siliconchip.com.au July 2019 101
of the capacitor, resistor and diode
connected from pin 9 to Vcc and GND.
Pins 10-16 of IC1 are shared be-
tween the alphanumeric LCD screen,
which operates in 4-bit mode, and
the 3x4 multiplexed keypad, which
has a 7-pin connector. The circuit is
powered from a 5V supply provided
by 5V low-dropout regulator REG1,
itself fed from a rectified and filtered
6V AC supply which can come from
a 6V AC plugpack or 12V AC centre-
tapped transformer.
Alternatively, it could be powered
from a 7.5V DC or 9V DC plugpack.
In operation, you use the numer-
ic keypad to type in the suffix of the
74LS device that you wish to test. If
the device selected is not found in the
micro's database, it displays a message
which reads “Not in Library”.
Otherwise, it then sets the I/O pins
which are routed to the socket as in-
puts and outputs as necessary, so that
each digital output of the logic IC is
connected to a digital input on the mi-
cro and vice versa.
All the pins on IC1 which are con-
figured as digital outputs are then
brought low and the digital outputs on
pins 25-27 are set so that the correct
transistor (Q1, Q2 or Q3) is enabled to
power the selected IC.
You are then prompted to insert
the IC in the socket and press a but-
ton when you have done so. It then
applies Vcc.
The micro then checks that the
states of its inputs, as driven by the
DUT, are correct for the DUT input
states being all zeroes.
Assuming that's OK, it cycles
through all the possible DUT input
states and compares the state of its in-puts to the truth table for the device be-
ing tested. If they all match, the device
passes the test. Otherwise, something
is wrong and it indicates a test failure.
It at any time an over-current condi-
tion is detected, the power to the DUT
is cut and an error is displayed on the
LCD screen.
You can build the device using
point-to-point wiring on protoboard.
The firmware and source code (ICtest-
er.bin, ICtester.hex and ICtester.asm)
are available for download from the
Silicon chip website, along with a list
of supported logic devices.
IC1 is programmed over a serial
port but you will need a suitable pro-
gramming rig. There's no need to al-
ter its configuration byte from its de-
fault value.
Noel Rios,
Manila, Philippines ($75).