Introduction to Industrial Automation

Figure 6.26  Applied block diagram of analog output module operations (Siemens).

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  Introduction to Industrial Automation

via an optical isolator. Analog output modules generally require an external supply unit with some 

requirements in terms of current and voltage.

Typical analog devices are small motors, servo valves, DC servo motors, drivers for high power 

motors with varying speed of rotation, analog instruments, etc. The voltages and the currents for an 

analog output module are always DC signals. However, this does not mean that an AC controller of a 

motor, for example, cannot be driven from the analog output of a PLC. Such AC controllers (e.g., the 

soft starters of motors) require a varying DC signal as a control input that can be the analog output 

of a corresponding module. Thus, with the application of a varying DC output voltage (e.g., 0–10 V 

DC) in the input of the AC controller, a corresponding AC motor speed alteration can be achieved. 

In Figure 6.27a, the typical connection of analog output devices in a corresponding output module 

is presented. Some analog output modules provide the ability to supply output device with a four-

wire connection (Figure 6.27b) for greater accuracy in the definition of the applied voltage that is, in 

many cases, different from the desired one. This is achieved with two additional wires (S

+

, S

–

) that are 

connected directly to the R

L

 load. Through those, the module is able to directly measure the voltage 

of the load (like a voltmeter) and automatically apply the needed corrections. The most common rea-

son for the deviation, among the desired output value and the real applied one in the output device, 

is the voltage drop in the connecting wires and bad contacts in the terminals.