[GreenKeys] Interface board for Model 15, schematic and parts list (RO version)

[Randy or Sherry Guttery]

Lester Veenstra wrote:

> How about an RS-232 interface powered opto isolated ciruit to go from 60 ma
> loop back to rs-232 for input to PC?

Not enough power available on even a "spec" RS-232 to generate the
needed loop supply for the older machines - which need at least 60V -
90V would be better.

60V @ 60ma is 3.6W, 90V @ 60ma is an even higher 5.4W.

RS-232 is current limited on each line (spec. calls for 50ma), and most
computers today run Maxim (or equiv. charge pump chips) which supply
+/-10V vs. +/-12V as in the old days.

Since there are (a maximum) of four data lines - thats only 200ma total
at somewhere between 5V and 10V each - unless it's a really old computer
with +/-12V supplies. The newer computers with the charge-pump chips
will sag under heavy loads - approaching the RS-232C minimum swing spec
of +/- 5V.

5V (swing) at .2A (4 X 50ma) is only 1W. Even 12V @ .2A is only 2.4W -
far from the 3 - 6W needed (and that's if the dc - dc converter were
100% efficient - which isn't going to happen).

Even in a minimal powered 60ma loop like I run on my 28 (which has a
"contact box" on the keyboard to keep "stuff" out of the contacts) - I'm
running 24V @ 60ma - which is 1.44W. That might be "doable" from an
older computer using 1488 driver(s) with full +/- 12V supplies - (i.e.
full 12V on the data lines @ 50ma) - but highly doubtful on modern
computers where the data lines can sag.

Of course- all of this "depends" on the actual equipment to be used,
etc. and is - just my .02

best regards...
-- 
randy guttery

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