Introduction to Industrial Automation

6.9.2   I/O System Design

With the design of the I/O system of a large PLC, the application engineer needs to determine the 

local and peripheral I/O modules, their number and type per local or peripheral category, the com-

munication hardware and any other auxiliary equipment, and generally the overall I/O hardware 

Peripheral extension unit

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Figure 6.36  Peripheral expansion of the I/O hardware of a PLC, with series topology (a) and 

star topology (b).

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Basic Operating Principles of PLCs 

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needed so that the PLC is connected to the controlled process. The design of the I/O system includes 

counting I/O devices and the recording of their operational characteristics; grouping I/O devices 

based on various criteria such as operating voltage, position, operational function, etc.; incorporation 

of the I/O devices into the local or peripheral I/O system; and determination of the number and type 

of required I/O modules and the required complementary communication equipment.

The design of the I/O system begins with counting the I/O devices and examining their opera-

tion. For example, we measure the power relays, various simple coils, light indicators, signaling 

devices, control buttons, selector switches, sensors and, in general, any device that can be an input 

or output to the PLC. By taking the positions and the characteristics of the I/O devices into con-

sideration, the ability to incorporate them into similar groups is investigated, either because they 

exist on the same machine or in the same production process; because they are topographically 

close and have a similar functionality; or because they have the same technical characteristics (e.g., 

nominal operating voltage). After determining the groups with the I/O devices, the selection of 

the groups that will constitute the local I/O system and those that will constitute the peripheral 

I/O system will follow. A prerequisite for defining the local and peripheral I/O system is the 

determination of the location in which the PLC will be installed, a decision taken on the basis of 

general technical and economic criteria.

An example would be the above design steps that are necessary for the configuration of the I/O 

system of a central PLC, which will be better understood by means of a hypothetical automation 

system. Figure 6.37 presents the topographic diagram of the departments of a hypothetical indus-

trial process. The PLC will be located in the area of the first department and will be connected to 

its devices with local I/O equipment, while for connecting to the corresponding second and third 

departments, peripheral I/O equipment will be utilized. Based on the I/O devices that have been 

1st department, basic production

Motors DS = 20

Motors IR = 5

Motors S/D = 8

Buttons* = 6

Lamps* = 20

Limit switches = 20

Valve coils = 12

Photocells = 10

Proximity switches = 5

Signaling devices = 2

Analog inputs = 4

Analog outputs = 2

(*) These devices are additional over those required for handling, indicating  operation of motors 

DS = direct starting, IR = inversible rotation, S/D = star-delta starting

PLC location

Motors DS = 6

Motors S/D = 6

Lamps* = 8

Limit switches = 10

Photocells = 6

Rotational switches = 3

3rd department, packaging 

2nd department, product processing 

Motors DS = 12

Motors IR = 2

Motors S/D = 10

Limit switches = 5

Valve coils = 20

Photocells = 10

Proximity switches = 20

Signaling devices = 10

Analog inputs = 2

Analog outputs = 2

Figure 6.37  A small production procedure consisting of three departments controlled by a 

central PLC.

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  Introduction to Industrial Automation

counted in each department and the examination of their technical characteristics, a detailed 

table of local and peripheral I/O devices can be synthesized. Table 6.2 shows the I/O devices 

divided into three groups (columns), one including the local I/O devices and two including the 

peripheral I/O devices for the second and third departments, respectively. This table shows also 

the type, quantity, and technical characteristics of each device. From the technical characteristics, 

we should only emphasize the ones that influence the choice of the I/O equipment of the PLC, 

such as the nominal operating voltage of the I/O devices. Also included in the table are those 

devices that have not been counted directly from consideration of the process, but are necessarily 

imported by the automation system (circuit or program of automation) that the PLC will real-

ize. For example, the control buttons of any kind of motors do not result from counting, but are 

imported necessarily by the automation circuit, since the latter provides the possibility of manual 

control of the motors in addition to any automatic operation. The same is valid for the indication 

lights of a motor’s operation. To meet the I/O equipment requirements identified in Table 6.2, 

PLCs with similar capabilities are assumed. These capabilities of PLCs concern the RAM (> 4 K), 

Table 6.2  Local and Peripheral I/O Devices of the Industrial Production Application