CDS Computer Output (Transmit) Data Cycle

interrupt on the input data lines and set the interrupt line. The interrupt is then processed by the CDS computer and the input buffer is closed. If the received stop code is a picket stop code, the DTS simply resets itself. If the stop code is a control station stop code, the DTS will compare the next two frames received with its own station address code. CDS Computer Output (Transmit) Data Cycle The output data cycle is initiated when the DTS detects its own station address, either in an interrogation message or at the end of an NCS report and interrogation message. When the DTS recognizes its own station address, it starts to transmit the preamble. During the first frame of the preamble, the DTS sets the prepare to transmit interrupt on the input data lines. The computer samples the interrupt and sends an IDA to acknowledge receipt of the interrupt. The DTS finishes sending the preamble and phase reference frames During the second frame of the start code, the DTS sets the output data request (ODR) active, requesting the first word of the tactical data. The CDS computer responds by placing 24 bits of data on the lines and then setting the output data acknowledge (ODA). The DTS samples the data and clears the ODR. The first frame of data is processed for transmission and the ODR line is then set to request the next data word. This procedure is repeated until all the data words have been transmitted. Once the CDS computer has completed sending all the data words, it will not acknowledge the ODR from the DTS. If the CDS computer has not acknowledged an ODR from the DTS in a preset amount of time, the DTS will clear the ODR line and generate a stop code. Upon transmission of the two-frame stop code, the DTS will return to the receive mode. Net Control Station (NCS) I/O Operations The station acting as NCS follows the same protocols when communicating with the CDS computer. Some differences exist in the generation of the control codes. The net control station is responsible for interrogating each station. Upon receipt of a picket stop code, the DTS checks the next station address and sends an interrogation message. After the interrogation message is transmitted, the DTS waits to receive a start code from the interrogated station. If a start code is not recognized after 15 frame intervals, the station will be reinterrogated. If a start code is not received after another 15 frame intervals, the address control unit will advance to the next active picket address and the interrogation process is repeated. The other major difference is when the net control station has completed its own tactical data transmission, a control stop code, followed by the next station address, is transmitted. Again, if a start code is not received within 15 frame intervals, a second interrogation is sent. This second interrogation is a normal interrogation message consisting of the preamble, phase reference frame, and address code. Modulator/Demodulator The modulator/demodulator function of the DTS provides the digital to analog and analog to digital conversion. During data transmission, the 24-bit binary data word is expanded to 30 bits by adding the six bits for error detection and correction. The 30 bits are then examined in pairs to determine the required phase angle shift for each of the 15 data-carrying tones in the audio package. At the frame boundary, the phase of each data tone is shifted with respect to the previous frame. Figure 4-19 shows the four possible phase shifts. A sixteenth tone, the 605-Hz Doppler correction tone, is added to the tone package. The Doppler tone is not phase modulated and is used to correct for Doppler shifts caused by the relative motion between the transmitting station and the receiving station. The 16 tones are combined into a composite audio signal and sent to the radio set. The radio set transmits the composite tone package on the carrier frequency in independent sideband form. 4 - 1 8