5 Feb
2006
5 Feb
'06
11:34 p.m.
vrs wrote:
From: "Don North" <ak6dn at
mindspring.com>
That is certainly true (7406 requires sinking 1.6ma on the input to
drive the output low; the DS8641/883x/29S10 all have maximum input
current specs of 100uA under all input voltage and power supply voltage
conditions).
It's not clear from the above discussion from 9000vax as to what he used
for the bus receiver; he just describes using the 7406 as an output
driver on QBUS. Not clear what he uses on input.
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