Question

 

Consider the following inputs and outputs associated with a DCS system. It is interesting to note that EXACTLY the same could be applicable to a SCADA system (although the SCADA system could use either a dedicated RTU or else a PLC).

Break the following signals up (either going to, or away from the RTU-type devices) and indicate which are:

i) digital inputs, ii) digital outputs, iii) analog inputs and iv) analog outputs:

18.1 ( 1 mark) START signal

18.2 ( 1 mark)

Speed control signal of a VSD (going to the VSD – Please think in terms of a SPEED UP button and a SLOW DOWN button).

18.3( 1 mark) An I/P (current to pressure) transducer for a butterfly valve (going to the valve).

18.4( 1 mark) Solenoid Valve (operating something in the field).

18.5( 1 mark) A light.

18.6( 1 mark) An electric motor’s contactor (which will be used to start a motor in the field)

18.7( 1 mark) An RTD temperature probe.

Note – This question is mandatory. It should be answered and students are expected to get this question completely right to be assessed as competent with a score for this module.

Answer 1

Solution: The following signals can be categorized as either digital input / output or Analog Input/output based on the type of signal applied.

18.1) START signal is a digital input to the DCS system because the status of START is Start/Not Start only two outcomes.

18.2) SLOW UP and SLOW DOWN are two buttons i.e digital inputs to control the speed of the motor.

18.3) I/P from the Pressure transducer (Current value) is an input Analog Value to the DCS system because it has range of values from lower bound to upper bound to be measured by the transducer.

18.4) Solenoid value is a digital output to control the solenoid from DCS system. It’s output is either ON/OFF.

18.5) A light indication is also digital output because the DCS will switch the light ON/OFF.

18.6) Electric motor contactor is operated on the analog supply voltage so it is an output analog signal to control the motor contactor.

18.7) RTD temperature probe gives analog value. So it is analog input to DCS system.