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TNETX4090
ThunderSWITCH II
9-PORT 100-/1000-MBIT/S ETHERNET
SWITCH
SPWS044E – DECEMBER 1997 – REVISED AUGUST 1999
41
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
ring-cascade topology (continued)
Frames received on a ring port must have an out-of-band pretag in the clock cycle before Mxx_RXDV is
asserted. The contents of the pretag are examined, and based on the results, are either forwarded normally,
or immediately discarded within the MAC. If discarded, the frame does not affect any of the statistics or
address-lookup database. Frames are forwarded normally, unless:
– The pretag is 0.
– The two IDs within the pretag are not the same.
– The ID within the received pretag is the same as ringid. This identifies a frame that originated at this
device, and that has passed completely around the ring.
The pretag format is shown in Figure 8.
Ring ID Preamble
M08_TXD3–
M08_TXD0/
M08_RXD3–
M08_RXD0
M08_TXEN
M08_RXDV
M08_GTCLK
Ring ID Preamble
M08_TXD7–
M08_TXD4/
M08_RXD7–
M08_RXD4
Figure 8. Ring-Topology Pretag Timing
The devices in the ring are connected as shown in Table 13.
Table 13. Ring-Topology Connectivity
SWITCH TERMINAL
n n + 1
M08_TXD7 → M08_RXD7
M08_TXD6 → M08_RXD6
M08_TXD5 → M08_RXD5
M08_TXD4 → M08_RXD4
M08_TXD3 → M08_RXD3
M08_TXD2 → M08_RXD2
M08_TXD1 → M08_RXD1
M08_TXD0 → M08_RXD0
M08_TXEN → M08_RXDV
M08_TXER → M08_RXER
M08_COL ← FLOW
NOTE: When a port is configured for the ring
topology, IEEE Std 802.3x flow control
should be disabled. Hardware-based
flow control is supported using the
FLOW and Mxx_COL terminals;
however, this must be enabled by
setting the flow bit in SysControl.