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2
2-4
EtherLink/MC:
Functional Description
Memory Cycle Arbitration
Because two devices (the Micro Channel and the 82586) require access to the same memory,
occasional resolutions of conflicts must occur. The adapter's arbitration circuitry is designed to
guarantee fair access to fhe RAM by both requestors.
During Micro Channel to the Packet Buffer when the 82586 is idle, the Micro Channel is allowed
free access to the memory and the arbitration circuit does not get involved at all. Similarly, the
82586 has free access to the RAM while the Micro Channel is not using it. However, all 82586
cycles require the assistance of the arbitration circuitry to allow it to access the adapter's address/
data bus.
Once the 82586 memory cycle has started, the arbitration circuitry checks, at a particular point in the
82586's cycle, whether or not the Micro Channel is going to be accessing the adapter. If the Micro
Channel is indicating activity, the 82586's cycle will be suspended until the Micro Channel has
completed its cycle. Once the Micro Channel cycle has been completed, the arbitration circuit
claims the adapter bus and allows the 82586 cycle to run to completion.
If the Micro Channel is not indicating adapter activity, the 82586 immediately proceeds with its
cycle. During the latter portion of the 82586 cycle, all Micro Channel accesses to the adapter will be
deferred until immediately after the 82586 has completed its cycle.
During typical operations, supporting the transfer of continuous 10 Mbit/s of data to or from the
adapter will consume approximately 35% of the available bandwidth of a 10MHz 80286 Micro
Channel system.
Transceiver
The EtherLink/MC adapter's transceiver is a 3Com thin Ethernet transceiver. The transceiver is
factory tuned and tested to guarantee operation on Ethernet segments that are twice as long as
specified by IEEE 802.3.
A control bit in the Programmable Option Select register is provided for"jumperless" selection of
either the on-board (BNC) or external (DIX) transceiver.
EtherStart ROM
A socket is provided on EtherLink/MC adapter for the installation of an optional "EtherStart" ROM.
This ROM contains software to allow the Micro Channel system to "boot up" without using local
disks. The EtherStart ROM is mapped into the upper 8K of the 24K Micro Channel memory space
used by the adapter. Reads from the ROM are byte-wide only. The byte-wide nature of the EPROM
cycles is transparent to the programmer and the system. Naturally, execution out of the Start ROM
will be slower than execution out of system RAM and, once the boot is complete, all software
required by the system should reside in system RAM.
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