TINY-2/MICRO-2 Technical Ref., 2. Ed 1989
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TROUBLESHOOTING
WARNING: Never remove or insert an IC with power on!
The PacComm packet controller is a complex piece of electronic equipment. Servicing must be ap-
proached in a logical manner. The best preparation for troubleshooting is to study the detailed
hardware description above. While it is not possible to present all possible problems, symptoms
and probable cures, this section of the manual will give direction to troubleshooting based on our
experience.
Press all socketed ICs into their socket along each edge. If there is any sign of corrosion on any IC
pins, remove each IC, one at a time, and burnish the pins. Often simply removing and reinstalling
the ICs will cure intermittent problems caused by vibration or humidity affecting the IC sockets. In-
spect all solder joints for a smooth shiny appearance. A dull gray appearance may be an indication
of a crystallized solder joint. Use a small soldering iron designed for printed circuit work and re-
solder the connection.
Remember that all the logic circuits operate at standard TTL levels (a "low" is less than plus 0.4 V
and a "high" is greater than plus 2.4 Volts), and all digital inputs and outputs switch between
these two levels. Thus, if you see logic signals switching between 0 and, say, 1 V, you can be sure
there is a problem (usually a short). On the other hand, do not mistake switching transients on
digital logic lines for improper operation - these show up as ringing and other distortions.
Verify that there is activity on the control bus READ and WRITE lines, the CHIP ENABLE lines on
the memories (U2 and U8 pins 20), the IOREQ line on CPU (U1) pin 20 and the INT line on CPU
(U1) pin 16. Each of these lines should show activity, and if any line is quiet this is a sign of trou-
ble. Address and data line problems may also show up as lack of activity on the control bus lines,
especially the chip selects. Check each of the 16 address and 8 data lines for activity. Any lines
showing a lack of activity are not operating properly.
If you suspect problems with address or data lines, try removing all the memory chips. Each ad-
dress and data line will now show a distinct pattern. The address lines should be (possibly distort-
ed) square waves whose periods increase by a factor of two on successive lines as you step line by
line from A0 to A15.
Logic lines that show no activity may often be traced to a short on the pc board, probably due to a
defective solder joint. Follow the instructions above for inspecting solder joints. If you decide to
use an Ohmmeter to check for shorted lines, use a low voltage/low current test instrument. (Most
modem DVMs are fine for this.) If in doubt, remove any ICs connected to the lines you are measur-
ing. If you suspect a short, check the high density areas of the PC board for the problem. In most
cases the short will be found there.
If an oscilloscope is not available, then a Voltmeter can help you find serious difficulty on digital
signal and address lines. Set the meter for the scale where a full 5 Volts can be displayed. A read-
ing of 1.5 to 3.5 volts indicates some varying digital signal. A reading of over 3.5 V or under 1.5 V
is most likely an indication of trouble. A very low, or very high voltage reading is a sign of no activ-
ity, or open and shorted circuits. If you cannot pinpoint the trouble using this set of guidelines,
then you must either obtain an oscilloscope, or send the board back to PacComm for repair.
If you have an oscilloscope available, the trouble shooting is much easier! The checks made above
still apply, but now you can "see" trouble on the oscilloscope waveforms. If you see a 'stair-step-
ped' waveform on two different pins, then it is likely they are shorted together. Also, square waves
of half amplitude, or lines with no activity are dead giveaways on the scope. The scope also allows
you to trace the analog sections of the device, including the modem.
