Signals.Txt

This file is for all the people wondering what the lights mean.  So here 
goes:

Baud Rate   - Speed of the Computer-Modem connection
TD          - On if there is data waiting to go from the UART to the modem
RD          - On if there is data in the UART for a program
CD          - Carrier Detect; usually used to tell if the modem-modem link
                is good, but can be forced with certain init strings.
RI          - Ring Indicator; will light at the end of a telephone ring.
DTR         - Data Terminal Ready; means that the computer is ready.  Usually
                used to hang up the modem.
DSR         - Data Set Ready; means that the modem is ready.  Usually on if
                the modem is on, but may change in self test modes.
RTS         - Request To Send; the computer holds this line high; it does not
                know when it will next have a chunk of data ready for the 
                modem.  You have to work to get this line low.
CTS         - Clear To Send; indicates the modem is ready for data from the
                computer.  Used with high speed modems for handshaking.

These are the signals currently monitored.  If the modem is powered up, you
can usually count on DSR being high.  If it is working A-OK, you will
hardly see a flicker (I haven't yet) on the DSR, RTS, or CTS lines.  The
CTS line might flicker if you have an expensive modem with a large on-board
buffer connected at a slow speed.  Otherwise, Monitor just isn't fast enough
to catch it.  Note that there isn't a pair for telling the modem that the
computer isn't ready (historically because CPU's are faster than telco lines
or printers, but that's changing)!

-Thor Johnson