[ltp] Serial port switch

[Richard Neill]

Dear All,

Just thought the following might be useful to someone wanting to do some
really trivial hardware hacking. I'm using it to connect a reed-switch
to an old ThinkPad, to make a networked burglar alarm.

The old-fashioned Serial port (or a USB-serial adaptor) has a couple of
status lines which are really easy to monitor and interface with. It's
also reasonable electrically "tough", being designed to work with large
voltage-swings, and long cables.

statserial  /dev/ttyS0    gives the following listing, for a
non-connected port:

-------------------
Device: /dev/ttyS0

Signal  Pin  Pin  Direction  Status  Full
Name    (25) (9)  (computer)         Name
-----   ---  ---  ---------  ------  -----
FG       1    -      -           -   Frame Ground
TxD      2    3      out         -   Transmit Data
RxD      3    2      in          -   Receive  Data
RTS      4    7      out         1   Request To Send
CTS      5    8      in          0   Clear To Send
DSR      6    6      in          0   Data Set Ready
GND      7    5      -           -   Signal Ground
DCD      8    1      in          0   Data Carrier Detect
DTR     20    4      out         1   Data Terminal Ready
RI      22    9      in          0   Ring Indicator
--------------------


Normal data flows through the TxD and RxD lines, which we don't use
here. However, the status lines are essentially logic levels(*).

So, if we connect a switch between pin 4 (RTS,out,logic 1), and pin 9
(RI,in,floats at 0), then we can very easily read the state of the
switch.  The circuit really is this simple:

   [. .]
   [. 9] -----------------------O--/ O----|
   [. .]                                  |
   [. 4] ---------------------------------|
   [. .]

   Female			switch
   DB9 connector

When the switch is closed, RI == 1; when it is open, RI == 0.

This makes it really easy to hook up a switch to a laptop, and monitor
all sorts of things. I'm using it to monitor something else, with a reed
switch, and a piece of thread connected to the magnet.

In practice, we have up to 4 inputs and 1 outputs available, as well as
a small amount of unregulated power.

[For more advanced or faster I/O, try the parallel port, or an
FTDI-USB245M USB device, which is about $15]

Scripting statserial is a bit of a pain, since it sends ANSI-escape
characters to the terminal.  Here's a kludge that works, and prints
either 0 or 1.
  statserial -n /dev/ttyS0  | cat -vT | \
  grep "Ring Indicator" | cut -b 29


Hope this is of interest.

Richard


(*)Serial ports use reverse logic: 1 = negative, 0 = positive. Also, the
voltage swing can be anything in the range +/-5V  to  +/- 30V.