Many PLC configurations are available, even from a single vendor. But, in each of
these there are common components and concepts. The most essential components are:
Power Supply - This can be built into the PLC or be an external unit. Common
voltage levels required by the PLC (with and without the power supply) are
24Vdc, 120Vac, 220Vac.
CPU (Central Processing Unit) - This is a computer where ladder logic is stored
and processed.
I/O (Input/Output) - A number of input/output terminals must be provided so that
the PLC can monitor the process and initiate actions.
Indicator lights - These indicate the status of the PLC including power on, program
running, and a fault. These are essential when diagnosing problems.
The configuration of the PLC refers to the packaging of the components. Typical
configurations are listed below from largest to smallest as shown in Figure 3.1.
Rack - A rack is often large (up to 18” by 30” by 10”) and can hold multiple cards.
When necessary, multiple racks can be connected together. These tend to be the
highest cost, but also the most flexible and easy to maintain.
Mini - These are similar in function to PLC racks, but about half the size.
Shoebox - A compact, all-in-one unit (about the size of a shoebox) that has limited
expansion capabilities. Lower cost, and compactness make these ideal for small
applications.
Micro - These units can be as small as a deck of cards. They tend to have fixed
quantities of I/O and limited abilities, but costs will be the lowest.
Software - A software based PLC requires a computer with an interface card, but
allows the PLC to be connected to sensors and other PLCs across a network.
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