Motorola MC68824 User Manual page 55

II
3.4 TX DATA FRAMES
Commands in this category are used to stop/restart transmission of data frames or to start trans-
mission from an empty queue.
3.4.1 STOP Command
The STOP command causes the TBC to suspend transmission of data frames in all queues. This
command does not affect the status of the four transmit queues (see 2.1.15 TX Queue Access
Class Status). If the TBC is holding the token when the STOP command is received, the TBC will
complete transmission of the current frame and then pass the token. If in_ring desired is true,
the TBC will remain in_ring or, if not currently in_ring, will continue to try to enter the ring. The
STOP command is equivalent to clearing the any_send_pending boolean defined in the IEEE 802.4
standard. The coding for the STOP command is EO and its format is shown below.
1
I
1
I
1
o
I
0
I I
0
3.4.2 RESTART Command
This command restarts transmission of data frames by the TBC. RESTART sets the
any_send_pending boolean and differs from the START command in that it only restarts trans-
mission of data frames and does not affect the status of the four transmit queues. The RESTART
command is used after transmission of data frames was suspended by an earlier STOP command.
If the TBC is not in_ring when this command is given, the TBC will enter the logical ring if
outstanding data frames remain to be transmitted. The coding for this command is C6 and its
format is shown below.
1
I
1
I
0
o
I
0
I
1
I
1
I
0
3.4.3 START Command
This command is used by the host processor to start transmission only when a new frame has
been added to an empty transmit queue. Before loading this command into the command register
(CR), the host must initialize the command parameter area located in the initialization table. The
first word of the CPA, CPA VALO (offset 90), must contain the code of the appropriate transmit
queue. These are:
Queue
6
4
2
o
CPA VALO (Hex)
0028
0030
0038
0040
The second and third words of the CPA, CPA VAL 1 and CPA VAL2 respectively, must contain a
pointer to the new head of the transmit queue. This pointer is the address of the first frame
descriptor in the queue, with the most significant word of the pointer in CPA VAL 1.
The TBC, when issued this command, sets its internal any_send_pending status to true. It also
zeroes the appropriate transmit queue status word in the private area. In other words, it sets the
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MC68824 USER'S MANUAL