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I know that PLC devices must have RTOS such as VxWorks, but is the same true with HMI devices that connect with PLC?

datum
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  • I don't know a single one of those acronyms from your question. Sorry – Dan Jan 03 '19 at 23:55
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    @Dan, PLC Programmable Logic Controller, brains of hardware devices(CNC machines, coffee machines...); RTOS Real-Time operating system, OS with guaranteed scheduling times(VxWorks is a proprietary RTOS); HMI Human to Machine Interface, a hardware version of a UI. –  Jan 04 '19 at 00:14

2 Answers2

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This depends on the hardware of the HMI. PLCs use RTOSs for the compactness and guaranteed latency. Typical HMIs are delayed by user input and therefore, do not need the strict timing provided by an RTOS, unless the hardware is not capable of running a full OS.

A full OS can communicate with a PLC.

  • I see. By that same logic SCADA systems do not need to be on a RTOS as well. Would that be correct? – datum Jan 04 '19 at 00:35
  • If there are **any** safety concerns, such as multiple coordinated PLCs with safety stops, use an RTOS. If you are only passively monitoring jobs and conditions and starting jobs, there is no need for an RTOS. If logging needs to be synchronized use an RTOS. –  Jan 04 '19 at 01:11
  • @datum, To more directly answer your question: Most SCADA systems are implemented on Linux or Windows. Critical systems are implemented redundantly with watchdog processes in RTOSs. It depends on your level of fault tolerance within the system. –  Jan 04 '19 at 02:49
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I've never seen a true Real-Time HMI, watchdogs are common for system critical processes. There's almost always a physical ESD button located within operator reach as well. For my money, an RTOS is unnecessary whenever you have human interaction through a display, simply based on the time it takes a human to move.

Michael White
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  • Exor advertises their HMI's to have a RT Linux distro on them. Would you consider this not a true RTOS? – datum Jan 21 '19 at 00:23
  • I guess I would concede that point, I've never encountered an Exor panel in the field. Although I have run across a few GE and AB panels that have real-time like IO. – Michael White Mar 27 '20 at 02:50