[GreenKeys] Modem with 20mA current loop interface

[drlegendre .]

Ah OK, so it's down to +two+ criteria, then?

1) SS vs. simple EM selector relays
2) Length of cable run

So then..

The devices with SS inputs don't have the issue of inductive rise-time in
the relay coils, so they don't need any kind of "high" voltage - in this
case, 120V - to drive them (closed / on) quickly. The only reason they
might need higher voltages (say, 24-48V) would be due to the length of the
loop.

Conversely, the devices with simple EM relays need the higher voltages just
to overcome the inductive rise vs. time - to say nothing of what might be
required to cope with the length of loop run, which is a separate matter.

Am I getting this right? Please let me know -  Like many others, I'm quite
interested to learn the gritty work-a-day details of current-loop
signaling.

On Mon, Dec 15, 2014 at 2:41 PM, Jim Haynes <jhhaynes at earthlink.net> wrote:
>
> On Mon, 15 Dec 2014, drlegendre . wrote:
>
>  What other 20mA 'levels' exist, and what differentiates them?
>> Would the 12V / 470-600R loop used with many simple level converters be
>> considered low-level 20mA?
>>
>>  Yeah, what I mean is that the M32/33/35 machines use electronic selector
> magnet drivers and the input to those is 20 or 60 ma but at low voltage,
> like 20 volts.  The older machines, M14/15/28 have the selector magnets
> brought out, not run through a solid state driver, so they require 60 or
> 20 ma at typically a high voltage like 120V and generate inductive kick
> when the circuit is opened.  A very few of them have solid state drivers
> incorporated so the input loop can run at low voltage.  And you can use
> less than 120V with a more complicated driver circuit, or if you have
> perfect signals to begin with.
>
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