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 is desirable.

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 to another.

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 "confused".

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.