Intel MCS-80/85 manual Cassette Recorder Interface

APPLICATION EXAMPLE 3

CASSETTE RECORDER INTERFACE

There are many situations where data has to be transmitted through a non-ideal medium. To give three typical examples, a system with electrically isolated elements might require that signals be AC coupled, communications through an audio net- work (such as telephone or radio) are greatly band- width limited, and some applications (such as a distributed network in an industrial environment) must tolerate random electrical noise. Attempting to record data on a cheap cassette recorder (the one used for this note cost $17.00) will reveal all of these shortcomings, plus one: The tape speed fluctuates significantly and varies as the batteries fun down, hence the data rate is inconsistent.

The recording scheme used here makes very few demands on the transmission medium. It makes no attempt to transmit DC voltage levels. Instead, data is transmitted by a series of variable length tone bursts. The dominant frequency of the tone used can be selected to be within the passband of the particular medium. Data is transmitted with each bit composed of a tone burst followed by a pause. The first third of a bit period is always a tone burst, the middle third is either a tone burst con- tinuous with the first or a pause corresponding to, respectively, a one or zero, and the final third is always a pause, as shown in Figure 25. Thus, data is distinguished by the burst/pause ratio.

Hardware Design

These tone bursts are obtained from the 8085 SOD line, using analog signal conditioning to eliminate the DC component of the waveform. (This low frequency component is due to the single-ended nature of the SOD line: it's deviations from ground are all positive, which unbalances the capacitive input stage of the recorder.) A suggested interface

circuit is shown in Figure 26, using one LM324 quad op amp and a few standard value discrete components which should be available in even a digital design laboratory. On playback, analog cir- cuitry is again used to detect the presence of a tone burst. In Figure 26, A2 buffers the incoming signal, and A3 inverts it. The peaks of these two signals are transmitted through 0 I or 02 and are filtered by an RC network. Comparator A4 then squares up the output and produces the logic signal read by the SID pin. Since the op amps are powered by the single 5-volt supply, a 2.0-volt reference level is obtained from a resistive voltage divider. The waveforms present at several points in the circuit are shown in Figure 27.

Software

The algorithm for reading a data bit off the tape is simple and straightforward: If the tone burst is longer than the pause, the bit is a one. Otherwise, it is a zero. Since only the time ratio is considered, any variation in tape speed will not affect the data determination.

VOLUME CONTROL

A question that arises with any audio cassette inter- face is how to set the volume control. (Recording level is usually determined internally.) When the play- back level is correct. the logk signal output from A4 will have either a one-third or two-thirds du ty cycle. This can be readily observed with an oscilloscope. In the field, an old-fashioned mechanical-type voltmeter could be connected to the A4 output, and the volume adjusted until the mete~ needle hovered somewhere between 1/3 and 2/3 the high level output voltage. With random data, the reading would be about 2 volts. There will be a fairly wide range of acceptable volume settings. (Since the quivering meter needle is being used here for inertial signal averaging, a digital voltmeter would not be very helpful in this applica- tion.)

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