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Wed Aug 17 18:26:21 EDT 2016

Remote access

How to make it work, sealed into a package at the end of a cable?

- RS485 serial line
- Power, Ground, Reset.

Use a 4-pair standard ethernet cable and use:
1: RS485
2: GND
3: 5V
4: reset current loop


Alternatively, to eliminate the RS485, use two current loops, one for
RX and one for TX.

How to drive a current loop like this?

Note that MIDI is supposed to be a current loop, but the current is
not regulated other than using a fixed current limiting resistor.  If
the cables become very long, the resistance increases and the current
drops.

What about making this compatible to MIDI, and for a fixed setup, make
the divider resistors programmable so they can be tuned to the cable
resistance.

MIDI is:

5V - 220R - cable - 220R - diode - cable - 220R - 5V/0V buffer.

Likely also works with open collector.
Typical 6N138 forward voltage is 1.4V

Max current is (/ (- 5.0 1.4) (* 3 220)) -> 5mA

Could be driven using open collector and NPN with emitter
degeneration.

What is the resistance of an Ethernet cable?
1 Ohm for every 10 metres

So there is no reason to make this use an active regulator.  Drive it
from open collector PIC pin with a current limiting resistor.




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