Radio Shack CCR-81 Reference Handbook

Cassette Audio Output
After the motor is turned on, the CPU may out-
put data for storage on tape. All data timing for
this output function is software controlled. Z59
is used to store data from the CPU and it
"builds" the output waveform using CPU data.
CPU data,
under software control, is applied to
latch Z59 on pins 4 and 5. Output pins 2 and 6
are
connected to a resistor network consisting of
R53 through R56. As QUTSIG* is clocking data
into Z59, the resulting output on the line label-
ed CASSOUT resembles a sine wave
built out of
square waves. Figure 10 is an illustration of one
bit time.
In Figure
10, the voltage output is a function of
the status of pins 2 and 6 of Z59. In the period
labeled
T1. the output is shown as 0.46 volt. T1
is when output pin 2 is zero and output pin 6 is
high (0(2)=(2); 01=(2)). The voltage during T2 is
outputted when pin 2 is high and pin 6 is high
(0(2)=1; 01=(2)). The voltage during T3 is out-
putted when pin 2 is low and pin 6 is low
(00=(2); 01=1). All "digital sine waves" are pro-
duced in this way.
Notice
the time periods shown in Figure 1(2).
From the start of one bit time to the start of
the next bit time is two milliseconds. A one or
46
zero is dependent upon the presence or absence
of a pulse between the start of two bit-times.
For example, when the CPU outputs a one-bit, it
will generate a
start pulse. One millisecond later,
another pulse will be generated. One milli-
second later, the start pulse of a new bit is gen-
erated. If
this bit is to be a zero, then there will
be a
two millisecond delay before another pulse
is generated; and this pulse starts the third bit-
time. The pu Ises are outputted to the Cassette
Recorder from pin
5 of J3. This pin is tied to
the AUX input of the Recorder. The CPU out-
puts all of the instructions in system RAM to
tape during the CSAVE function. When the
function is complete, audio to the Recorder is
disabled and a low is outputted at D2, shutting
off the Recorder's motor.
Data is written on the tape in the following for-
mat: Upon CSAVE,
the CPU forces Z59 to out-
put 128 zero bits. It then outputs hex code A5
used by the CPU during CLOAO for synchroni-
zation. A
two-byte starting address and a two-
byte ending address is added next. Then the data
follows, however long it is. After the data, the
last portion to be stored on tape is the check
sum. This one byte number is the sum of all data
added together. It is used by the CPU to ensure
what it CLOADed-in is what was CSAVEd-out.
If the check sums don't match up, then there
was a load error.

Radio Shack CCR-81 Specifications

General

Brand	Radio Shack
Model	CCR-81
Category	Desktop
Language	English