|
Parallel communication is used
between the master and the slave. There are four data lines
and three control lines for transfer and control of data
between the master and the slave. Port 3 of the slave IC and
port 2 of the master IC are used for parallel communication.
To test a
particular digital IC, one needs to insert the IC into the IC
socket and enter the IC number using the keyboard and then
press the “ENTER” key. The IC number gets displayed in the
7-segment display unit. The IC number gets transferred to the
slave using parallel communication.
The process of parallel
communication is as follows:
§
The slave sends a ready
bit to the master IC when it is ready to receive data.
§
If the slave is ready,
the master places the data on the data lines.
§
A control bit gets set as
soon the master transmits the data.
§
The slave checks whether
the bit is set, and then receives the data.
§
Another control bit gets
set as soon as the slave receives the data.
§
A third control bit gets
set whenever the slave is ready to accept another set of
data.
As soon as the slave gets the IC number, it compares the IC
number with the stored list and then goes to the corresponding
service routine. The service routine checks the particular IC.
Depending to the IC, it assigns some ports as input ports and
some as output ports. It gives the corresponding inputs and
checks for the output according to the IC logic. If the output
is according to the IC logic, the slave sends the data to the
master IC. If the IC is a logic gate, then the data will
consist of 4 bits, where each bit corresponds to each gate of
the IC. In case the IC is not a logic gate, the data consists
of a single bit, which may be a ‘0’ or a ‘1’. Corresponding to
these four bits, the master sets or resets the lower port of
P3, which is connected to the indicators. Four LEDs are
interfaced to the master IC as indicators. If the IC being
tested is a logic gate, then each of the 4 indicator LEDs
correspond to the 4 gates of the IC. In any other case wherein
the inserted IC is not a logic gate, all the 4 LEDs work as a
single indicator.
Using this IC tester,
any digital IC can be tested provided its program is written.
For a particular IC, the corresponding program must be written
in the slave. The total number of ICs that can be tested using
this IC tester, depends on the memory available in the slave.
The 8951 microcontroller consists of a 4k ROM, using which
around 150 ICs can be tested. To test more number of ICs, a
microcontroller with an 8k ROM can be used.
The keyboard is
provided with a ‘RESET’ button, which when pressed resets both
the microcontrollers and the 4 shift registers. The shift
registers require a low pulse to get reset, whereas the
microcontrollers require a high pulse to get reset. To
synchronize this, a NOT gate is used.
DISPLAY UNIT:
The display unit is used to
display the IC number. This unit consists of five common anode
7-segment displays, five 7447 ICs(BCD to 7-segment converter)
and four 74194 ICs(4-bit bidirectional universal shift
register).
Here, the shift
registers are used in parallel load (parallel in parallel out)
mode. These shift registers are used to shift the digits of
the IC number. When the first digit is pressed, the master
recognizes it and places it in the lower pins of port 0. These
four bits are connected to the input of the first shift
register and also to the input of the first 7447 IC. The 7447
is a BCD to 7-segment converter, which converts the four input
bits (BCD) to their corresponding 7-segment codes.
The outputs of the 7447 are
connected to the 7-segment displays. Hence the first digit
gets displayed in the right-most 7-segment display. As the
next number is typed in, the master gives a clock pulse to the
shift registers to shift the data. Then the master places the
second digit on the lower pins of port 0. Hence the new digit
gets displayed in the right-most 7-segment display and the
previous digit gets shifted to the left.
S0 and S1 of the
74194 IC are connected to +Vcc, to work in parallel mode. P3.4
of the master is connected to the clock pins of all the shift
registers. These shift registers require a low to high clock
pulse. The ‘RESET’ button provided in the keyboard resets all
the shift registers. The common pins of the 7-segment display
are connected to +Vcc.
Click
here to study more about interfacing 7-segment display.
The entire
display section is shown in the figure below.
|
