The Digital Interface Module (DIM) was originally designed as a companion product to the NT transmitter; however, this versatile interface enables FSK digital data transmission on all of Cobham Surveillance (GMS Products) FM transmitters and FM receivers. It can be used to convert an audio sub-carrier into a FSK data sub-carrier at data rates up to 38.4 Kbaud. Since the system is DC-coupled there is no sensitivity to the data’s structure.
The DIM supports both transmit and receive operations. In transmit operations, it accepts either RS-232C or RS-485 data and converts it to filtered bi-polar alternating DC voltages that are applied to the audio sub-carrier input. These voltages cause the sub-carrier frequency to shift +100 KHz of the subcarrier center frequency for a logic one input and -100 KHz for a logic zero input. Other FSK offsets are available via special order. The DIM can be installed within the ST series transmitter or used externally with any Cobham Surveillance (GMS Products) FM transmitter.
In receive mode, the DIM works with the RX to convert corresponding bi-polar voltages from the subcarrier emodulator’s output into RS-232C or RS-485 compatible logic signals. The DIM can be installed within the MR series receiver or used externally with any Cobham Surveillance (GMS Products) FM receiver.