Introduction to Industrial Automation

Figure 6.34  Two possible ways for connecting remote I/O devices in a PLC: through power 

lines (a) and through communication processors (b).

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Basic Operating Principles of PLCs 

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235

The I/O hardware of a PLC is designated as “local” when the corresponding I/O modules are 

located in the immediate neighborhood of the CPU. The distance that defines the designation “local 

I/O hardware” is determined by the distance of the interconnecting cable between the CPU and 

the I/O hardware, generally defined in the order of a few meters (20–50 m). When it is desirable to 

place the I/O modules at a relatively greater distance for the reasons explained above, then the I/O 

hardware is designated as “peripheral” and special communication equipment is required for the 

operation of the remote modules. The spacing in the placing of the peripheral I/O hardware is in the 

order of a few kilometers (1–10 Km). Figure 6.35 presents the characteristics and differences between 

the local and peripheral I/O hardware, while it is obvious that the local I/O modules are no different 

PS

C

P

U

1st local extension unit

2nd local extension unit

1st peripheral extension unit

2nd peripheral extension unit

C

P

C

P

C

P

C

P

PS

C

P

PS

C

P

C

P

Mounting base

of I/O hardware

Mounting base

of I/O hardware

Mounting base

of I/O hardware

Local

communication

modules  

Local I/O modules

Local interconnecting cable

Local I/O extension system

Peripheral

interconnecting

cable  

Peripheral

communication

modules  

Peripheral I/O extension system

Local extension of

a peripheral unit 

Peripheral I/O modules

Central PLC

with I/O extension  

Last local extension

unit up to a few meters

Last peripheral

extension unit up to

 a few hundred meters