> When there were real Teletype machines on line it was not unusual to see
> 100 volts on an open line. Add in the inductive spikes and it got more
> interesting. I hope there are more reasonable specs today.

Of course this is a DIFFERENT kind of 20mA loop, but is interesting
nonetheless. Teletype machines used "selector magnets" that drove a
mechanical USART (it's amazing what they did with mechanics). Selector
magnets were either holding or pulling. For holding magnets, a cam would
move the armature up to the electromagnet, then release it at the bit
sample time. If the armature stuck, the line was in the mark condition. If
it did not hold, it was in the space condition. Pulling magnets did not
have this cam, so a higher current was required to pull the armature in at
the correct time.

The selector magnets had two coils. For holding magnets, these could be
wired in series for a 20mA loop or in parallel for a 60mA loop. Pulling
magnets were always wired in series and required 60mA.

Now, about the high voltage... It was quite possible to get enough current
with 12V. I did this in high school (about 45 years ago). But, you get
lots of errors. This is when I learned that besides T=3DRC, T is also L/R.
If you do not have a large resistance in the loop, the current would not
build up quickly enough through the inductive selector magnets for there
to be sufficient force to hold or pull the armature at the correct time.
So, we'd run a 100V loop with the required resistance to yield the desired
loop current.

This was all done with Teletype model 15 printers a long time ago...

Harold


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