[GreenKeys] Current Loop to RS-232 Conversion on ASR33

[Rob Jarratt]

From: Teletypeparts [mailto:teletypeparts at aol.com] 
Sent: 25 March 2013 01:42
To: epvgk at limpoc.com; r.jarratt at computer.org
Cc: greenkeys at mailman.qth.net
Subject: Re: [GreenKeys] Current Loop to RS-232 Conversion on ASR33

 

Rob,

 

The only thing between the 33 and its output in Line mode is the Line/Local
relay and some wires.  If it works in Local then likely nothing was damaged.


 

Is the board behind the Line/Local switch vertical with a vertical metal
relay on it? 

 

 

Yes, there is.

 

 

If so its just the tape reader control card that was used with PDP
computers.  You need a closure and 15 volts from the computer to run the
tape reader in Line mode.  You can check this by seeing if the reader runs
in Line mode. 

 

The reader was the part that didn't look too good when I got the machine, I
am not sure if it works properly, and I don't even remember how to make it
work. I dare not try to make it work at the moment because of its condition.

 

 If the extra card is for reader control, it wont run in Line mode without a
connection to the computer but will work in Local.  

 

Well I have been trying it connected to a computer, but I don't get the bit
about the 15 volts you mention, perhaps that is what is wrong, if there was
a fault here would it stop the machine receiving characters? Actually,
although the reader is plugged into the reader control card, there is a
green wire coming out of the assembly that looks like some kind of earthing
connection but it is not connected to anything. I know I did not disconnect
it but I am not sure if it really is an earth or where it should be
connected.

 

That is unless someone has bypassed the normal wiring.  

 

Wayne  



-----Original Message-----
From: epvgk <epvgk at limpoc.com>
To: r.jarratt <r.jarratt at computer.org>
Cc: greenkeys <greenkeys at mailman.qth.net>
Sent: Sun, Mar 24, 2013 8:32 pm
Subject: Re: [GreenKeys] Current Loop to RS-232 Conversion on ASR33

On Sun, Mar 24, 2013 at 11:44:04PM -0000, Rob Jarratt wrote:
> Yes, I think what you say may be right (although mine does not have a
> telephone panel). But even if you are right and it does have a loop supply
> inside the ASR33 why does it only appear to output about 10mA and then
drop
> to the point that it starts to chatter as if it was open? Yet, when in
local
> mode it works perfectly, so the current would *appear* to be enough.
> 
 
It's also worth considering that if you did at some point connect more than
one highish-voltage supply into the loop in series, you may have damaged
either
the current loop converter or the tty's solenoid driver? 
 
eric
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