Multibus arbiter design
n0body.h0me at inbox.com
Mon Jul 20 01:04:39 CDT 2015
> -----Original Message-----
> From: spacewar at gmail.com
> Sent: Sun, 19 Jul 2015 21:14:45 -0600
> To: cctalk at classiccmp.org
> Subject: Re: Multibus arbiter design
> On Sun, Jul 19, 2015 at 4:01 PM, N0body H0me <n0body.h0me at inbox.com>
>> 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.
I just happened to see the parts you mentioned on EPay, so I posted that.
Apparently, the 8289 is something of a square wheel (at least, for your
application, anyway) and not a good fit.
Anyways, just trying to be helpful.
That's why I'm......
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