[GreenKeys] 20mA circuit

[Lee Mushel]

A perennial favorite topic:   The loop supply.

Now, going back to the sixties and continuing for 15-20 years  everyone used 
a Triad N-48X 115v:115v  15VA transformer (available back then) and then 
added a silicon rectifier, a filter capacitor and a 2500 ohm adjustable 
resistor, and, once adjusted for 20 or 60 mils (mostly 60) you had the "loop 
problem" solved and no one gave any thought to the subject and everyone, 
including me, had several of those transformers in "inventory."   But then, 
of course, we called a terminal unit just that (and not "converter" as 
promoted by the ARRL).  And, yesterday, while looking through my "shed" of 
old junk I discovered a price list of Elliot Buchanan!   If you needed a 88 
mH toroid you went to Elliott.   Did people have to beg someone to take a 
machine so it wouldn't be sent to the junkyard?   Nope!   Elliott would sell 
you a 28KSR for $1,400.

So what, you say?   Simply this:   if you correct for inflation, Elliott was 
asking for close to $10,000!   Maybe a bit more.  It's like they say,  "It 
ain't your grandfather's ham radio."

73

Lee   K9WRU   WD2XSH/32   41 year life member ARRL   51 year member of the 
ARRL   TAPR
----- Original Message ----- 
From: "Nico de Jong" <nico at farumdata.dk>
To: "B M" <iamvirtual at gmail.com>; <GreenKeys at mailman.qth.net>
Sent: Wednesday, March 24, 2010 2:26 AM
Subject: Re: [GreenKeys] 20mA circuit


> >I am almost ready to get my ASR33 hooked up to my PDP-11 computer.
>>
>> I am looking for a circuit that uses optoisolators to ensure both
>> sides of the connection are protected from each other.  I do want a
>> full duplex connection.  I will be using an active 20mA port on the
>> PDP-11 and a passive 20mA connection on the TTY.  I would prefer to
>> use as low a voltage that will work on the TTY.  I suspect the PDP-11
>> uses +/-12v, but I have not yet confirmed that detail.
>>
>> I only have a basic knowledge of electronics and can build the circuit
>> if I have a schematic drawing.  I would also be looking for tips on
>> how I might test the circuit before I actually use it.
>>
>
> I can recommend http://www.baudot.net/ I'm right now installing a
> teleprinter net in our data museum (Denmark). The tleprinter connects to a
> standard COM port. The part between the RS232 chip and the exit to the PC
> port mas been modified a bit to allow for Connect, Disconnect, and "Local"
> buttons. I wrote the software in Delphi 6. The TTY will need more then
> +12VDC, I use a 110-110 isolation transformer. This keeps the contacts
> clean. Everything is built into a box previously used for a SCSI drive.
> Most of the hardware is available through Mouser, but the eurocards are
> bought locally. Prints are not available anymore.
>
> I'd be happy to lend a (remote) hand
>
> All the best
>
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