The analog interface is a very simple analog to digital converter which accepts the analog video output of the RF/IF board and puts it into a form that can be read by a PC Games port. The joystick input of the games port essentially converts a resistance in the range 0 to 100k ohms into a binary number from 0 to 255. The interface transistor looks

to the PC as such a variable resistance, whose value is roughly proportional to the input voltage. The proportionality is not linear, but then neither is the receiver output. It can be used as a relative value, or it can be calibrated if a good quality signal generator is available. However, this is not necessary. It is necessary however, to set up the interface in accordance with the output values that it will receive from the RF board. This process is described later. The eight data outputs of register one are used to tune the radiospectrograph across the frequency band. These are fed to the digital to analog converter interface as previously described to produce a voltage that varies from 2 to 28 volts. This is applied to the tuning pin of the RF tuner. The two least significant bits of register 3 are used to select either low band (VHF-Lo) or the high band (VHF-Hi) of the RF tuner. Asserting bit 0 selects VHF-Lo, while asserting bit 1 selects VHF-Hi. Note that both bits 0 and 1 must not be asserted together. Also realise that the hardware does an inversion of the values presented by the software to these two register bits. A small QBASIC program that cycles the parallel port data byte from 0 to 255 can be used to set the max and min voltages output by the digital to analog converter (sweep generator). The code is listed below. `File name DACTEST. BAS `DAC test `produces a sawtooth ramp dly = 0 `increase to increase...

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