Experimental Physics and
Industrial Control System

Mark Rivers <[email protected]> · Thu, 6 Sep 2018 13:42:53 +0000

Hi Emilio,

> I found two ways of achieving the goal:
>    1. Set pollMsec  to a value less or equal to 0 and add an extra record for each register that has periodic scan and writes to MODBUS_READ (disable button would disable this record)
>    2. Use absolute addressing (modbusStartAddress=-1) and change all the records to be periodic scanned

> The downside of the first solution is that I need to have double records for each modbus register.

That is not quite true.  You need an additional record for each contiguous block of registers that are read by a single Modbus asyn driver, not one record for each register.  In many cases there are just a few such blocks of registers that need to be read, so there are only a few new records required.

> The downside of the second solution is that this might be a feature of absolute addressing that might change, as documentation
> is not explicit on the point that absolute addressing mode disables the readPoller thread and calls doModbusIO on record's demand.

I think you can be certain that drivers using absolute addressing will never poll.  If they did poll they would need to poll the entire 64KB modus address space, which would require multiple read operations and be very slow.  This is because the driver cannot know what Modbus addresses will actually be read when a record processes.  Furthermore it would almost always result in Modbus errors because it would attempt to read Modbus addresses that are invalid for a particular piece of hardware.

So I think either method will work.  Method 1) may require fewer periodically scanned records to change their SCAN rates when you want to do a read.

Mark

-----Original Message-----
From: [email protected] <[email protected]> On Behalf Of [email protected]
Sent: Thursday, September 6, 2018 4:16 AM
To: [email protected]
Subject: Modbus Polling disable/enable

Hello Tech-talk

I'm currently working in a module which relies on the modbus module and I need to create a button in the user

interface that allows to disable the read polling.

As it is working now, readPoller still reads the values even if the associated record is disabled.

I found two ways of achieving the goal:

    1. Set pollMsec  to a value less or equal to 0 and add an extra record for each register that has periodic scan and writes to MODBUS_READ (disable button would disable this record)

    2. Use absolute addressing (modbusStartAddress=-1) and change all the records to be periodic scanned

The downside of the first solution is that I need to have double records for each modbus register.

The downside of the second solution is that this might be a feature of absolute addressing that might change, as documentation

is not explicit on the point that absolute addressing mode disables the readPoller thread and calls doModbusIO on record's demand.

My question is:  what is the recommended way?

Best regards and thank you,

Emilio

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Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System