AM79C970A Advanced Micro Devices, AM79C970A Datasheet - Page 80

• Manufacturer Part Number: AM79C970A
• Description: PCnet-PCI II Single-Chip Full-Duplex Ethernet Controller for PCI Local Bus Product
• Manufacturer: Advanced Micro Devices

Abnormal network conditions include:
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-
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FCS errors
Late Collision
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P R E L I M I N A R Y
Am79C970A
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