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TNETX4090
ThunderSWITCH II
9-PORT 100-/1000-MBIT/S ETHERNET
SWITCH
SPWS044E – DECEMBER 1997 – REVISED AUGUST 1999
40
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
Table 12. Directed Format Receive Pretag Bit Definitions
BIT NAME FUNCTION
31 1
One. Indicates directed format. The frame is routed to port(s) specified in portvector that are enabled (disabled in
portxcontrol = 0), regardless of whether the destination address is unicast or multicast (i.e., the destination address
is not examined).
The internal frame-routing algorithm is bypassed and has no effect on frame routing. The TxBlockPorts,
RxUniBlockPorts, and RxMultiBlockPorts registers also have no effect on frame reception or transmission.
Portvector is not examined to see if the source port has been specified as a destination port, so it is possible to send
a frame back out of port 8. If portvector is 0, the frame is discarded.
30–10 reserved Reserved. Bits 30–10 are ignored.
9–0 portvector
Destination port vector. A bit for each port on this device. A 1 in position n indicates the frame is destined for port n
on this device.
ring-cascade topology
The ringports register allows port 8 to be used to cascade multiple TNETX4090 devices using a full-duplex
ring-cascade topology (see Figure 7).
TNETX4090
0123 4567
Port 8
RXD
FLOW
COL
TXD
TNETX4090
0123 4567
Port 8
RXD
FLOW
COL
TXD
TNETX4090
0123 4567
Port 8
RXD
FLOW
COL
TXD
Figure 7. Ring-Cascade Topology
This configuration provides a way to construct a higher port-density system using three or more TNETX4090s.
Setting the portvector bit corresponding to this port in RingPorts modifies the normal behavior of the switch in
several ways to enable the ring topology.
Frames received on this port can be retransmitted out through the same port. This allows frames destined
for a nonring port on another switch in the ring to pass through this switch.
Frames transmitted from a ring port have an out-of-band pretag in the clock cycle before Mxx_TXEN is
asserted. The contents of the pretag are determined as follows:
– If the transmitted frame was received on a nonring port, the pretag consists of the ringid field from
RingPorts, replicated on Mxx_RXD3–Mxx_RXD0 and Mxx_RXD7–Mxx_RXD4.
– If the transmitted frame was received on a ring port, the pretag is the same as the received pretag.