100 Silicon chip Australia’s electronics magazine siliconchip.com.ausiliconchip.com.au
This circuit checks digital logic ICs
which are plugged into an IC socket
and displays the results on an LCD
screen. It performs a thorough check,
applying all possible input combina-
tions and checking that the output pin
states are correct. It also automatically
switches off power to the device if it
draws too much current. It’s controlled
via a 12-key numeric keypad.
It has support for 70 different
types of logic ICs from 74LS00 up
to 74LS374. Many of the 74C, 74HC,
4000B series and so on have the same
function and pinouts, and as long as
they will run with a 5V supply (most
will), this circuit can check them, too.
It's based on a DS89C430 microcon-
troller from Dallas Semiconductor.
It is pin compatible with the vener-
able 8052 and also uses the same in-
struction set but it has a much larger
16KB of program memory than older
8052-compatible chips.
The 89C430 has a high-speed archi-
tecture. It runs at one clock per ma-
chine cycle, with a maximum clock
rate of 33MHz.
The DS89C430 provides three 16-bit
timer/counter, two full-duplex serial
ports, 256 bytes of direct RAM plus
1KB of extra MOVX RAM.
The lower-left pin of the socket is
permanently connected to GND (0V)
as all the ICs in the supported list
(and most digital logic ICs) have their
ground pin at lower-left and power pin
at upper-right.
Since the IC pin count can vary,
three different socket pins can be con-nected to Vcc via one of PNP transistors
Q1-Q3, under the control of micro IC1.
This allows for 14-pin, 16-pin and 20-
pin ICs to be powered up and tested.
PNP transistor Q4 is used to switch
overall Vcc power to the DUT (device
under test). That power flows through
a 1W resistor. The voltage across it is
monitored by op amp IC2a and if the
current drawn by the DUT exceeds a
threshold set by trimpot VR1, IC2a
pulls the base of Q4 high, switching
it off and cutting power. This is also
fed to pin 24 of IC1, so it knows that
a fault was detected.
The other pins of the DUT are routed
to digital I/O pins on microcontroller
IC1. Each one has a 10kW pull-up to
Vcc. IC1 requires external power-on
reset circuitry and this is the purpose74LS- and 74HC-series logic IC tester