5 Feb
2006
5 Feb
'06
11:13 p.m.
From: "Don North" <ak6dn at mindspring.com>
9000 VAX wrote:
> I have done extensive research on this issue. If you happen to take a
> look of the schematic of the mscp scsi project, you know my choice. It
> is the plain TTL 7406. I can defend this choice for Qbus. It satisfies
> the majority of the requirements, and has work around of the rest.
> 1. availability, price
> 2. leak current satisfies requirement (Yes, I measured. 0.25mA
> specification is for 30V, not 5V)
> 3. rise/fall time satisfies requirement (This is the most important
factor)
> 4. drain current work around. use one for single
terminated QBUS,
> parallel two for double terminated QBUS. (when in parallel, leak
> current doubles; but the QBUS tolerance doubles too.)
The 7406 works for the output/driver side (most any of
the older OC
drivers can sink enough current, and have slow enough edge rates), but
the issue is with the receiver L/H input thresholds. Standard TTL levels
of 0.8V/2.0V VIL/VIH do not have enough noise margin on the low side;
that is why the original DS8641/DS883x etc parts were designed to
support 1.7V/2.5V thresholds.
For a small single slot QBUS backplane I could agree up front the 7406
probably works just fine. But I would be doubtful it works reliably for
larger UNIBUS configurations (multiple 9-slot with bus jumpers, for
example).
Isn't the Iil value a couple of orders of magnitude out of specification
(for a receiver) too?
Vince