TNETX3100 datasheet(17/80 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

uplink RX pretagging to support an external routing engine

When port 00 (uplink) is operated in 100-Mbit/s mode, a pretag is placed one cycle prior to MRXDV going high for

the frame; tag data is applied to M00RXD3–M00RXD0. This pretag is not operated on internally by the

TNETX3100 although it is used by an external address-lookup engine. The tag is passed to the DRAM interface,

where it appears on bits 31–28 of the forward pointer; this provides the ability for an upstream device, connected

to the uplink, to pass information through the TNETX3100 to the DRAM bus where an external address-lookup

device (TNETX15VEPGE) operates on that information. The external address-lookup device makes a

forwarding decision based on destination address and VLAN register value (see Figure 3).

The pretag is qualified with the port that received the frame. Only pretags received on port 00 are valid. A pretag

received on the uplink (port 00) is retained for the duration of the frame.

M00RCLK

M00RXD(3–0)

M00RXDV

Tag

Preamble, DA, SA, Data, CRC

Pretag Nibble

Ethernet Frame

Figure 3. Data Format for Uplink RX Frames With Pretag

using TX pretagging and RX posttagging to support an external routing engine

If the TNETX3100 relies on the external routing engine to make switching decisions, no local switching is

performed and port 00 forwards all frames to a routing engine; port 00 also provides a pretag on all forwarded

frames. The pretag provides source-port information to the routing engine. The routing engine then makes

forwarding decisions and transmits a frame and posttag to port 00; the posttag provides destination port

information to the TNETX3100. To allow this, the TAGON bit (bit 4 of RSIZE register 0x00D2) must be a 1.

uplink RX posttagging to support an external routing engine

The external routing engine hardware must provide port 00 with an indication of the destination-port information;

this information is used by the TNETX3100 to route frames to ports 01–09. This indication consists of three

nibbles. When posttagging is enabled, the tag bytes overwrite the value received on the EAM interface (EAM

interface has no effect). When posttagging is turned off (designed for local switching), the posttag is ignored,

and the EAM interface or the internal address match supplies the routing information.

There is no handshake or flow control for the receive uplink path on the TNETX3100. If required, this must be

implemented in upstream devices. No preamble is expected on data received by the uplink at 100 Mbit/s (see

Figure 4). If RX posttagging is used, the receive side of port 00 must be configured for store-and-forward mode.

TNETX3100 数据表(PDF) 17 Page - Texas Instruments

TNETX3100 数据表(HTML) 17 Page - Texas Instruments

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