Experimental Physics and
Industrial Control System

"Kasemir, Kay" <[email protected]> · Wed, 24 Aug 2011 17:47:30 -0400

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

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System