DCS Alarms & Pump signal double question!!

Guys,

Is a DCS alarm a Digital Input? For instance, if a PT reaches a High alarm level of 5 bar or say 18mA, is this set point then converted into a digital input to the system/graphics/HMIs where ever it is displayed or logged?

Also... if the DCS is receiving multiple pump status signals from the MCC via profibus, is each status/command (such as start, stop, running etc) its own input, or is the profibus signal itself one input? as it is only terminating into one card?? im a little confused by this. I would imagine it is multiple seperate signals that are merely sharing the same comms cable / technology to get to where they need to be.

Thankyou

Anthony
 
if a PT reaches a High alarm level of 5 bar or say 18mA, is this set point then converted into a digital input to the system/graphics/HMIs where ever it is displayed or logged?

No.

The transmitter sends its process variable (PV) to the DCS via analog (4-20mA) or digital (FF, Profibus, Modbus) protocol. The value goes into a process variable tag. The tag also has properties, one of which is the engineering units, bar. The tag can be used anywhere in the PLC/DCS or by digital communication to the HMI/graphics.

For an alarm, a comparator function in the control logic reads some tagged value (on its input), compares value to a setpoint (preconfigured value, or some other tagged variable), and if the comparison meets alarm conditions then the alarm's digital output tag changes state to reflect the alarm state.

The alarm tag is monitored by the HMI via some digital protocol. An HMI interprets and displays the alarm in some fashion, in an alarm list, as a flashing widget, or displaying an alarm message.

This all involves internal software functions; no I/O hardware is involved except the original analog input.

If an alarm annunciator is needed, then I/O is needed; control logic can change the state of a hardware discrete output to drive an annunciator (light or horn).
 
profibus, is each status/command (such as start, stop, running etc) its own input, or is the profibus signal itself one input? as it is only terminating into one card?
A master asks for data, the slave provides the data. Then the master moves onto the next slave and asks for data. If the field device is a final control element, then the master will write data to it, like a position value written to a positioner on a valve.

All the data is moving 'back and forth' on the cabling with only one comm port on each device (redundancy is a whole different game).

See page 12 & 13 on the Profibus summary posted by ISA for a description of data exchange on the bus.

https://www.isa.org/getmedia/f6e94b...845/Basics-of-profibus-operation-chapter1.pdf
 
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