[GreenKeys] Anyone need a loaner 28-TD (100-wpm to rs232)?

[gil smith]

Hi Lester:

The 28TD got converted to 232 with just an ac adapter, a transistor, and a 
resistor.  Take a peek at:

http://www.baudot.net/tty-232/my-28TD-d.jpg
...to...
http://www.baudot.net/tty-232/my-28TD-h.jpg

I removed the original connector/bracket inside, and was able to squeeze a 
12VDC adapter in it's place.  I connected the adapter to switched ac power 
using the end of an extension cord.  The 232 output line simply has a 2.7K 
pulldown resistor to ground (for the mark state), and a 2N4126 PNP 
transistor to drive it up to 12VDC (for the space state).  The tempest 
contacts are SPDT, and the filter board provides series resistance for base 
drive -- so I just connected the base of the pnp to the appropriate side of 
the switch.

The RS-232 mark level is technically supposed to be below -5VDC, but 
virtually every 232 input chip on a pc will accept ground as a valid mark 
(negative) state.

gil

At 04:37 AM 2/16/2005, you wrote:
>Sounds like just the thing !
>What was the mod to get rs-232 out of the lowlevel box?
>
>gil smith wrote:
>
>>Hi Lester:
>>
>>I have a standalone 28TD I could loan to you for a while.  It is small; 
>>maybe 20 pounds.  It was a tempest unit with low-voltage contacts, so 
>>could not be used on a standard high-volt loop.
>>
>>I modified it to send rs232 directly.  It just has two pigtail 
>>connectors:  one for an ac power cord and one DB9F for connection to a PC 
>>via a straight cable.  The TD rums at 100-wpm, so tape reads are a bit 
>>faster than you would get on a model 14 at 60-wpm.



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