[GreenKeys] 28 TD questions

[gil smith]

Hi Bob:

Continuing this wiring discussion from Jack's email:

At 12:17 AM 1/22/02 -0500, you wrote:
>The three wire, gold plated contact assembly was designed for +/- 7V at 
>1mA. It is a Tempest rated unit. The three wires connect to a polar relay 
>package, and everything is shielded to minimize radiation of the signals.
>But that makes it pretty simple to use it to drive an RS-232 port directly. 
>Just put +6 on one side, and -6 on the other side, and connect the common 
>to RD through enough resistor to make the total load about 6K ohms. I 
>believe the -6 goes on the NC contact.

I suppose this is indeed a tempest unit, as it has an RC filter for the
contacts and gold-plated contacts for dry-circuit (low-current) switching.
But this unit only brings out the mark contact.  The switch is on pins A
and C, and the shield is on pin B.

A mark switch could drive a 232 port by pulling the TXD line high with a
resistor and using the mark contact to switch the line low.  For 232, high
(space) is anything from +3V to +15V, and low (mark) is anything from -3V
to -15V.  Practically every 232 chip made has a "fail-safe" mode where the
technically-undefined negative region from -3V up to at least ground is
considered low.  So using a pullup resistor (a few K or so) from TXD to +5V
or +12V, and switching to ground, is a very easy way to do it, without a
negative supply.

This unit has an RC filter on the mark contact that has a total of 12.2K of
series resistance, so it would need a pullup resistor of maybe 100K.  A 232
chip may load this too much for proper levels to be maintained -- a buffer
transistor could be used, or the RC filter in the can could just be
jumpered out.

gil


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