TNETX3100 datasheet(14/80 Pages) TI1

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TNETX3100

ThunderSWITCH™ 10-PORT 10-/100-MBIT/S ETHERNET™ SWITCH

SPWS031D – JUNE 1997 – REVISED OCTOBER 1997

POST OFFICE BOX 655303

• DALLAS, TEXAS 75265

flow control

The TNETX3100 incorporates a collision-based flow-control mechanism; this mechanism is activated by setting

the FLOW bit (bit 5) in the RAM size register to a 1. Flow control affects only the ports that are in half duplex.

The TNETX3100 detects that it is becoming congested by monitoring the size of the free-buffer queue. When

there are fewer than 256 buffers on the free queue, the TNETX3100 prevents frames from entering the device

by forcing a collision with all frames on all ports (except any frames that the TNETX3100 is transmitting). When

the flow control is active, the FLOW LED is illuminated.

When the TNETX3100 has transmitted some of the backlog from the DRAM, and the free-buffer queue

increases to 256 or more, forced collisions cease, and frames can again enter the device.

This simplified approach maximizes the use of the DRAM for the mix of port activity. The 256-buffer threshold

ensures that all ports can receive and complete a maximum-length frame when there are 256 free buffers.

The purpose of flow control is to reduce the risk of data loss if a long burst of activity causes the TNETX3100

to backlog frames until the DRAM is full. Flow control has no mechanism to allow the TNETX3100 to operate

on a sustained basis when the activity level exceeds the device internal bandwidth. Transmit queue length

limiting is disabled if FLOW = 1.

giant (long) frames

The TNETX3100 can handle frames up to 1531 bytes to support IEEE Std 802.10. All frames longer than 1535

bytes are truncated by the TNETX3100. Frames greater than 1535 bytes received in cut-through mode are

passed. Frames greater than 1535 bytes received in store-and-forward mode are discarded and not

transmitted. The LONG option bit governs how the statistics for long frames are recorded. The byte count for

long frames with good CRCs is recorded in the RX + TX-frames 1024–1518 statistic, which effectively becomes

RX + TX-frames 1024–1531 when LONG is set. Long frames received with bad CRCs are recorded as jabbers.

short frames

All frames less than 64 bytes received into any port are filtered by the TNETX3100 within the RX FIFOs. They

do not appear on the DRAM bus.

RX filtering of no-cut-through frames

The TNETX3100 filters RX frames that are in error (CRC, alignment, jabber, etc.) when operating in the

store-and-forward mode. A frame that might be cut-through can be non-cut-through if its destination is busy.

If it contains any error, it is filtered. The error is recorded in the relevant statistic counter, with all used buffers

being recovered and returned to the free queue.

TNETX3100 数据表(PDF) 14 Page - Texas Instruments

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