[GreenKeys] Fwd: New Teletype Model 28 ASR

Sun Jul 28 14:06:01 EDT 2019

Me again.

I've shown this device on GreenKeys in the past.  It is about the simplest way to convert
RS-232 voltage levels to a 20 ma output, and/or taking a 20 ma signal and converting it 
to RS-232 voltage levels.

I am attaching three 'screen snapshots':  One is the schematic; one is the front side of
the PC board and the last is the 'solder side' of the PC board.

With just seven components, all of which should be available economically, it should not
be too much of a stretch to put one in between say, a ST-8000A TU to and from the connector
which has the I/O terminals for the RS-232 voltages.

It could be point to point wired on perf board and one could add as many heat sinks to any
component, just in case, and have the voltages and current(s) as needed.  A simple +/- 12V
power supply should not be a problem.  Just make sure that the transformer is capable of
providing the needed current on a continuous basis.  (I would personally look for something
in the 80 to 100 ma range.  Higher, if size is not a great concern.)

The PC card could easily fit inside of the ST-8000A and a loop jack could be added to the
back panel.

Hope something good comes from this.

73,

Ralph

PS    Electronic Systems, the creator of this design and item is long gone
      from the sales world, at least under that name.  (The copyright date
      of 1978 should be a clue.)

R

-   -   -   -

On Jul 28, 2019, at 12:03 PM, William Wuttke wrote:

> Hi John,
> 
> I’ve read articles on the 20 ma vs voltage LR time constant problems problems, but I have to give what I have a try. If it doesn’t work, I’ll have to try something else. In the end, I need to connect the 28 to the ST-8000, one way or another. Experimentation is what makes all this fun. The learning that comes from that helps to keep my mind from growing old.
> 
> Bill
> KE3BK
> 
>> On Jul 28, 2019, at 8:48 AM, w9ddd at tapr.org wrote:
>> 
>> I meant to send this to the list as well as to Bill. 
>> 
>> John, W9DDD
>> 
>> 
>> Begin forwarded message:
>> 
>>> From: "w9ddd at tapr.org" <w9ddd at tapr.org>
>>> Date: July 28, 2019 at 8:41:57 AM CDT
>>> To: William Wuttke <wcwuttke at att.net>
>>> Subject: Re: [GreenKeys] New Teletype Model 28 ASR
>>> 
>>> Bear in mind the usual RS232 to 20 mA converter is based on the idea of connecting to a 33.  The 33 by default contains a selector magnet driver.  That isolates the inductance of the selector magnet from the loop, so the LR time Constance is not a problem.  Connecting the RE232 to 20 mA converter directly to the 28 selector magnets will have to deal with the LR time constant.  It probably doesn't have a high enough voltage supply for the 20 mA loop to properly drive the 28 selector magnets.  Of course YMMV.
>>> 
>>> John, W9DDD
>>> 
>>> 
>>>> On Jul 27, 2019, at 11:24 PM, William Wuttke <wcwuttke at att.net> wrote:
>>>> 
>>>> Hi Paul,
>>>> 
>>>> I’m currently using a Model 33 ASR with a HAL ST-8000 which doesn’t have a current loop interface. I made a homebrew 20 ma loop to RS-232 converter using HCPL-4100 and HCPL-4200 optoisolators. I don’t have anything that interfaces with a 60 ma loop. The quickest and easiest thing for me to do is convert the 28 to 20 ma.
>>>> 
>>>> 73,
>>>> Bill
>>>> KE3BK
>>>> 
>>>>> On Jul 27, 2019, at 8:07 PM, Paul Heller <paul0926 at comcast.net> wrote:
>>>>> 
>>>>> Glad to hear it  I’m curious why you want to run at 20ma.
>>>>> 
>>>>> Paul 
>>>>> W2TTY
>>>> 
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> 
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