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signal lagging

Guys,

 

I have a LS module that is lagging.

 

 

A valve close indication comes into my SIS logic module.  We had someone in the field, someone in the I/O room, and someone at the DCS.  As soon as the valve closed, the LS IS DI CHARM lit up.  However, in control studio (online viewing of SIS module) showed the parameter value stayed at (0) for a count of between 2~3 seconds.  When watching the CHARM in diagnose, the change of state can be seen immediately.  We have the module executing at 50ms. 

 

The CHARM in question also is referenced to our BPCS.  There is an SFC on the SZ controller that references the same LS CHARM. 

 

 

I would like to know how it is possible to have this long a delay in an SIS module.

Petrisky

• 3 Aug 2021 8:22 PM
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3 Replies

• Tadeu Batista

  

  • 4 Aug 2021 1:22 PM

  Hello Petrisky,
  
  If I understood correct, a person monitoring the physical LS CHARM notice an immediate change in the LED when a valve changes it's state, while another person in the control room, monitoring the SIS Control Module open in Control Studio notice a delay in the value change. The same person monitoring the SIS Control Module checks the DeltaV Diagnostics, and also notice a difference between the update in Control Studio vs the update in DeltaV Diagnostics.
  
  The first thing I'd like to point out, is there's a delay between the immediate transition noticed in the physical LS CHARM, and what is presented in Control Studio or DeltaV Diagnostics. You may notice difference between Diagnostics and Control Studio update rate, but neither is "immediate".
  
  Apart from the visual observation, is there any other indication that the LS module is lagging?
  
  Rgs,

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• Tadeu Batista

  

  • 4 Aug 2021 1:22 PM

  Hello Petrisky,
  
  If I understood correct, a person monitoring the physical LS CHARM notice an immediate change in the LED when a valve changes it's state, while another person in the control room, monitoring the SIS Control Module open in Control Studio notice a delay in the value change. The same person monitoring the SIS Control Module checks the DeltaV Diagnostics, and also notice a difference between the update in Control Studio vs the update in DeltaV Diagnostics.
  
  The first thing I'd like to point out, is there's a delay between the immediate transition noticed in the physical LS CHARM, and what is presented in Control Studio or DeltaV Diagnostics. You may notice difference between Diagnostics and Control Studio update rate, but neither is "immediate".
  
  Apart from the visual observation, is there any other indication that the LS module is lagging?
  
  Rgs,

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• Andre Dicaire

  

  • 4 Aug 2021 11:21 PM

  As Tadeu points out, the HMI will show a lag. The SIS Module in the LS must execute to "see" the input change state. This module then reports its data to the SZ controller which then passes it on to the HMI and the HMI display is updated. The HMI requests data to be reported every 1 second, by default. The SZ and SIS Module will serve the data at this requested rate. The tasks are asynchronous so yes there will be some latency.
  
  If the input condition triggers an alarm configured in the SIS module, this event will be time stamped and you can use this to see the latency between the CHARM LED and module execution/read of the channel. If there is no alarm or event in the SIS Module, you can add one temporarily or add a test module for this purpose, if you are not on a commissioned system.
  
  The CSLS scans its local CHARMS every 50 ms and publishes them to the Local Safety network for all CSLS's to see. It then executes its SIS modules, which may run at 50 ms, 100 ms or 150ms depending on the overall complexity. The CHARM will "see" the input change be scanned within 50 ms and publish the input. The SIS Module will see this on its next scan, so the SIS module execution scan time will dictate the possible latency. The output scan to drive any output charms would follow the module execution within 50 ms. Screw to screw time would take all these scan times into consideration.
  
  The HMI however has normal unsolicited exception reporting that can present as 2 - 3 seconds due to the SIS module to SZ and SZ to HMI updates.
  
  Note that if the SZ controller is referencing a CHARM, the CHARM can be enabled to report to the SZ controller at a 100 ms update rate as an IO signal, rather than an SIS Module parameter. This allows the SZ to use the Input signal for control in the BPCS, or in your case an SFC to drive a decision.

  Andre Dicaire

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