Experimental Physics and
Industrial Control System

David Dudley <[email protected]> · Wed, 24 Aug 2011 19:49:54 -0400

I have a fix for that that I've been testing.

I'll generate a patch and send it to you.

The solution is that in the initialization portion of the driver, if the tag
is just a boolean tag by itself, set the mask to 255 so that all the bits
get set and reset.

David

On 8/24/11 5:47 PM, "Kasemir, Kay" <[email protected]> wrote:

> Hi:
> 
> Martin Smith @ APS has found a quirk in the way the ether_ip driver for
> ControlLogix PLCs and the PLC handle 'BOOL' tags that might be of interest
> to those that use the driver.
> 
> The short summary is that you cannot use BI or BO records with plain BOOL
> tags on the PLC.
> 
> The binary record support in the ether_ip driver is meant for efficient
> handling of _bits_. It can read and write bits in a BOOL[], DINT, DINT[]
> tag. In those cases it reads or writes the whole numbers over the network,
> then dispatches the individual bits to records.
> 
> For example, say record A is attached to BOOL[0] or DINT.0,
> record B is attached to BOOL[1] or DINT.1 and so on.
> Meaning the records are attached to the bits of a DINT, DINT[] or BOOL[].
> That all works fine: The driver reads or writes the whole DINT or DINT array
> or BOOL array and performs the appropriate bit fiddling to interface the
> bits to records.
> 
> But if you attach a binary record to a plain, scalar BOOL tag on the PLC,
> the following happens:
> The driver still wants to read or write individual bits.
> A BO record with OUT="@plc MyBOOLTag" will read or write 0 and 1 values.
> The PLC, however, returns 0 for 'false' and 255 for 'true'.
> 
> The mismatch between 1 and 255 is the issue at the root of the problem that
> Martin found in a setup:
> 
> A BO record would write a "1". The PLC now actually sets the BOOL tag to
> 255, and the driver from now on sees 255 as a value.
> If the driver is instructed to write a "0" for the BO record, it will write
> 254, i.e. the received value of 255 with the first bit set to 0.
> The PLC, however, considers 254!=0 also as 'true' and keeps the BOOL tag at
> 255.
> 
> Longer summary:
> Use binary records (BI, BO, MBBI, MBBO) with BOOL[], DINT or DINT[].
> Not with individual BOOL tags, which - even if it were supported - would be
> slower than array transfers anyway.
> 
> Thanks,
> Kay
> 
> 
> 

David

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"... The Crossbow Project. There's No Defense Like a Good Offense. " - Real
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Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System