Abstract: A Frequency Shift Keying (FSK) system to transmit and receive data as a sequence of dual frequency short duration tone bursts. Normally the data is transmitted from an underground location, as in a mine, to the surface or to another underground location. At the receiving end the transmitted data is detected as marks or spaces that are used to clock the appropriate data bits into a storage shift register. Erroneous data from wideband noise transients at the receiver are eliminated by the use of an exclusive OR circuit which permits shift pulses to appear at the data register only when one or the other of the two shift pulses are present but not both. To further insure accurate copy of the digital data at the receiver, a receiver alert code is transmitted prior to the telemetering of the actual data. Only after recognition of the receiver alert code is the receiver able to receive the transmitted data bit stream.