am79c961 Advanced Micro Devices, am79c961 Datasheet - Page 36

• Manufacturer Part Number: am79c961
• Description: Pcnettm-isa Jumperless Single-chip Ethernet Controller For Isa
• Manufacturer: Advanced Micro Devices

Cards must be assigned a CSN before they will respond
to the other commands defined in the specification.
It should be noted that the protocol permits the 8-bit
checksum to be stored in non-volatile memory on the
card or generated by the on-card logic in real-time. The
same LFSR algorithm described in the initiation key sec-
tion of the Plug and Play specification is used in the
checksum generation.
Software Protocol
The Plug and Play software sends the initiation key to all
Plug and Play cards to place them into configuration
mode. The software is then ready to perform the isola-
tion protocol.
The Plug and Play software generates 72 pairs of l/O
read cycles from the READ_DATA port. The software
checks the data returned from each pair of I/O reads for
the 0x55 and 0xAA driven by the hardware. If both 0x55
and 0xAA are read back, then the software assumes
that the hardware had a “1” bit in that position. All other
results are assumed to be a “0.”
During the first 64 bits, software generates a checksum
using the received data. The checksum is compared
with the checksum read back in the last 8 bits of the
sequence.
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AMD
P R E L I M I N A R Y
Am79C961
There are two other special considerations for the soft-
ware protocol. During an iteration, it is possible that the
0x55 and 0xAA combination is never detected. It is also
possible that the checksum does not match If either of
these cases occur on the first iteration, it must be as-
sumed that the READ_DATA port is in conflict. If a
conflict is detected, then the READ_DATA port is relo-
cated. The above process is repeated until a non-
conflicting location for the READ_DATA port is found.
The entire range between 0x203 and 0x3FF is available,
however in practice it is expected that only a few loca-
tions will be tried before software determines that no
Plug and Play cards are present.
During subsequent iterations, the occurrence of either
of these two special cases should be interpreted as the
absence of any further Plug and Play cards (i.e. the last
card was found in the previous iteration). This termi-
nates the isolation protocol.
NOTE: The software must delay 1 ms prior to starting
the first pair of isolation reads, and must wait 250 m sec
between each subsequent pair of isolation reads. This
delay gives the ISA card time to access information from
possibly very slow storage devices.
Plug and Play Card Control Registers
The state transitions and card control commands for the
PCnet-ISA
+
controller are shown in the following figure.