Lesson 3 - Principles of Process Control SAQ
This lesson allows you to test yourself on the material covered in the Principles of Process Control lesson and determine the level to which you have met the learning outcomes.
Suggested Response - Q2
** What is the difference between analogue and digital output and analogue and digital communication? Give examples of each. Which category does HART fit into?
Response:
An analogue output covers a continuous range of values, whilst a digital output only outputs one of two values: either ‘on’ or ‘off’. Therefore, a flow transmitter (which has analogue output), for example, will output over a range. By comparison, a proximity switch will only output an on or off value, as it has digital output.
Analogue communication uses conventional 4-20 mA as the signalling medium. Digital communication sends information in the form of data across a network. The analogue network will thus consist of 4-20 mA wiring. The digital network will be fieldbus or profibus.
HART networking uses the 4-20 mA network (analogue communication) and transmits digital values across it by adjusting the frequency. It can thus communicate both analogue and digital values. HART can still only have one device per wire.