5 Feb
2006
5 Feb
'06
11 p.m.
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.)
vax, 9000
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).