driving Unibus an Qbus - withan fpga? Spartan 2/2e/3?

[Eric Smith]

> I'm looking to drive both Unibus and Qbus with  5v tolerant
> Spartan2 (2.5v) fpga I/Os... anyone done this before?

Yes, using the DS3662.  Unfortunately it has been discontinued, as was the
DS8641.  Surplus DS8641 chips seem to be not that hard to find, I'd
probably use those.

> I'v heard of people using 4000 series cmos(??)  in instead
> of the approved  DS8640/1, DC005, DEC9881, DEC380 etc

CD4000 CMOS drivers and receivers have nowhere near the required specs.

> Spartan2 3.3V CMOS I/O is 5v tolerant without external resistors,

FPGA port pins have nowhere near the required specs either.

> but since lines may have to be series source terminated anyways,

No.  Series termination on a multidrop bus is NOT viable.  You
have to use Thevenin or Thevenin-equivalent termination on the
ends of the bus only.

> Spartan2e or Spartan3 might work as well..

Still nowhere near the required specs.

> Ideally I'm looking
> to use the same part for both Unibus and Qbus, possibly fiddling  Vref.

AFAIK, if you get drivers and receivers that meet the required specs,
they will work fine for Qbus, Unibus, and Omnibus.

The required specs are:

Driver:
       Vol: 0.7V max at 70 mA  - rules out 4000 CMOS and all FPGA pins
       leakage: 25 uA max, with Vcc from 0V to 5.25V
       capacitive load: 10 pF max
       rise and fall times (10%-90% voltage) must be no faster than 5 ns

Receiver:
       Vil: 1.3V max
       Vih: 1.7V min
       capacitive load: 10 pF max
       leakage: 80 uA max at 3.8Vdc, with Vcc from 0V to 5.25V

I don't know of any current production parts that meet the Qbus specs.
Most drivers don't meet the Vol spec at full current.  The receiver Vil
and Vih specs are hard to meet, as they aren't typical of logic parts.
The leakage specs are also a problem for many parts.  DEC used specially
selected DS8641 chips.

The closest current production chips I've found do not fully meet the
specs, but at least come close in many regards:
    TI SN75138
    TI AM26S10

Eric