TRS-80 curiosity

Fred Cisin cisin at
Fri Apr 14 09:54:53 CDT 2017

On Fri, 14 Apr 2017, Bill Gunshannon via cctalk wrote:
> The PC board shows two resistors and two transistors. Mine has
> four resistors and two disk capacitors.
> Obviously not even similar.

Possibly very similar,  Have you counted the pin numbers to determine 
which traces the parts are connected to?
If the component resistors, transistors and capacitors perform compatible 
"buffering" functions, they may be functionally compatible.
Both designs sound like kludges to deal with problems that surfaced when 
straight-through cables were used.
If you count pins and identify the components, or take a picture, then 
Allison might be able to identify the details!

> On another note, the box is labeled "To EI Screen Printer Port".
> Not sure what that means as the EI has a "Line Printer Card Edge"
> and not something specific to the Screen Printer.

That's due to a fluke in TRS-80 history.
Even before the EI was available, RS had a "screen printer" that connected 
to the 40 pin expansion bus of the CPU.
"To avoid confusion", and to avoid explaining what a bus was, 
RS therefore, called that 40 pin connector the "Screen Printer Port".
It sold for $599 (as much as the computer!)
That continued to be available for a while after the EI came out.  Owners 
of it "had to be" supported.  The EI has a continuation of the 40 pin 
expansion bus.  But, rather than try to explain it, RS simply showed them 
a picture of where the EI 40 pin expansion bus "continuation"/"output" 
connector was, and told them to plug their 40 pin screen printer into that 
connector.  "To avoid confusion", and to avoid explaining what a bus was, 
RS therefore, called that 40 pin connector the "EI Screen Printer Port".

Because RS had PROBLEMS with the EI,
they ended up coming out with multiple versions of the cabling, 
including the "Screen Printer" cable specifically including "buffered" 
cable with a box.
I think that the screen printer cable was 40 pin to 40 pin?

They also made a 40 pin to 34 pin adapter for connecting "Centronics 
style" printers to the CPU bus.  It also would probably have been labelled 
"screen printer port",

One 50 pin to 40 pin adapter would be the one to connect the screen printer to 
the model 3.  Insane thing to do, but we, the TRS-80 users, were rarely 
accused of sanity.

Another 50 pin to 40 pin adapter would be Model 1 to hard disk.  The 
connector on that for the model 1 would probably have been labeled "EI 
Screen Printer Port".

All of these cables would have been easiest to build as a board with two 
gold-plated card-edge connectors, and then use female card-edge to female 
card-edge cables for connection (except 26-1401 female card-edge to 
"blue-ribbon" for "Centronics style" printers).  But, RS cut corners and 
did not gold-plate the EI connectors!  That came back to bite them on the 
ass (similarly to the way Apple's crappy sockets bit them), and therefore, 
rather than risk another card-edge bad connection, some of their stuff was 
made with attached cables, instead.

Grumpy Ol' Fred     		cisin at

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