AM79C970AVCW datasheet(80/219 Pages) AMD

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AMD

P R E L I M I N A R Y

Am79C970A

Abnormal network conditions include:

s FCS errors

s Late Collision

Host related receive exception conditions include MISS,

BUFF, and OFLO. These are described in the section

“Buffer Management Unit”.

Loopback Operation

Loopback is a mode of operation intended for system di-

agnostics. In this mode, the transmitter and receiver are

both operating at the same time so that the controller re-

ceives its own transmissions. The controller provides

two basic types of loopback. In internal loopback mode,

the transmitted data is looped back to the receiver inside

the controller without actually transmitting any data to

the external network. The receiver will move the re-

ceived data to the next receive buffer, where it can be

examined by software. Alternatively, in external loop-

back mode, data can be transmitted to and received

from the external network.

Loopback operation is enabled by setting LOOP

(CSR15, bit 2) to ONE. The mode of loopback operation

is dependent on the active network port and on the set-

tings of the control bits INTL (CSR15, bit 6), MENDECL

(CSR15, bit 10) and TMAULOOP (BCR2, bit 14). The

setting of the full-duplex control bits in BCR9 has no ef-

fect on the loopback operation.

GPSI Loopback Modes

When GPSI is the active network port there are only two

modes of loopback operation: internal and external

loopback. The settings of MENDECL and TMAULOOP

have no effect for this port.

When INTL is set to ONE, internal loopback is selected.

Data coming out of the transmit FIFO is fed directly to

the receive FIFO. All GPSI outputs are inactive, inputs

are ignored.

External loopback operation is selected by setting INTL

to ZERO. Data is transmitted to the network and is ex-

pected to be looped back to the GPSI receive pins out-

side the chip. Collision detection is active in this mode.

AUI Loopback Modes

When AUI is the active network port there are three

modes of loopback operation: internal with and without

MENDEC and external loopback. The setting of

TMAULOOP has no effect for this port.

When INTL and MENDECL are set to ONE, internal

loopback without MENDEC is selected. Data coming

out of the transmit FIFO is fed directly to the receive

FIFO. The AUI transmitter is disabled and signals on the

receive and collision inputs are ignored.

When INTL is set to ONE and MENDECL is cleared to

ZERO, internal loopback including the MENDEC is se-

lected. Data is routed from the transmit FIFO through

the MENDEC back to the receive FIFO. No data is trans-

mitted to the network. All signals on the receive and colli-

sion inputs are ignored.

External loopback operation is selected by setting INTL

to ZERO. The programming of MENDECL has no effect

in this mode. The AUI transmitter is enabled and data is

transmitted to the network. The PCnet-PCI II controller

expects data to be looped back to the receive inputs out-

side the chip. Collision detection is active in this mode.

T-MAU Loopback Modes

When T-MAU is the active network port there are four

modes of loopback operation: internal loopback with

and without MENDEC and two external loopback

modes.

When INTL and MENDECL are set to ONE, internal

loopback without MENDEC is selected. Data coming

out of the transmit FIFO is fed directly to the receive

FIFO. The T-MAU does not transmit any data to the net-

work, but it continues to send link pulses. All signals on

the receive inputs are ignored. LCAR (TMD2, bit 27) will

always read ZERO, regardless of the link state. The pro-

gramming of TMAULOOP has no effect.

When INTL is set to ONE and MENDECL is cleared to

ZERO, internal loopback including the MENDEC is se-

lected. Data is routed from the transmit FIFO through

the MENDEC back to the receive FIFO. The T-MAU

does not transmit any data to the network, but it contin-

ues to send link pulses. All signals on the receive inputs

are ignored. LCAR (TMD2, bit 27) will always read

ZERO, regardless of the link state. The programming of

TMAULOOP has no effect.

External loopback operation works slightly different

when the T-MAU is the active network port. In a

10BASE-T network, the hub does not generate a re-

ceive carrier back to the PCnet-PCI II controller while

the chip is transmitting. The T-MAU provides this func-

tion internally. A true external loopback covering all the

components on the printed circuit board can only be per-

formed by using a special connector that connects the

transmit pins of the RJ-45 jack to its receive pins. When

INTL is cleared to ZERO and TMAULOOP is set to ONE,

data is transmitted to the network and is expected to be

routed back to the chip. Collision detection is disabled in

this mode. The link state machine is forced into the link

pass state. LCAR will always read ZERO. The program-

ming of MENDECL has no effect in this mode.

The PCnet-PCI II controller provides a special external

loopback mode that allows the device to be connected

to a live 10BASE-T network. The virtual external loop-

back mode is invoked by setting INTL and TMAULOOP

to ZERO. In this mode, data coming out of the transmit

FIFO is fed directly into the receive FIFO. Additionally,

all transmit data is output to the network. The link state

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