Hardware Reference
In-Depth Information
is sometimes essential to minimize the time taken to correct the failure of a
PC-based instrumentation or process control system. An ability to make on-site
repairs, at least to board level, is thus highly desirable.
At first sight, the prospect of fault-finding a PC-based instrumentation or
control system can be somewhat daunting. This is especially true when those
having to carry out the repairs may be relatively unfamiliar with electronic
circuitry. However, in the author's experience, the vast majority of faults are
attributable to failure of external devices (such as sensors, cables, and connec-
tors) rather than with the board and cards themselves. Furthermore, even when
dealing with boards within the system enclosure, most faults can be detected
without recourse to sophisticated test gear.
When component rather than board level servicing has to be undertaken, it
is useful to obtain a circuit diagram and service information on the equipment
before starting work. This information will be invaluable when identifying
components and establishing their function within the system as a whole.
Certain 'stock faults' (such as chip failure) may be prevalent on some boards
and these should be known to manufacturers and their service agents. A tele-
phone enquiry, describing the symptoms and clearly stating the type and version
number of the card or board, will often save much time and effort. Furthermore,
manufacturers are usually very receptive to information which leads to improve-
ment of their products and may also be prepared to offer retrofit components
and/or circuit modifications to overcome commonly identified problems.
Test equipment
A few items of basic test gear will be required by anyone attempting to perform
fault location on bus systems. None of the basic items is particularly costly and
most will already be available in an electronics laboratory or workshop. For the
benefit of the newcomer to electronics we will briefly describe each item and
explain how it is used in the context of PC-based system fault-finding.
Multi-range meters
Multi-range meters provide either analogue or digital indications of voltage,
current, and resistance. Such instruments are usually battery-powered and are
thus eminently portable. Connection to the circuit under test is made via a pair
of test leads fitted with probes or clips. Controls and adjustments are extremely
straightforward and a typical meter layout is shown in Figure 13.19.
The following specification is typical of a modern digital multi-range meter:
DC voltage
200 mV, 2 V, 20 V, 200 V, and 1.5 kV full-scale
Accuracy
0.5%
Input resistance 10 M
AC voltage
2 V, 20 V, 200 V, and 1 kV full-scale
Accuracy
2%
Input resistance 10 M
DC current
200
µ
A, 2 mA, 20 mA, 200 mA, and 2 A full-scale
Accuracy
1%
AC current
200
µ
A, 2 mA, 20 mA, 200 mA, and 2 A full-scale
Accuracy
2%
Resistance
200
,2k
,20k
, 200 k
,2M
full-scale
Accuracy
2%
