ACD81024 データシートの表示（PDF） - Unspecified

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ACD81024

24 Ports Single Chip 10Base-T Ethernet Switch Designed For Desktop Environment

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verify if the received frame has a valid FCS value.

Wrong FCS value is an indication of a fragmented

frame or a frame with frame bit error. The method of

calculating the CRC value of the frame data is by

using following polynomial

G(x) = x32 + x26 + x23 + x22 + x16 + x12 + x11

+ x10 + x8 + x7 + x5 + x4 + x2 + x + 1

destination port(s).

Inter Frame Spacing Handling

If the silent time between frames coming from a DTE

is less than the minimum IFS requirement of 96BT,

ACD81024 will send a jam pattern to the correspond-

ing source DTE just like a congestion has occurred.

as a divider to divide the bit sequence of the incoming

frame, beginning with the first bit of destination

address field, up to the end of the data field. The

result of the calculation, which is the residue after the

polynomial division, is the value of frame check

sequence. The value should be equal to the FCS field

appended at the end of the frame. If the value does

not match with the FCS field of the frame, the Frame

Bit Error LED of the port will be turned on once.

Short Frame Handling

If a frame is shorter than 140 bits after the SFD fields,

it will not be forwarded to the destination port(s).

Frames longer than 140 bits will be forwarded to the

destination port(s) no matter if it passes the FCS

checking or not.

Collision Detection and Handling

While the MAC logic of a destination port is transmit-

ting data to the destination DTE, if there is any data

coming from the destination DTE (or say if the carrier

sense signal is raised up by the PHY module of the

destination port), collision is detected. Collision will

cause the MAC logic of both the destination port and

the source port to enter a collision handling state. On

the destination port side, current transmit process is

aborted and a Jam pattern is appended to the end of

the broken frame. On the source port side, a Jam

pattern is sent to the source DTE, beginning with 56

bit preamble and a valid Start of Frame Delimiter. The

Jam pattern will cause the source DTE to stop

transmitting and retransmit the frame after proper

back-off time.

Jabber Lockup Protection

Congestion Detection and Handling

If an incoming frame is detected to be longer than

50,000 bits, it will be broken by the Jabber Protection

circuitry inside the MAC logic. The frame bits later

than 50,000 bit will be forwarded to nowhere. A

Jabber error is reported through the Jabber error LED

of the port. After proper inter-frame-spacing time, the

MAC logic of the port will be ready to receive next

frame.

Transmit Time Watchdog Control

The watchdog circuitry inside each MAC logic keeps

on monitoring the number of bits transmitted by the

destination MAC logic. If the length of the transmitted

frame is longer than 38,000 bit, the watchdog circuitry

will break the frame and append a Jam pattern to the

end. This function can be disabled by pulling the

WCHE signal low.

If a destination port headed by an incoming frame is

in the process of receiving or transmitting or deferring

on inter-frame-spacing time, the Fabric Control logic

signals the source port MAC logic that the destination

port is not ready. The source port MAC logic then

enters a Congestion Control state. In Congestion

Control state, the MAC logic of the source port first

sends a Jam pattern to the source DTE to force a

collision, and then, after proper inter-frame-spacing

time, sends a continuous Back Pressure signal to the

source DTE. If collision is detected on the Back

Pressure signal, the MAC logic will end the Back

Pressure pattern with a Jam pattern, wait for mini-

mum IFS time, and send the Back Pressure signal

again. When the destination port(s) is available, the

Back Pressure signal is ended with a Jam pattern.

Expansion Port Handling

Preamble Bit Handling

The preamble bit in the header of each frame will be

used to synchronize the MAC logic circuitry with the

incoming bit stream. The minimum length of the

preamble is 30 bits and there is no limit on the

maximum length of preamble. The preamble will be

regenerated by MAC logic of ACD81024 with exact

length of 56 bits as the frame is transmitted out of the

Port 24 of ACD81024 is designed to be the expansion

port (or say the dumping port). It can be used for

uplink connection or for ordinary desktop connection.

That is, port 24 can be connected with a repeater

hub, a work-group switch, a router, or any type of

interconnection device compliance with IEEE 802.3

10Base-T standard. ACD81024 will direct following

frames to the expansion port:

ACD Confidential. Do Not Reproduce. Use under Non-Disclosure agreement only.

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