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4-56
The channel STOP TRANSMIT command disables the transmission of
frames on the transmit channel. If this command is received by the HDLC
controller during frame transmission, transmission of that frame is aborted.
No new BD is accessed, and no new frames are transmitted for this channel.
The transmitter will transmit an abort sequence (if the command was given
during frame transmission) and then begin to transmit flags or idles as
indicated by the HDLC mode register.
This command is useful for performing frame retransmission. The M68000
core may issue the STOP TRANSMIT command, reorganize the transmit
BD table, and issue the RESTART TRANSMIT command. The STOP TRANS-
MIT command may also be used in the X.25 protocol to send a reject frame
or a link reset command.
The STOP TRANSMIT command must be issued before the SCC mode
register is used to disable the transmitter if the transmitter is to be re-
enabled at a later time.
RESTART TRANSMIT Command
The RESTART TRANSMIT command re-enables the transmission of char-
acters on the transmit channel. This command is expected by the HDLC
controller after a STOP TRANSMIT command, after a STOP TRANSMIT
command and disabling the channel in its SCC mode register, or after
transmitter error (underrun or CTS lost). The HDLC controller will resume
transmission from the current transmitter BD (TBD#) in the channel's trans-
mit BD table.
If the transmitter is being re-enabled, the RESTART TRANSMIT command
must be used and should be followed by the enabling of the transmitter
in the
sec
mode register.
ENTER HUNT MODE Command
After a hardware or software reset and the enabling of the channel by its
SCC mode register, the channel is in the receive enable mode and will use
the first BD in the table.
The ENTER HUNT MODE command is generally used to force the HDLC
receiver to abort reception of the current frame and enter the hunt mode.
In the hunt mode, the HDLC controller continually scans the input data
stream for the flag sequence. After receiving the command, the current
receive buffer is closed, and the CRC is reset. Further frame reception will
use the next BD.
MC68302 USER'S MANUAL
MOTOROLA
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