Multibus arbiter design

[Eric Smith]

On Sun, Jul 19, 2015 at 4:01 PM, N0body H0me <n0body.h0me at inbox.com> wrote:
> A quick search on the "Site Which Must Not Be Named" shows
> some NOS 8289's for about $7 each, including postage.  I dunno,
> just sayin-- why re-invent the wheel?

1.  The 8289, being bipolar, draws an absurd amount of current on +5V.
Well over 100mA IIRC.  If I were going that route, I'd want to use the
82C89, which is even harder to find and more expensive.

2.  The 8289 is specifically designed for the 8086 maximum-mode bus
interface, which is essentially unlike any other microprocessor's bus
interface, so it will take nearly as much extra logic to convert the
signals to what's needed for the 8289 as it would to just implement a
suitable bus arbiter.

3.  $7 is a lot to pay for something that I can implement in the FPGA
for no incremental cost.

4.  If I want to build more cards in five years, how much with a NOS
8289 cost then?

5.  If I'm going to sell the card, which there's some chance of
happening (admittedly small), I don't want to depend on availability
and price of NOS parts.

If there was any particular magic involved in implementing a Multibus
arbiter, I'd shy away from doing it. However, it appears to actually
be rather simple. The reason for my request for examples was so that I
could study a few known working designs and make sure that there isn't
some trick to it that I've overlooked.