AEA PAKRATT PK-232, Техническое справочное руководство

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PK-232 TECHNICAL MANUAL CHAPTER 7 – TROUBLESHOOTING
PK232TM Rev. A 5/87 7-3 Page 42
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:
ADDRESS LINE A0 U1 Pin 30
DATA LINE D0 U1 Pin 14
ROM CHIP ENABLE CE* U20 Pin 20
OUTPUT ENABLE OE* U2 Pin 22
CE*/OE* TIMING U2 Pin 20, U2 Pin22
READ LINE RD* U1 Pin 21
WRITE LINE WR* U1 Pin 22
PCLK U7 Pin 20
CLOCK TRXCA U7 Pin 14
CLOCK RTXCA U7 Pin 12
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.