Ed,
Thanks for the suggestions! I will dig a bit further into the box when I
can get to it - it's sitting behind several large boxes of stuff.
Steve H.
On Mon, Nov 30, 2020 at 3:42 PM Ed Breya via groups.io <edbreya=
[email protected]> wrote:
Steve, the digital circuitry in the bigger Comdyna box could be there for
a number of reasons. By the end of the analog computer era, digital stuff -
computing and otherwise - had already been around for a long time, so it
was natural to combine them and get the synergy of both technologies.
Lacking any documentation, you may be able to at least tell kind of what
the extra stuff was for, by studying the types of parts included, and doing
a little reverse engineering to see what things go back and forth between
sections. Here are a few thoughts on what it could be:
1. Digital readout of values. In the old days, it took quite a lot of
circuitry to build a DVM - even a whole board full of stuff, for say a four
digit A-D converter, and more stuff to select which signals to look at,
etc. The digital results of old-time DVMs were often provided to external
equipment such as digital computers for further processing, storage, and
printout. These were parallel interfaces, with four bits per digit, plus
possibly range and polarity info, and handshake lines. Also, sometimes the
computer could control the DVM's ranges. If there's quite a large (pin
count) interface connector/port on there somewhere, this could be a
possibility.
2. Digital storage/equivalent time conversion for display. One of the same
reasons as today. Digitizing the X-Y or T-Y signals could save and display
the results of a very slow compute cycle, or present flicker-free display
of a medium speed repetitive cycle.
3. Function generator. This is not the kind typically pictured when we
hear the name today, that generates CW square, triangle, and sine waves. In
the olden days there were all sorts of schemes to include multiplying and
dividing, and nonlinear and transcendental functions in analog computers,
from mechanical servos and cams, to semiconductor device characteristics,
to stepped diode (even tube diodes!) line/curve fit approximations. Then
there were special analog hybrids and ICs that could do log amps, and more
complicated combinations for all sorts of functions. All this can be done
discretely, but ICs greatly simplified it.
With the availability of more digital stuff like DACs, ADCs, and ROMs, it
became possible to make an arbitrary function generator. A tracking ADC
follows an analog input signal, and its result addresses a ROM, which is
programmed for various functions. The ROM data out drives the DAC,
resulting in the desired analog function response output.
I vaguely recall seeing info about this kind of FG for analog computers,
but not necessarily related to Comdyna. I think by this time, the digital
computers were common enough and capable enough to leave analog ones in the
dust. The look-up table method is of course commonly used today for AWGs
and such. I think that every modern DDS chip has buried somewhere inside, a
quarter-sine ROM table and DAC to produce the desired sine output.
If the Comdyna box has no microprocessor, but one or more ROMs, and ADCs
or DACs, then it could be a digital function generator.
4. The last category is one where a digital computer was used to set up an
analog computer - anything from the overall layout, to initial conditions,
to timing control. The analog computer would then do its thing, effectively
and relatively quickly, then the results would be digitized and read back
to the digital computer. This seems crazy today, but there was a time when
early digital just didn't have the compute power or memory, and an analog
one could be used as a peripheral device to make it easier - like a math
co-processor or DSP chip.
If the Comdyna box has lots of relays or JFETs (set up as signal
switches), or lots of more modern IC analog switches, then it could be for
remote control/peripheral use, or pre-programed setups. I do vaguely recall
some Comdyna literature about something like this.
Ed
