Introduction to Industrial Automation

6.9   PLC Expansion and I/O Configuration

Small PLCs with a compact structure have a predetermined set of inputs and outputs, which is 

not subject to any kind of change. The opposite is true in the case of large PLCs, whose CPU has 

the ability to control a few hundred (or thousands) of inputs or outputs. In the latter case, the I/O 

system (which is the PLC hardware including any kind of I/O modules) is not fully predetermined 

by the manufacturer; instead, the engineer can configure it according to the application’s needs. 

The need for a large number of I/Os served from a CPU requires the extension of the PLC in 

additional racks of I/O modules. Multiple possibilities of expandability for PLCs of this category 

include the possibility of local or remote I/O modules, the variable number of I/O modules that 

can be supported in both cases, as well as the selection of I/O modules with a different number 

of I/O channels in each, which define the problem of proper design and configuration of the I/O 

system of a central PLC.

Bit

7

6

5

4

3

2

1

0

Absolute

address

Byte 0 

0000

Byte 1

0001

Byte 2

0010

…

…

I2.5

I1.1

Input mapping memory