DEC bus transceivers

allison ajp166 at
Sun Oct 23 16:35:25 CDT 2016

On 10/23/2016 04:57 PM, Toby Thain wrote:
> On 2016-10-23 2:50 PM, shadoooo wrote:
>> Hello,
>> surely the old transceivers are the most compatible solution, however
>> you
>> still need to convert the voltages back and forth...
>> Plus the solution is not the cheaper, and a little uncomfortable too, as
>> you need to find these old chips, hoping not to buy fake chinese
>> duplicates
>> (it happened to me more time unfortunately).
>> So I was searching a solution with modern components, but not using
>> components too much specific and difficult to be found.
>> As we need 3.3v logic, but able to work in 5v bus, I'm thinking about 5v
>> tolerant standard logic as TI LVC or LVT.
>> 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.
>> So identifying one or maybe two codes would be enough for all the
>> components needed for the board.
>> The idea of using bare transistors seems to me too much simple.
>> Not that it couldn't work, but it would be almost impossible to
>> satisfy all
>> the specifications of the bus in this way... unless you use a more
>> complex
>> circuit with precise current sources and resistors to grant correct
>> voltage
>> biases, impedances and slew rates, which in the end is a logic
>> integrated
>> circuit.
>> Andrea
> As an electronics noob, I'm really waiting for somebody to publish
> their findings on this, comprehensively, so I can steal their labour.
> Has anyone done so? Is anyone planning to do so? I know that this
> topic flares up on the list every 6 months ago in a series of
> disjointed posts and observations. The gold is hard to find
> (especially for aforesaid noobs).
> --Toby

I've watched many flail on this when simple solutions work.  They seem
to read into the driver/receiver
specs a lot of imagined should be x when its simple.  The specs are
those of Utilogic and TTL of the day.

DEC buses relied on two things; pull-ups to assert the high voltage
level and a strong pull down to ground
at low cost.  All of this was developed in the late 60s and early 70s. 
To add to that the idea of open
collector was  to prevent damage if two drivers asserted the same
line.    Also in some cases to allow
wired OR logic.  We forget that a transceiver in 1970 was the level of
complex logic and the only thing
more complex was a flipflop as around then the tech was not there for
more than a handful of transistors
on the die.  By time the tech could do more the parts were firmly
entrenched and provided the basic
bits often needed.

DEC always suggested their part for the base reason is they work, there
were examples in use to study
and internal engineering was expected to!  That and if a customer needed
DEC support it was easier.

I keep my supply of those for board repair.  For new boards I make for
myself (QBUS or Omnibus)  I use
bog simple 74LS14, 74LS240, 74LS241, 74LS245  and occasionally a open
collector part like 7438.

The 3.3 volt problem is  modern logic and not modern in conflict.   The
trick in engineering around that
is to keep the interfaces limited to reduce the needed transitions back
and forth.

Former MilRat

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