Problem in pi-card driver (+ fix!)

[Dave.Perry@Software.Mitel.COM (Dave Perry)]

The reason the PI card uses DMA on only one port, and half duplex DMA,
was to keep the requirements on the host machine down to a single DMA
channel.  As I've said elsewhere, the card was really designed with
specific requirements in mind - to provide end-user access to a high
speed lan, to work on a lowly XT class machine (with possibly only
one free dma channel), and to be relatively cheap.

Two DMA channels could have been used (as in the PacketTwin, for example),
but the hardware would have been more complex. There is a timing constraint
with the SCC that defines a minimum time between accesses to the chip,
including DMA accesses. With one channel supported I was able to meet
this requirement with software delays. Two DMA channels would have
required additional hardware to insert wait states, because the
accesses from the 2 DMA channels would occur asynchronously and not
under software control.

Dave