[GreenKeys] Why high loop voltages?

[John Nagle]

    Something I don't get: why were such high loop voltages used?

    I just got a Model 15, and just finished degreasing, cleaning and
re-oiling it, but don't have anything to connect it to yet.
(It was mechanically in good shape, but out of use for years.)
The magnets are wired in parallel and measure 55.3 ohms at the
input to the printer.   This is a 60mA current loop device, so
it should run on 3.3V.  3 D batteries in series will run the thing,
drawing 75mA; I should put a 20 ohm resistor in to get the
current right.

    So what is the rationale for huge loop voltages?  I realize
the drive is supposed to be constant current, but making a
current source by putting 120VDC in series with 2K seems like
doing it the hard way.  Yes, you get the option of plugging in
extra devices in series without adjusting the current.
And you get some contact cleaning as higher voltages burn
through oil and dirt.  But 120VDC?  I could see 12VDC;
that's enough to get through automotive-type dirt on contacts.
Thanks.

				John Nagle