24 Oct
2016
24 Oct
'16
10:23 p.m.
On 10/24/2016 1:18 PM, David Bridgham wrote:
On 10/24/2016 01:37 PM, allison wrote:
FYI here is an I/O plot I did of the bus input receiver of an 8641 device (the
blue line). A threshold of about 1.5V.
Also included is a plot of a 26S10 transceiver (which has a higher input voltage
threshold) which I think is used
in later designs (SBI? IIRC).
http://www.ak6dn.com/stuff/26S10vs8641.jpg
The voltages are based on TTL levels. What are
the unique voltages?
The QBUS spec from the 1979 Bus Handbook (the Unibus levels
are the same):
Input low voltage (maximum): 1.3 V
Input high voltage (minimum): 1.7 V
And from the TI datasheet for the 74LS74:
Vil - low-level input voltage 0.8 V (maximum)
Vih - high-level input voltage 2 V (minimum)
So no, the DEC bus voltage levels are not TTL levels. Yeah, TTL might
work on a smaller system but you can see that if you push it out to its
limits, TTL could start getting flaky. That's the kind of bug I'm happy
to have DEC's engineers figure out and not have to track down myself. 