Some Features of the HC-278 Modem

The recently developed HC-278 digital data modem provides full-duplex data transmission at rates up to 1200 bit/s over voice bandwidth channels in fixed or transportable ground electronic systems. Using phase continuous frequency shift modulation, the modem meets the operational requirements of JCS Publ. 10 (TADIL-B) and the interface requirements of MIL-STD188B. Suitable transmission channels include wire lines and tropospheric scatter radio equipment. Maximum usage of digital techniques and integrated circuits provides economy, reliability, simplicity, and compactness. The modulator accepts synchronous serial digital data at standard rates of 75, 150, 300, 600, or 1200 bit/s and converts the bits into tones representing MARK frequency (logical 1) and SPACE frequency (logical 0). At the receiving terminal, the composite line signal containing the above tones is decoded into the original serial bit stream. System timing is derived from a crystal oscillator with a frequency of 1.0752 MHz and stability of per month.