TINY-2/MICRO-2 Technical Ref., 2. Ed 1989
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Be very careful about shorting pins on ICs when applying meter or scope probes to the board. It is
a good idea to attach a secure ground lead to the meter or scope, one that won't accidentally
short across components on the board. A good place to pick up this ground is on the ground bus
that completely encircles the perimeter of the board. Avoid connecting in the area where power
leads run to and from the power switch to avoid the possibility of a short.
Loopback testing
Loopback testing consists of having the transmit section of the packet controller communicate with
the receive section. If this procedure is successful, the correct operation of both transmit and re-
ceive sections can be verified. If data is not successfully passed, clues to the malfunction may be
obtained.
There are two types of loopback testing, analog and digital. For both types of test, the command
FULLDUP must be ON and some type of connection must be made between the transmit output
and the receive input. Once the proper arrangements are made, testing consists of attempting to
pass information from the transmit section to the receive section of the packet controller. One
method is to send data to and from the PMS (if the PMS option is installed) by connecting to MYP-
CALL. You may also connect to MYCALL and send and receive from the terminal, or send UI frames
and monitor them. Be sure to undo all the loopback steps once the testing is completed.
Analog loopback
Remove JPR to place the input circuit in a high impedance state. Install a homemade jumper be-
tween pins 1 and 4 of the radio connector J2. Send UI frames and monitor for their reception or
try a connection as described above. The output level trimpot R12 may have to be turned to near
maximum to allow the modem to hear itself. Failure of the analog loopback test does not necessa-
rily mean there is a modem problem since the transmit circuits and receive circuits are not opti-
mized for direct connection.
Digital loopback
Move the JP L jumper to 'LOOP' and attempt to pass UI frames or connect. Digital loopback is very
reliable and should work every time if there is no problem in the digital portion of the board.
General Tests
The TINY-2/MICRO-2 has status LEDs which are powerful tools in troubleshooting. Many suspected
problems may be disproved by the following simple tests. Connect a power source to the TINY-
2/MICRO-2 and a Voltmeter (15 Volt scale) to pin 2 of the RS-232 connector. Do not connect any
computer or radio cables. Turn on the TINY-2/MICRO-2. Observe the LEDs. If the TINY-2/MICRO-2
powers up with the PWR, CON and STA LEDs lit, followed by the CON and STA LEDs extinguishing
a second or so later, and the Voltmeter shows a lot of activity for about ½ second, the TINY-2/
MICRO-2 digital section is operating and attempting to send a sign on message out the serial port.
Step 1: Obvious Problems
Look for any unusual physical symptoms. Is the unit drawing excessive current? Are any compo-
nents discolored? Does something smell burnt? Do any of the parts seems excessively warm? In
general, their normal temperature (NMOS) will be quite warm to the touch. CMOS parts should not
show any heating.
