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Control Port
The control register output consists of two 74LS175s;
one of which is used to hold the four least significant
bits of the data for use in the output jumper area and
the other of which is used to hold the four most signifi
cant bits for controlling the internal operation.
The internal operations use bit 4 asserted to indicate
that a write operation is being performed on the CRI
and bit 5 to indicate that a read operation is being per
formed on the CRI.
modes: 1) to select the status input lines for SIOS and
PIOS by providing the A and B inputs to the 74LS153 dual
4 to 1 selector; and 2) to multiplex the PIO select lines
and the status signal, CRIS.
the PIO ports by having /CR7 as the DS2 input to the 8212s
for PIO»Port 1; and CR7 as the DS2 input to the 8212s for
PIO Port 2.
Bit 6 is used to select the Byte Ready (/CR6)
or Bit Ready (CR6) signal for input to the CRIS signal.
The input to the control port is accomplished by selecting
the appropriate jumpers in the input area as described in
the User Guide.
74LS367s for gating onto the internal data bus.
Cassette Recorder Interface
The Cassette Recorder interface uses the ANSI standard bi
phase encoding technique to record data on the tape by using
a square wave clock to shift the data and EXCLUSIVE ORing
the clock with the output data.
Timing
Figure 1 shows a timing illustration of how the CRI inter
face works with respect to shifting, recording, and recover
ing the data.
The top line shows the serial data which is
to be written on the tape.
The third line shows the serialized data as clocked out of
U36.
The fourth line shows. EXCLUSIVE OR of the clock and
the data.
Notice that there are two flux reversals or one
complete cycle per bit when a 'constant data stream is being
written and only one flux reversal or one cycle per two
bits when alternating ones and zeros are being written.
The fifth line shows the EXCLUSIVE OR of the data and the
inverted clock.
soidal wave form which is written on the tape.
also be considered to be the data read directly back from
the tape.
The seventh line shows the output of the 8T20
Bits 6 and 7 are used in two different
Bit 7 is used to multiplex
These jumpered inputs are input to the
Below this is the clock pulse.
The sixth line shows the resultant sinu
Theory of Operation
This can
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