Specifications

CONSTRUCTION
8085 µP-KIT BASED
SIMPLE IC TESTER
S. RAJKUMAR
RUPANJANA
A
ll electronic laboratories in engi-
neering colleges and other insti-
tutions need a digital IC tester to
verify the serviceability of frequently used
digital ICs, e.g., ICs 7400 (NAND), 7408
(AND), 7432 (OR), 7486 (EXOR), 7404
(Hex Inverter), 7407 (Buffer) etc. The
truth tables of all such ICs are available
in digital IC data books. Based on their
truth tables one can write suitable sub-
routines to test them using an 8085 mi-
croprocessor kit and a minimal of inter-
face circuitry. An 8085 microprocessor kit,
having requisite peripheral devices, is nor-
mally available in most electronic labs,
and as such one does not have to buy
costly IC testers for testing simple type
of ICs, as mentioned above.
It is assumed that the kit has at least
two 8255 PPI (programmable peripheral
interface) ICs whose input/output ports
have been extended via suitable connec-
tors, for external usage. The configura-
tion of the interface circuit required for
testing of the digital 14-pin ICs using 8085
microprocessor kit is shown in Fig. 1. The
interface circuit comprises simply a 14-
pin ZIF socket and two hex buffer 7407
ICs. The +5V supply needed for the inter-
face circuit (and ground) is obtained from
the kit’s power supply itself. The total
cost of the interface circuit would be less
than Rs 300.
Both the 8255s have been configured
for mode ‘0’ operation (which is a basic
input/output mode) with registers A and
B as output and register C
(both upper and lower half)
as input. The required con-
trol word for the mentioned
configuration is 89 hex. The
characteristics of mode ‘0’
operation of 8255 are:
1. Two 8-bit ports
referred to as registers A
and B respectively.
2. Two 4-bit ports
referred to as C register
(lower-comprising bits C0
through C3) and C register
(upper–comprising bits C4
through C7).
3. Ports configured as
output have latched outputs
while input ports are not
latched.
4. Any port can be made input or out-
put. There are 16 possible input/output
configurations. (Please refer Table I for a
summary of the configurations and the
control word required to be used during
initialisation of an 8255 for each configu-
ration.) Control word can also be formed
with the help of Fig. 2.
The hex buffer/driver IC 7407 has
open collector outputs. The outputs of ‘IC
under test,’ which is placed in the ZIF
socket, are combined with those of 7407
in a wired-OR (actually wired-AND) fash-
ion. To realise this function, a logic 1 is
always output on the 7407 gates connected
to output pins of ‘IC under test.’ All pos-
sible logic input combinations are given
to input pins of ‘IC under test’, while logic
1 is placed at all its output pins via 8255’s
registers A and B, through IC 7407 buff-
ers. For each input combination, the logic
state of the ZIF socket pins (as modified
by the ‘IC under test’) is read (after a
short delay) via ‘C’ registers of the two
8255s. The expected results for each com-
bination of inputs, for above-mentioned
ICs, are shown in Table II in hex digits.
These are stored in memory, in consecu-
TABLE I
Control Words
S.No. Port A Port C Port B Port C Control Word
(Upper) (Lower) (Hex)
1OOOO 80
2OOOI 81
3OOIO 82
4OOII 83
5OIOO 88
6OIOI 89
7OIIO 8A
8OIII 8B
9IOOO 90
10 I O O I 91
11 I O I O 92
12 I O I I 93
13 I I O O 98
14 I I O I 99
15 I I I O 9A
16 I I I I 9B
Note: O = Output; I = Input
Fig. 1: Circuit for interfacing IC under test to 8255 PPIs on 8085 microprocessor kit
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