Hardware Reference
In-Depth Information
Photo 13.3
A drive power connector makes a convenient test point for
measuring the
+
12 V and
+
5 V power rails
I/O lines however, in situations where logic levels are continuously changing,
a multi-meter cannot provide a reliable indication of the state of a line.
Where logic levels do remain static for several seconds, the multi-range
meter may be used on the DC voltage ranges to sense the presence of logic
0 or 1 states according to the following table which gives the conventional volt-
age levels associated with TTL logic:
Logic level
Voltage present (V)
1
>
2.0
<
0.8
0
indeterminate
0.8 to 2.0
It should be noted that an 'indeterminate' logic level may result from a
tri-state condition in which bus drivers are simultaneously in a high impedance
state. Modern high-impedance instruments will usually produce a misleading
fluctuating indication in such circumstances and this can sometimes be confused
with an actively pulsing bus line.
Logic probes
The simplest and most convenient method of tracing logic states involves the use
of a logic probe rather than a multi-range meter. This invaluable tool comprises
a hand-held probe fitted with LEDs to indicate the logical state of its probe tip.
Unlike multi-range meters, logic probes can generally distinguish between
lines which are actively pulsing and those which are in a permanently tri-state
