I want to know how to wire DB-9 cable such that when one end is
connected to my RS-232 serial port, the other end can be connected to
a normally-closed boolean sensor and the state of the circuit can be
detected by software running on the computer. I need to know only
about the hardware configurations necessary, I can write the software
to detect data from the serial port.

My ultimate objective is to build a "door alarm". The software running
on my computer will detect signals sent to the serial port. The DB-9
cable will run from the serial port to a normally-closed door sensor
which will be of course connected to the door. When the door is
closed, the circuit is closed. When the door opens, I want to transmit
a signal to the serial port. When it closes, I want to transmit a
different signal to the serial port.

I hope I have provided enough details. Please let me know if my
question is confusing or if you need more information. Thanks!

Hi,

Here is what you need:

http://www.zytrax.com/tech/layer_1/cables/heavy.htm

Have your TD pin 3 send data, and complete the circut at the door and
send it to RD just loop back the Gnd. Then when your program receives
data then the door is open.

MORE:

http://www.zytrax.com/tech/layer_1/cables/tech_rs232.htm


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