Introduction to Industrial Automation

Figure 6.23  Applied block diagram of an analog input module operation (Siemens).

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Basic Operating Principles of PLCs 

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functional block diagram of a real analog input module with eight input channels is presented, 

while the complexity of the design with respect to the digital input module is straightforward. 

The overall functionality of the module is orchestrated by its microprocessor, which checks the 

operation of all the related components, like the multiplexer, the A/D converter, and the memory. 

The multiplexer and the A/D converter are electrically isolated from the rest of the electronic 

units. As depicted i
into two parts, totally electrically isolated, with analog signals on one side and digital ones on 

the other, so that the microprocessor of the module, as well as the central processing unit of the 

PLC, can be protected. The communication buses among the module and the PLC are utilized 

for the transfer of data, addresses, and control signals. The time duration of one operational cycle 

for the analog input module is independent of the scanning time of the PLC. The time needed for 

the conversion of an analog input, from the A/D converter, is dependent directly on the conver-

sion method (e.g., a successive-approximation or integrating method). Since the conversion of 

the analog signals takes place in a serial manner in a one-by-one channel, the overall operational 

cycle time is the summation of the corresponding conversion times of all the active input chan-

nels, as shown i

μ

s, 

the operational cycle of a module with eight analog input channels will last less than 1 

μ

s, a time 

that is much less when compared with the scanning time for most of the commonly found real-

life applications and the usual scanning times of PLCs.

In the modules of analog inputs, depending on the manufacturer and the model, various levels of 

resolution can be found that are a direct measurement of the accuracy in the conversion of the ana-

log signals. The resolution of an A/D converter expresses the smallest difference that this converter 

is able to detect, with respect to the varying input signal, and is related to the number of bits of the 

binary word that represents the digital value of the analog signal. Since the number of bits has a 

direct connection to the maximum number and the size of steps into the range of variation of the 

input signal, the determination of the desired resolution for an A/D converter is a very important 

factor for the selection of the corresponding hardware. An A/D converter that utilizes data words 

of 8 bits will have a resolution of 1 step in 256 steps of the full-scale variation of the input signal. 

Conversion time 

for Channel 1

Total time of

operation cycle

Conversion time 

for Channel 2

Conversion time 

for Channel N

Conversion time

for channel 1

Conversion time

for channel 2

Conversion time

for channel N