Microcomputer Circuits and Processes

A CONTROL AND DATA AQUISITION
COMPUTER
Most of the ideas of this chapter can be put to use in building up
CONDAC. CONDAC's purpose is Control and Data Aquisition.
Before we see how CONDAC could be used in the laboratory, let's look
at its circuitry. Don't be put off if it looks complicated, it's not; it's
simply a combination of some of the circuits you have just studied.
Here is a description of CONDAC, shown in figure 4.38. To the
bottom left of the CPU are the memory boards, and to the bottom right is
the keyboard with its sixteen keys. Next to this is the display. This is an
eight-digit, seven-segment display. At the top right of the circuit
diagram, there are two D-to-A converters, labelled x and y. To the right
of these is the A-to-D converter, a successive approximation type.
Finally, in the bottom right of the circuit there is an input buffer which
can read the states of eight logical inputs, and also an output latch
which can output eight logical signals.
Now for a closer look at some features of the circuit. Firstly, the
decoding. In Chapter 2 you learned how different memory boards were
selected using a decoder. In the last section of the same chapter, you saw
how input and output devices can be selected by using a decoder.
CONDAC has two decoders, one reserved for memory, and the other
for input-output. The decoders both have 'enable' connections; if
enable is low, the decoders' outputs are all low. If enable is high, then
the correct output of the decoder will go high. Notice that the MilO
signal is used to enable the memory decoder, and the inverse of this
signal is used to enable the input-output decoder. So either memory or
input-output decoder is selected at any time. It turns out that this use of
two decoders saves a handful of gates elsewhere in the circuit.
Two outputs from the memory decoder are used. One selects a
RAM board, which also receives both RD and WR signals, and the
other output selects the ROM board, which, being read only, needs
only the RD signal to work. The ROM in CONDAC contains the
operating system. This is a chunk of program which actually gets the
machine running; scanning the keyboard, allowing you to enter and run
programs. It also looks after the multiplexing of the display, allowing
you to read results, check memory contents, and the like. In addition, it
makes the cassette interface function, and may even contain subroutines
to make the D-to-A and A-to-D converters function. (On larger
machines, the operating system would run disk drives, and enable you
to edit programs shown on a television screen.)
Next, take a look at the D-to-A ports. These are selected by outputs
1 and 2 of the input-output decoder. The x-channel is selected by 1,and
the y-channel by 2. Since these channels are OUTputting data, they are
controlled by the WR signal, which is ANDed with the decoder outputs
using the two AND gates shown.
The A-to-D converter is selected by decoder output number 7. Two
commands must be sent to the A-to-D converter. First, a 'start
conversion', which is an OUTput command. The signal is thus generated
by ANDing decoder output number 7 with WR. When the conversion
is done, the A-to-D converter must inform the CPU of this fact.
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