Subject: Re: NetBSD/vax QBUS IDE?!
To: None <firstname.lastname@example.org>
From: Eric Smith <email@example.com>
Date: 11/22/2002 19:12:05
Carl Lowenstein <firstname.lastname@example.org> wrote:
> 2) You don't have to duplicate the original interface chips, just their
> functionality. A quick look at a current catalog shows the
> TI 74LVT logic series. It looks to me like this satisfies both the bus
> receiver and bus driver specifications of the original TTL parts. Except
> I can't tell from a one-line description whether open-collector (or
> whatever is the CMOS equivalent) is available.
> Some more catalog scanning makes me believe that any of the logic
> families with CMOS inputs will satisfy the bus receiver requirement for
> low current drain, and the Schmitt trigger units will have the
> hysteresis for noise rejection.
The relevant Qbus specifications are:
Vol: 0.7 V max at 70 mA
Leakage current: 25 uA max
Vil: 1.3 V max
Vih: 1.7 V min
Leakage current: 80 uA max
Most CMOS drivers are spec'd for 64 mA or less. And most receivers don't
meet those thresholds. The leakage current is met by most CMOS parts.
If I wasn't concerned with being slightly out of spec, I'd probably use
the 74HCT14 as a receiver, and the 74ABT125 as a driver. The 74ABT125
is 3-state rather than open-collector (or open-drain), but by tying the
input to ground and using the control input to enable and disable it,
the behavior is equivalent to OC/OD.
Aside from fully meeting the specs, the other advantage of the DS3662/3862
was that it had "trapezoidal drive", which means that they carefully
controlled the slew rate rather than having sharp edges. This is very
desirable for large busses, but difficult to achieve with other drivers.