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CHAPTER 5 SAR FUNCTION
222
(9) Closing the ABR channel
Transmit and receive VCs are always used as a pair when the ABR service is used. When the host
terminates the use of ABR channels, it must close the corresponding transmit and receive VCs in a
specified sequence, described below:
<1>
Reset the RE bit in the receive VC table.
<2>
Close the transmit channel.
<3>
Close the receive channel.
Resetting the RE bit in the receive VC table
To close an ABR channel, it is necessary to first reset the RE bit in the receive VC table to 0. The RE
bit is used to specify whether to perform rate calculation/turn-around processing when an BRM/FRM
cell is received. If the RE bit is set to 0, the
μ
PD98405 does not access the transmit VC indicated by
the Tx VC TABLE POINTER in the receive VC table. When an ABR channel is closed, the transmit
VC is closed before the receive VC. When an RM cell is received at a receive VC, if a transmit VC
that is accessed has already been closed, a malfunction will occur in the
μ
PD98405.
Closing the transmit channel
To close an ABR channel, it is necessary to close the transmit VC before the receive VC. Resetting
the RE bit disables rate calculation for subsequent RM cell reception, but performing rate calculation
for an RM cell received just before the RE bit is reset may cause the
μ
PD98405 to access the receive
VC indicated by the Rx VC TABLE POINTER in the transmit VC table. If the receive VC has been
closed, however, a malfunction will occur in the
μ
PD98405.
The ABR transmit channel is closed in the same way as the VBR VC transmit channel. See
Section
5.4.3
for details. A Deactivate_Channel command can also be used to force the transmit channel into
the idle state.
Closing the receive channel
The receive channel must be closed after the transmit channel is closed.
The ABR receive channel is closed in the same way as the VBR VC receive channel. See
Section
5.5.3
for details. Basically, the ABR receive channel is closed by first disabling the receive lookup
table entry and issuing a NOP command twice, followed by the Deactivate_Channel and
Close_Channel commands.