RE: Substituting every period scan in EPICS MODBUS by I/O Interrupt

[Mark Rivers via Tech-talk <tech-talk at aps.anl.gov>]

Ø  Does this mean that, whatever the modbus device is that i am communicating, i can always substitute my 'field(SCAN, "1 Second")' to 'field(SCAN, "I/O Intr")' and go out unpunished?

 

Yes, that is true.  This assumes that you are not using the “absolute” addressing option, because in that case there is no poller thread.  When you are not using absolute addressing then the poller thread is constantly reading a block of Modbus registers.  If your record has SCAN=1 second, for example, then it calls the driver and retrieves the most recent poll value.  It does not cause a Modbus transaction to occur.  If you use SCAN=I/O Intr then your record will only process when the driver finds that the value has changed.  This will be much more efficient for bits and registers that rarely change state.

 

Ø  Will this work against me in the future?

 

No, it is the recommended way to run.

 

Mark

 

 

 

From: Tech-talk <tech-talk-bounces at aps.anl.gov> On Behalf Of Marco A. Barra Montevechi Filho via Tech-talk
Sent: Monday, July 25, 2022 4:40 PM
To: tech-talk at aps.anl.gov
Subject: Substituting every period scan in EPICS MODBUS by I/O Interrupt

 

Good evening for all.

I was reading the EPICS MODBUS documentation after realising that i had misspelled a SCAN field in my modbus application and came across the following:

> modbus supports pseudo-interrupt mode on inputs. The poller thread calls device support when there is a new value, so the EPICS records can have >SCAN=I/O Intr and do not need to periodically read the inputs. This is more efficient.

Does this mean that, whatever the modbus device is that i am communicating, i can always substitute my 'field(SCAN, "1 Second")' to 'field(SCAN, "I/O Intr")' and go out unpunished? Will this work against me in the future?

Thanks for any tips.

Marco.

 

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