Motorola MC68824 User Manual page 52

IRND - In_Ring Desired
1 In_ring desired
a
In_ring not desired
This bit determines the TBC's steady state condition when it is not OFFLINE and has no
queued transmission requests. The initial value of this mode is set in the initialization table
(offset 7A). This command should only be used at initialization time or for testing purposes.
The preferred way to modify the in_ring desired flag is by using the SET/CLEAR IN_RING
DESIRED command (see 3.3.4 SET IN_RING DESIRED Command).
SRLB - Source Routing Limited Broadcast
1 In source routing limited broadcast mode
a
Not in source routing limited broadcast mode
This bit has meaning only when recognize source routing (RSR) is enabled. Source routing
frames are of two types: broadcast and non-broadcast. A broadcast frame may be limited
broadcast or unlimited broadcast. Unlimited broadcast frames are accepted by all bridges
while limited broadcast frames are accepted only by bridges which are in limited broadcast
mode. This option can be used to reduce the amount of traffic generated by broadcast frames
or to specify default routing paths. More details may be found in APPENDIX C BRIDGING.
BRG - Bridge Delay Mode
1 Enable bridge delay mode
a
Disable bridge delay mode
This mode is used when the TBC transmits 'aliased' data frames; i.e., SA not equal to TS.
This occurs in bridges, which can transmit frames originating on other segments. This bit
should only be set in broadband networks where the TBC is located far enough from the
head-end remodulator so that the entire echo of a minimal length frame transmitted by the
TBC could be heard by it after it has completed transmission. This mode solves a problem
caused by hearing a data frame with SA not equal to TS, which could cause the TBC to
incorrectly believe that another station is transmitting. In this mode, the TBC will wait ap-
proximately one slot time before transmitting a solicit successor or token frame after trans-
mitting a data frame, or before transmitting a non-retry request with response data frame.
RSR - Recognize Source Routing
1 Recognize source routing
a
Do not recognize source routing
This mode allows the TBC to act as a part of a source routing MAC bridge between inter-
connected networks. The meaning of the least significant bit of the source address of a frame
having a value of one is not defined in the IEEE 802 standards. The source routing method
uses this bit in data frames to indicate the presence of a routing information field in the frame
which describes the segment routing of the frame. This is a non-address based routing
scheme, meaning that the destination address does not itself contain information on where
the destination station is located. When the least significant bit of the source address bit is
zero, the frame is treated as an ordinary frame. If this bit is a one in a control frame, the
frame is considered invalid and is treated as a noise burst. If this bit is a one in a data frame
and if RSR mode is disabled, the TBC treats the frame as a normal frame (i.e., if the destination
address matches the criteria set by the host, the frame is accepted and the routing information
field is treated as normal data). If the RSR mode is enabled, the frame is accepted if the
destination address matches the specified criteria or if the routing information field specifies
that the frame is to be routed to another segment via the TBC. The routing information field
is copied to the first data buffer like normal data. The length of the buffers in the free pool
MC68824 USER'S MANUAL
MOTOROLA
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