[GreenKeys] A reliable current-loop-to-RS232 converter

[Mike Douglas via GreenKeys]

In this particular case - interfacing with an ASR-33 - the loop current does not drive the selector coil directly, but instead drives a solid state circuit that, in turn, drives the selector coil. As Gabriel noted, even this converter was able to drive the typing unit of the 33. 



I believe the reason it can't receive from the teletype is that 5v just isn't enough potential to reliably operate across the drop of the carbon wiper and contacts of the distributor, the keyboard contacts, and then across the Vf of the LED in the opto receiver.  If 12v could be put across the receive loop, along with an additional resistor to maintain 20ma in the loop, I think this converter would probably work with the 33.
Mike

On Aug 13, 2015, at 9:25 PM, Jeffrey D Angus <jdangus at att.net> wrote:


 On 8/13/2015 9:15 PM, Gil Smith wrote:
  
 Shhesh.  Really nothing to go on unless someone recognizes this unit.
 Likely some Chinese thing in many stores.  I would open it up and
 trace out the schematic, but that is often not an easy task, especially
 if it has more than two layers (though I doubt it does). 
  Does this look familiar to anybody? 
 This one in particular no, but I've seen similar.
 
 The problem is NONE of these are designed to work with an inductive
 load. They are designed to run either a 20 or 60 mA current loop using
 a matching converter at both ends of the cable.
 
 The "give away" on any of these converters is the "Will work up to 19K
 baud." That and being powered off a USB cable or external +5v supply.
 
 Similar problems with the "Black Box Inc." current loop converters."
 They were designed with a maximum loop voltage of around 65 vdc, 
 NOT 120 vdc. And they get unhappy REALLY fast driving a 4 Hy magnet
 with huge spikes from the switching.
 
 
 
 
 
 -- 
Jeff-1.0
wa6fwi
http://www.foxsmercantile.com 

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