1.2 readings per second). The digital signal is output in a bit-parallel
byte serial form. The A/D converter output is multiplexed by U27 and U28
and input by the microprocessor.
In addition to the A/D information, the microprocessor inputs the
front panel control information, external switch position, set point and
optional inputs (BCD or IEEE), and outputs display information and set
point voltage.
The microprocessor unit (MPU) used
in
the DRC-84C is a 38P70 which
utilizes a piggy-back memory architecture. A 3870µp was originally a mask-
only part with the user tied to one program form when the part was fabri-
cated. The piggy-back variation of the part allows for variable memory
space (between 8K and
64K
of PROM) to be placed on top of the µp. This
allows all lines that were used for addresses and data to be used for
input/output.
The microprocessor unit has an internal RAM scratchpad memory used
for programming. The unit derives its internal clock from resistor R23
(and C18, if needed). The MPU has four
(4)
8-bit bi-directional ports
used for communication to and from the processor. Two of these ports,
Ports
0
and 5, are used for internal control of the instrument (A/D input,
BCD temperature output, set point voltage output, etc.). The remaining
two ports, Ports
1
and 4, are used for option access (IEEE-488).
The CPU uses one of its
I/O
ports to control Tri-State 8-line to 4-line
multiplexers (74LS257) controlled by a decoder/demultiplexer (74139) to input
the information needed. The second of the 2 ports handles the input of data
and manipulation of display data.
The CPU outputs the set point information through one of its
I/O
ports
as well.
The DRC-84C software is discussed in detail in Section 4.3.4.
4.3.4 Software
Refer to Figure 4.2 as an aid in the following discussion. The flow
chart gives the major steps of the program. When the instrument is turned
on, the program does a power on reset and starts the program at the beginning.
At this point, the program initializes internal registers to be used in the
program.
inputs multiplexed A/D information when the A/D tells it there is fresh data
ready (the program loops until the A/D information is ready).
then verifies that there are no illegal characters that were input and stores
the reading. After the first cycle,
a
calculation is done with each A/D
reading.
The program first checks for an A/D converter overrange and then
The program
The program then inputs display sensor information. There is one stan-
dard curve for the instrument and there are a maximum of twelve additional
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