. When the FSI aborts transmission of the data frame, or
4. When the MAC is forced to stop transmission of the frame because of a claiming or
beaconing process, a MAC_RESET, or certain modifications to control register B.
The MAC chip must not assert TXRDY until the previous packet can no longer be
aborted. Hence, the assertion of TABORT after the first assertion of TXRDY means that
the MAC is aborting the new packet, never the last packet. The assertion of TABORT
before or at the same time as TXRDY indicates that the MAC is aborting the Iast packet, a
FILLER transfer, or the TX_START of the new packet.


Every packet to be transmitted by the MAC has an associated control field of three bytes
called the packet request header. The MAC uses the packet request header to determine
how and when this packet should be sent. The packet request header must be passed to
the MAC before the MAC can send the packet data. When the last byte of data in the
packet has been passed to the MAC (and before the frame status indicators are sent),
the MAC requests the next packet request header by asserting TXRDY. ln addition to the
packet request header, the FSI must also pass the packet to the MAC. The packet
passed to the MAC contains the following fields:
1. FC field
2. DA field
3. SA field
4. INFO field
5. FCS field (unless the MAC has been requested to generate CRC).


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