The Model 400 Modem comprises a configured set of seven (7) VME modules contained in a Modular Subcarrier Subsystem (MSS) chassis. A functional block diagram of the Model 400 is shown below.

The Modulator section of the unit consists of the FSK003 AM-FSK (SGLS) Modulator, PSK007 UQPSK Modulator, SUM004 Summer and the PMM004 Phase Modulator. The Demodulator section consists of the FDT003 AM/FSK (SGLS) Detector, and the RDM020 Receiver/Demodulator which includes a Linear Phase Demodulator and UQPSK Demodulator with I Channel Bit Synchronizer. The Q Channel produces an analog output. 

The CPU010 Control Processor provides control of the unit via either the local front panel VF Display and keypad or the Remote Control Interface.