Abstract: A digital data radio transmission system, such as may be used in communicating point-to-point by radio, in which a keyboard is used to insert a multi-decimal-digit number representing the called station. This is fed to a decimal-to-binary converter, forming a train of binary bits which are then coded in a selected manner so as to be substantially noise immune. The output of the coder then goes to the radio transmitter. At the receiving end, a similar means is provided for obtaining and storing in a 24-bit register the selected number of the called station. The output of the radio detector is then decoded to obtain the 24-bit word representing the called station, which is stored in a second register. A comparator compares the numbers stored in the two registers. When they compare positively, an alarm signal is enabled, or the receiver squelch is broken.