[GreenKeys] Loop electronics - a request for assistance

[gil smith]

Hi Don:

 From this description, I would presume that the loop keyer is an NPN 
transistor to ground.  They say it is open-collector, so it will be 
grounding the loop to complete the circuit.  Since there is no mention of 
opto-isolation, there is no floating return output.  Be sure to provide 
current-limiting resistance or you will blow the transistor.

I would connect the negative side of the loop supply to their circuit 
ground (pins 10/11/12/13/14), connect the positive side of the loop supply 
to one side of the tty, connect the other side of the tty to one side of 
the loop resistor, and connect the other side of the loop resistor to the 
open-collector output (pin 15).

Try it with a low-volt supply first -- no tty, just the supply and the 
resistor.  Maybe a 12VDC adapter and a 10K resistor (1/8 watt is plenty), 
for a measly 1.2 ma that won't hurt anything.  Connect a scope ground to 
their circuit ground, and connect the scope to the transistor output -- you 
should see a nice clean keying waveform from 0V (mark) to 12V (space).  If 
you don't have a scope handy, but you can set the unit's output to a static 
space or mark, put a meter on it -- you should see 12V (no current) for 
space, and 0V (current flowing) for mark.  Remove the meter or scope before 
connecting a high-voltage supply.

You can now try the full HV tty if you'd like, or you can test a bit more 
incrementally -- eg: drop the resistor to 200 ohms (at 1W) to see if it is 
sinking 60-ma properly (provided your 12VDC source can provide it).  If all 
is well you can put a full loop supply on it, with the appropriate resistor 
(eg: 120V with  2000-ohm/10W, 60V with 1000-ohm/5W...)

Be careful with those nasty loop voltages!  I'd opt for 60 to 80V to keep 
it a bit safer, and keep the heat down, and I'd bet that even 30V would be 
just fine for most machines.  I have not seen a tty yet that needed 120V.

gil


At 06:31 PM 1/8/2006, Don Robert House wrote:
>My friends,
>
>I am somewhat puzzled by the lack of a specific ground/return point
>for connecting my Info-Tech M6000 to a high voltage loop supply and
>the selector magnets of my Teletype machines.  The Info-Tech M200F is
>easy because the manual calls out TWO specific pins and shows a
>diagram which includes the loop supply complete with polarity
>markings!  Cool.  I can do that super easy.
>
>The M-6000 assumes I am up to snuff as a bachelor of electronics and
>I guess I was about 30 or 40 years ago.  But the descriptions are
>incomplete and there is no diagram.   They call out ONE pin as the Hi
>voltage loop keyer and NO SPECIFIC return path.
>
>Here is what the manual shows:
>
>APPENDIX  I
>SERIAL PRINTER OUTPUT SELECTION
>
>The M-6000 offers three different serial printer output levels:
>
>J6-  4  Mil-188  Mark = +5V,  Space = -5V
>J6-  5  RS-232 (EIA)  Mark =  -7V,  Space = +7V
>J6-15  High voltage, open collector transistor for 20 or 60 mA.
>loops,  Mark = Low
>Note:  Maximum open circuit voltage on this transistor is 175 V.D.C.
>maximum current is 70 mA. and must be current limited (underlined).
>
>APPENDIX  II
>J-6  PINOUTS  &  FUNCTIONS (15 pin D-sub connector)
>
>Pin          Function
>1              Handshake from computer
>2              Data from computer
>3              Data to computer
>4              Serial printer output MIL-level
>5              Serial printer output EIA-level
>6              Auto-start output
>7              Serial printer handshake
>8              Auxiliary data input
>9              Spare
>10            Ground
>11            Ground
>12            Ground
>13            Ground
>14            Ground
>15            Serial printer output (Loop Keyer)
>
>I assume that voltage, current flow, and polarity are important but
>they are not called out in the manual, nor is the correct ground, or
>"return" as we telephone people call it.
>
>Sadly I have the service manual for the M200F but only the owners
>manual for the M-6000.
>
>If any of you can be of assistance please reply.
>
>Many thanks,
>
>Don
>
>Don R. House K9TTY
>drhouse at nadcomm.com
>
>
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