DEC bus transceivers

allison ajp166 at
Mon Oct 24 12:37:14 CDT 2016

On 10/23/16 2:59 PM, Al Kossow wrote:
> On 10/23/16 11:50 AM, shadoooo wrote:
>> The problem is that there aren't open drain bus transceivers, but the
>> problem could be solved simply using input-only and output-only components,
>> connecting two in parallel but opposite direction on bidirectional pins.
> The reason for using the old parts is the logic thresholds are unique to
> the Unibus to handle worst-case bus loading and the termination voltage they
> used.
The voltages are based on TTL levels.  What are the unique voltages?

The key was limited leakage current and input current to not load the 
bus by inserting or removing
current from a node (there is a specified maximum in per node and total 
nodes).  That cover input
to card devices and bus driver leakage.

Logic low voltage is typical of TTL and the driver device has to sink 
that current and meet that value.
Logic High was set by the terminator devices at 3.36V but the threshold 
is lower based on the bus

By late 1970 it was an easy spec to meet,  When first used (pdp8e) it 
was new and the ICs
were not so great with leakage current and output device saturation current.

Every time this comes up the world is supposed to stop if not met. The 
LSI-11 bus (qbus)
was actually harder as it was 120 ohm terminated and HeathKit did it 
with common TTL
and the CPU was DEC standard LSI-11 and it worked out to 18 slot backplanes.


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