S*TJ
JUNE 1981
6
ANALYSIS (Part 2)
by Jim Snead, General Manager, Electronics Division,
Kurz-Kasch, Inc., Dayton, Ohio
To test a CPU we must give it an instruction which it can execute in a repetitive fashion and generate signatures which are stable and the same for all
like CPU's (8080's, Z-80's, 6502's, etc.). This instruction is "NOP''. (See figure 1 ). In order that we can be assured the CPU will only see this
instruction we must break the data bus and "hardwire" the NOP into the CPU. This is done by a NOP fixture.
(This NOP fixture does a few other things such as disable the interrupts so the processor will run.)
FIGURE 1 - CPU NOP FIXTURE CONNECTIONS
1
A .,
2
A,,
A,,
~ A,>
RESET
TSC
NC
~ A .'
i2
~ ;-•
1
D,
RESET
DBE
NC
6
D,
R/W
R/W
D,
8
o,
o,
9
D,
10
D,
D,
D, r - = - - t
D,
D,
D,
Ml
RESET
BUSRO
BUSAR
A,
A,
A,,
A,.
A,,
A,,
A,,
A,
·A,
D,
D,
31
JO
D,
29
D,
D,
D,
A,,
28
21
26
A,
A,
-SV
RESET
A,
'
+12V
A,
A,
A,
A,.
WAIT
WR
SYNC
+SV
A,,
Z 80
A,,
i2
DBE
NC
D,
WAIT r - + - - ~
A,,
GND
TSC
NC
6502
6800
READY
8080
0 -THESE PINS CONNECT DIRECT FROM LOGIC BOARD CPU SOCKET TO FIXTURE SOCKET PINS
WITHOUT 0- ARE INTERCONNECTED AS SHOWN. THESE PINS ARE NOT CONNECTED TO LOGIC BOARD CPU SOCKET.
FIGURE 1A - NOP FIXTURE SIGNATURES - FOR PROCESSORS
PROCESSOR
Z-80
8080
6502
CLOCK
PIN 21
PIN 17
PIN 37
START/STOP
5
36
25
GROUND
29
2
21
AO
30
25
uuuu
9
uuuu
A1
FFFF
31
5555
26
10
A2
32
cccc
27
11
8484
A3
33
7F7F
29
12
P763
A4
34
5H21
30
13
1U5P
AS
OAFA
35
31
14
0356
A6
36
UPFH
32
15
U759
A?
37
52F8
33
16
6F9A
A8
38
HC89
34
17
7791
A9
2H70
39
35
18
6321
A10
HPPO
40
1
19
37C5
A11
1
1293
40
20
6U28
A12
HAP?
37
4FCA
2
22
A13
3C96
38
23
4868
3
A14
24
9UP1
4
3827
39
A15
5
755P
36
25
0002
vcc
0001
0003
6800
PIN 37
25
21
9
10
11
12
13
14
15
16
17
18
19
20
22
23
24
25
The NOP instruction causes the CPU to become a
65K counter with its 16 address lines being the
outputs. If the address outputs are counted through
from all "O's" to all "1 's", then the address bus has
been exercised for all possible combinations and it
does this repeatedly. Each of the address lines Ao -
A15 have a unique signature unless there is a fault
with the processor or a trace or a device connected to
that address line. (Figure 1 A).
To obtain the signature for the address lines we
use the system clock and as in all counters (remem-
ber the CPU is now a counter) the start and stop go to
the most significant bit. Since we wish to input the
maximum number of clock pulses we start and stop
on the A 15 leading edge. (See 5* TJ Vol. 3, No. 3.)
SOME COMMON FAULT SIGNATURES
I HOW TO DEAL WITH THEM
Figure 2 shows a normal inverter chain with signatures
you might expect
Figure 3 we see a proper signature on the first
inverter's input and a "stuck at O" at the output. As
shown at points A, B & C we can have 3 failure modes.
Point A is a failed output on the inverter, Point B
would be a physical short to ground and C, a failed
inverter input