(*) The voltage 230 V AC refers to the nominal operation voltage of the relay coil feeding the corre-
Basic Operating Principles of PLCs
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the local or peripheral expansions, the maximum number of inputs or outputs that they can cover,
their specific control and communication modules that are available, etc. It is also assumed that
the PLCs have I/O modules with different numbers of inputs or outputs per module (e.g., 4, 8, 16,
and 32, with I or O per module), both digital and analog ones.
The next design step for I/O system configuration is the classification of each I/O point (I/O
device) per group (column) in Table 6.2. The various I/O points of Table 6.2 are classified according
to whether they are digital or analog I/Os, the level of the nominal operating voltage (230 V AC, 24
V DC, etc.), and the type of point (input or output). The result of such a classification is presented
in Table 6.3, from which it is possible to precisely determine the required I/O equipment, namely
the I/O modules needed, the mounting racks of the modules in the local and peripheral system, the
communication modules, the power supplies, etc. The existence of I/O modules with different I/O
densities, allows for the adoption of many alternative solutions, from which the most economical
and functional should be selected. An additional characteristic from the various PLC manufactur-
ers is the number of I/O modules that could be placed on an extension hardware rack. Based on the
module capacity per rack and considering the inevitable occupation of positions by power supplies
and communication modules, the required expansion units in both the local and the peripheral I/O
system can be determined. Since it is beyond the scope of this book to provide a detailed description
of all the alternatives, it will be assumed only digital I/O modules with a density of 16 inputs or
outputs per module, analog I/O modules with a density of 4 analog inputs or outputs per module,
and expansion bases or racks with a capacity of 8 modules per base, will be assumed. Based on
these data, as well as the data in Table 6.3, we can easily estimate that the I/O modules presented
in Table 6.4 are required. Since the 24 V DC digital inputs are only 6 in the local I/O system, a
special input module with 8 digital inputs is selected in this case. The final equipment specified in
Table 6.4 is not the optimal one. Several combinations of modules with different I/O densities can
be made. For example, a total of fourteen 24 V DC digital outputs to the local I/O system will
remain unused for the future, a number that can be considered excessive. Thus, it would be pos-
sible to select 2 modules with 16 outputs per module, and 1 module with 4 outputs, so that with
36 total outputs it could more economically cover the need for 34 outputs (2 digital outputs for
future use). Also, in the peripheral I/O system of the third department, the one expansion base will
be almost empty, since it will contain only two I/O modules. A different choice of the I/O density
of the modules may provide a more functional design. Figure 6.38 shows the electrical diagram of
the interconnection of all the I/O equipment, including the central PLC. It should be highlighted
that the specific configuration of the I/O equipment corresponds to the hypothetical choice made
for the I/O density per unit. Different density options will give us different configurations. The
examination of all possible configurations and the search for the most economical of them is now
taking the form of a techno-economical study that the engineer should be able to carry out. At this
point the design configuration of the I/O system for this example is finished.
The above I/O system design, but also any similar one for any other industrial application,
should be made in relation to the existing PLC systems in the market. This means that the design
engineer should always be updated on the existing PLCs and their corresponding size and capa-
bilities, so that during the selection study, the final design will converge towards the most eco-
nomical PLC selection. However, it is obvious that, independently of the final selected PLC, the
final design cannot have exactly the required size of the controlled industrial process. In general,
there will always be some redundant equipment, either I/O modules or expansion slot positions,
that can be accepted as further availability for future use.
Apart from the cost comparison between the various PLCs for choosing the most cost-effective
one, one must account for the cost of the equipment required to install the PLC. Both the PLC’s
