hello............,

at the moment i am studying a step7 project and i came up to something that i don't understand.

in an fc piw500 is moved to mw500.

after that the MSB and LSB are swobbed.

but after that they use m500.0, m500.1, m500.3, m500.7 and m501.1  some as 'normaly open'  and some as 'normaly closed'    see attachment

later on they use m501.3 as an individual bool

anyone an idea what this is

gr gerard

Hello gerard;

The only thing I can think of is a Big Endian/Little Endian problem when exchanging data with machines (your PLC and the transmitter) that support different formats of byte ordering (for example, if the transmitter is microprocessor based, the byte order sent to the PLC is reversed compared to what the PLC is expecting).

See this Wikipedia explanation for technical details:

https://en.wikipedia.org/wiki/Endianness

It happens often that data sent over Modbus RTU has to be byte swapped to allow the PLC to make sense of the data trsansmitted. A flowmeter sends 120.0, the PLC interprets this as 1.033 e-47, for example, because the bytes are reversed and must be swapped in the user program to recover the original value.

In your case, I believe the bytes were swapped so data (alarm bits, for example) from the transmitter match the expected bit order (from the manual) describing the alarms.

Hope this helps,

Daniel Chartier