Did you know that embedded landline modems
can be as little as one inch wide and less than
one half inch high? Ideal for telemetry applications
where a phone line is available and small footprint
Sometimes there are a few "gotchas"
when using a new modem model, but generally speaking
landline dial up modems have been around for a
long time and are a stable technology. There are
some nice features available like modem size,
speed, smart modem behaviors, and auto-bauding.
The first thing you want to do when communicating
with a modem is to make sure both devices can
communicate. Make sure the baud rates on the device
serial port are the same, and that the Rx on one
device is connected to the Tx on the other. Test
the connection with an AT command and you should
receive and "OK" response. First test
your modem commands on HyperTerminal before programming.
The modem manufacturer should include modem initialization
strings, which must be programmed or entered before
the modem dials another modem. Modems have two
communications modes, command mode and data mode.
Command communications include dialing, auto answer,
hang up, etc. The modem must be in data communications
mode before data is transferred from one host
Once the modems connect a "CONNECT"
response with the connection speed baud rate displayed.
At this point the modem is in data mode and data
can be transmitted. Modem test commands cannot
be issued in data mode.
When the modem does not detect data being transmitted
it will usually disconnect after a pre-configured
timeout, and return to command mode. To return
to command mode (or escape from data mode ) send
"+++" three plus characters without the
quotes about 100 milliseconds apart. Wait for
the modem to return to command mode. Then hang
up "ATH". Some modems do not like
being immediately queried and will reboot when
When you receive your manufacturer's embedded
modem guide, make sure you have the initialization
strings and the full command set. You may need
assistance with new setups or development boards
so also make sure the modem manufacturer has professional
and responsive technical support.