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PK-232 TECHNICAL MANUAL
Oscillator and Reset Circuits
If no LEDs are illuminated during the startup or reset cycle:
o
Verify that system clock crystal oscillator Y1 is operating and that a square wave sig-
nal (4 MHz, 0 to +5 volts) exists at U1 pin 6. The clock signal should be a mode-
rately-distorted square wave.
o
Verfiy that baud-rate generator crystal oscillator Y2 is operating and that a square
wave signal (2.4576 MHz, 0 to +5 volts) exists at U7 pin 29.
o
Verify line receiver U19 and diodes D1 and D2 in the reset timer circuits.
Digital Logic Lines
All logic circuits operate at standard TTL levels. "Low" is less than +0.8 V; "High" is great-
er than +2.4 volts. All digital inputs and outputs alternate between these two levels.
o
If logic signals are alternating between 0 and perhaps 1 volt, there is a problem,
usually a short circuit.
o
Do not mistake switching transients on digital logic lines for improper operation –
such as transients appear as ringing anf other distortions.
o
Verify with the oscilloscope and the waveform diagrams shown in APPENDIX D that
there is activity on the following lines:
Each of these lines should show activity. If any line is inactive, this is a sign of trouble.
Logic lines that do not show activity can often be traced to a short circuit on the printed
circuit board.
short circuits an the address and data lines can also appear as lack of activity on the con-
trol bus lines, especially device select lines.
o
Verify each of the 16 address and 8 data lines for activity. Any lines showing a lack
of activity are not operating properly.
o
Remove all memory chips if you suspect problems with address or data lines. Each
address and data line will now show a distinct pattern. The address lines should be
(possibly distorted) square waves whose periods increase by a fyctor of two on suc-
cessive lines as you move line by line from A0 to A15.
o
Use a low-voltage, low-current test instrument if you decide to use an ohmmeter to
verify short-circuited lines. Most modern DVMs are adequate for such tests. Remove
any integrated circuits connected to the lines being measured if in doubt.
o
Verify the high-density areas of the printed circuit board for the problem if you sus-
pect a short circuit. In most cases the short will be found there.
PK232TM Rev. A 5/87
ADDRESS LINE A0
DATA LINE D0
ROM CHIP ENABLE CE*
OUTPUT ENABLE OE*
CE*/OE* TIMING
READ LINE RD*
WRITE LINE WR*
PCLK
CLOCK TRXCA
CLOCK RTXCA
U1 Pin 30
U1 Pin 14
U20 Pin 20
U2 Pin 22
U2 Pin 20, U2 Pin22
U1 Pin 21
U1 Pin 22
U7 Pin 20
U7 Pin 14
U7 Pin 12
7-3
CHAPTER 7 – TROUBLESHOOTING
Page 42
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Table of Contents
Troubleshooting
