AM79C960KCW datasheet(55/127 Pages) AMD

55 / 127 page

P R E L I M I N A R Y

AMD

1-397

Am79C960

It is the responsibility of upper layer software to correctly

define the actual length field contained in the message

to correspond to the total number of LLC Data bytes en-

capsulated in the packet (length field as defined in the

IEEE 802.3 standard). The length value contained in the

message is not used by the PCnet-ISA controller to

compute the actual number of pad bytes to be inserted.

The PCnet-ISA controller will append pad bytes de-

pendent on the actual number of bits transmitted onto

the network. Once the last data byte of the frame has

completed prior to appending the FCS, the PCnet-ISA

controller will check to ensure that 544 bits have been

transmitted. If not, pad bytes are added to extend the

frame size to this value, and the FCS is then added.

The 544 bit count is derived from the following:

Minimum frame size (excluding preamble,

including FCS)

64 bytes

512 bits

Preamble/SFD size

8 bytes

64 bits

FCS size

4 bytes

32 bits

To be classed as a minimum-size frame at the receiver,

the transmitted frame must contain:

Preamble

(Min Frame Size + FCS) bits

At the point that FCS is to be appended, the transmitted

frame should contain:

Preamble

(Min Frame Size - FCS) bits

(512

32) bits

A minimum-length transmit frame from the PCnet-ISA

controller will, therefore, be 576 bits after the FCS is

appended.

Transmit FCS Generation

Automatic generation and transmission of FCS for a

transmit frame depends on the value of DXMTFCS bit in

CSR15. When DXMTFCS = 0 the transmitter will gener-

ate and append the FCS to the transmitted frame. If the

automatic padding feature is invoked (APAD_XMT is

SET in CSR4), the FCS will be appended by the

PCnet-ISA controller regardless of the state of

DXMTFCS. Note that the calculated FCS is transmitted

most-significant bit first. The default value of DXMTFCS

is 0 after RESET.

Transmit Exception Conditions

Exception conditions for frame transmission fall into two

distinct categories; those which are the result of normal

network operation, and those which occur due to abnor-

mal network and/or host related events.

Normal events which may occur and which are handled

autonomously by the PCnet-ISA controller are basically

collisions within the slot time with automatic retry. The

PCnet-ISA controller will ensure that collisions which

occur within 512 bit times from the start of transmission

(including preamble) will be automatically retried with no

host intervention. The transmit FIFO ensures this by

guaranteeing that data contained within the FIFO will

not be overwritten until at least 64 bytes (512 bits) of

data have been successfully transmitted onto the net-

work.

If 16 total attempts (initial attempt plus 15 retries) fail, the

PCnet-ISA controller sets the RTRY bit in the current

transmit TDTE in host memory (TMD2), gives up owner-

ship (sets the OWN bit to zero) for this packet, and

processes the next packet in the transmit ring for trans-

mission.

Abnormal network conditions include:

s Loss of carrier

s Late collision

s SQE Test Error (does not apply to 10BASE-T port)

These should not occur on a correctly configured 802.3

network, and will be reported if they do.

When an error occurs in the middle of a multi-buffer

frame transmission, the error status will be written in the

current descriptor. The OWN bit(s) in the subsequent

descriptor(s) will be reset until the STP (the next frame)

is found.

Loss of Carrier

A loss of carrier condition will be reported if the

PCnet-ISA controller cannot observe receive activity

while it is transmitting on the AUI port. After the

PCnet-ISA controller initiates a transmission, it will

expect to see data “looped back” on the DI

± pair. This

will internally generate a “carrier sense,” indicating that

the integrity of the data path to and from the MAU is in-

tact, and that the MAU is operating correctly. This

“carrier sense” signal must be asserted before the end

of the transmission. If “carrier sense” does not become

active in response to the data transmission, or becomes

inactive before the end of transmission, the loss of car-

rier (LCAR) error bit will be set in TMD2 after the frame

has been transmitted. The frame will not be re-tried on

the basis of an LCAR error. In 10BASE-T mode LCAR

will indicate that Jabber or Link Fail state has occurred.

Late Collision

A late collision will be reported if a collision condition oc-

curs after one slot time (512 bit times) after the transmit

process was initiated (first bit of preamble commenced).

The PCnet-ISA controller will abandon the transmit

process for the particular frame, set Late Collision

(LCOL) in the associated TMD3, and process the next

transmit frame in the ring. Frames experiencing a late

collision will not be re-tried. Recovery from this condition

must be performed by upper-layer software.

SQE Test Error

During the inter packet gap time following the comple-

tion of a transmitted message, the AUI CI

± pair is

asserted by some transceivers as a self-test. The inte-

gral Manchester Encoder/Decoder will expect the SQE

Test Message (nominal 10 MHz sequence) to be re-

turned via the CI

± pair within a 40 network bit time period

after DI

± pair goes inactive. If the CI± inputs are not

asserted within the 40 network bit time period following

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