Why does PLC not read (4-20) directly without converting from (4-20 mA) to (1-5 volt)?
The fact that PLCs do not read 4-20 mA signals directly is due to historical and practical factors. Let’s break it down:
Historical context:
When PLCs were first developed, voltage inputs were more common and easier to work with as compared to current inputs. Many systems have maintained the use of voltage inputs.
Analog-to-digital conversion:
Analog signals are read by PLCs through the use of Analog to Digital Converters abbreviated as ADCs. Many ADCs are intended for voltage measurement, not current. It is easier to have current converted to voltage before it gets into the PLC to reduce internal circuit complexity.
Noise resistance:
Current loops such as the 4-20 mA are very useful in transmitting signals over large distances because they are immune to electromagnetic interference. However, after the signal enters the PLC, it is possible to convert it to voltage, which can minimize the noise even more.
Standardization:
It is now common to find the 1-5V range in most of the industrial applications. This standardization makes it easier to integrate different components and systems, in this case, the architecture of the building and its systems.
Flexibility:
Employing a current to voltage converter makes it easier to amplify and/or filter the current before it gets to the PLC. This may be helpful in calibration and range settings.
Cable length:
Although 4-20 mA signals can be transmitted over long distances, voltage signals are used for short distances in control panels where PLCs are usually housed.
Power considerations:
Current inputs would entail the PLC to provide power to the loop which may prove cumbersome in power management in the system.
However, today, most of the new PLCs have provisions for direct current inputs; however, the practice of converting to voltage is still widely used because of its simplicity, efficiency and compatibility with the existing systems.
Remember, in industrial automation, the changes are gradual because it is expensive and dangerous to alter the existing systems. This is why you still see this conversion step in many PLC setups, even though direct current input is technically possible.
Refer the below link for how a PLC reads the data from field transmitters?