5 Feb
2006
5 Feb
'06
8:06 p.m.
> I have been looking at suitable driver chips for
Unibus/Qbus with
Richard,
and it looks
like the SN75ALS057 is both suitable and readily available.
The bus driver/receiver voltage spec (absolute maximum voltage of 2.5V)
of these parts is NOT suitable for UNIBUS/QBUS. These parts are designed
for low voltage swing busses and may work in the short term on UNIBUS,
but will eventually fail, as the UNIBUS termination voltage (static high
level) is 3.5V, 1V over the abs max rating. I've looked at this problem a bit and the most
suitable modern day (ie,
still in active production) part I've found is the Am26S10C. TI now
produces this older AMD part, and it is available in both 16DIP and
16SOIC. It is a quad part not unlike the DS8641. You can get them at
Digikey for about US$1.35 each. Even in Pb-free packages.
The 26S10 has 100ma OC drivers, and the bus receiver input threshold is
in the 1.75-2.25V range, very close to UNIBUS specification. The only
issue I see in using this part is that the specs of the output rise/fall
time are sub 10ns transitions. This may cause problems with reflections,
especially on UNIBUS systems with longer cables.
I think the driver part of the Am26S10 is pretty good, except the rise time
can be too fast (might have to select parts more toward the "typical"
values,
rather than the minimum values from the datasheet).
I'm a little concerned about the input thresholds on the Am261S0, though.
The 1.8V Vil is actually in the allowable range for a UNIBUS high! OTOH,
the DS8838/8T38/MC3438 that were used in the RX emulator won't sense
voltages that low as a high, either!
(I don't know why the bus spec Vih is so low; is it just because the DS8641
was that way? Is it is really possible to have that much leakage on a real
world bus?)
Looking at the bus specs, 105ua of leakage are allowed per bus load. I
calculate that over 9ma of load would be required to drag the bus down
to the Vih 0f 2.3V for the Am26S10. That works out to 87 bus loads!?
For the DS8838 (2.5V), I get 71 bus loads. Am I doing this right? If
so, I am guessing that the bus can't get that big, and the Am26S10 should
work fine?
Vince