Radio Shack TRS-80 Service Manual page 79

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5.4.2.1 E and Q
Check for the presence of E and Q. These signals should
look like the ones in Figure 5-4. If either clock is
missing, the CPU will not operate. Also make sure that the
clocks have good logic levels. The E clock is more
demanding than the Q clock, and requires a high logic level
of at least 4 volts. These clocks are produced by the SAM
chip (U22). Eis sent to both PIAs (U2 and U7), the CPU
(U23), the NOR gate CU10) and the edge connector. The Q
clock goes only to the processor and the edge connector.
Both clocks have small value capacitors on them to prevent
harmonic radiation. If any of these components connected to
these clock lines become shorted, it is likely to reduce the
amplitude of the clock signal.
5.4.2.2 Interrupt/Halt/Reset
After power up, without a cartridge installed and without
executing any program, the NMI* line (pin 2), the FIRQ* line
(pin 4), the HALT* line (pin 40), and the RESET* line (pin
34) of the processor should all be high. In addition, the
IRQ* line (pin 3) should have a 60 Hz pulse on it Cit should
have a short low-going pulse about every 16 msec). If any
of these lines are stuck low, you have found the problem.
Replace the processor. If the condition still exists, find
out what is pulling the line low, and repair it. If the IRQ
line is low, the PIA U7 could be bad, or a problem could
exist in the address decoding section. (Once the PIA
generates an interrupt, the processor must access the PIA to
clear the interrupt.)
5.4.2.3 Address/Data Lines
It is rather difficult to define exact patterns of address
and data lines on the CPU as these are dependent upon which
address is being accessed, and the data to be read or
written. All address and data lines should be capable of
activity, however, and the output levels should be valid
TTL. Figures 5-13 and 5-14 show typical address and data
patterns. Note that some of the address lines may not show
activity when the processor is in a loop. However, by
depressing the RESET button while scoping the address lines,
at least momentary activity should be displayed. Another
clue is that typically (although certainly not always) the
address lines should act more or less like a frequency
divider -- that is, the frequency of activity on Al should
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