Step7: Accessing individual bits of a double word (DBD)

[MartyW]

A scale module the PLC communicates with feeds back a double word (DBD) that contains status information: 32 defined status bits. In this case it dumps it into a data block at DB21.DBD26. I assumed that I could address DB21.DBX 26.0, 26.1, etc. to look at the individual bits. Looking at the binary representation of the DBD, the information is correct. However may attempts at addressing each item aren't giving the proper values. I have tried assuming that 26.0 has the most significant bit and assuming it has the least significant bit. Either way seems to give incorrect results.

Any ideas?

 

[Jeebs]

DoubleWord is Word1..Word2
Word is Byte1..Byte2
Byte is 7..0

This means it goes:

26.7..26.0, 27.7..27.0, 28.7..28.0, 29.7..29.0
MSB - - - - - - - - - - - - - - - - - - - - LSB


Open the LAD/STL/FBD editor and do a help search on: DWORD Data Type

 

[MartyW]

I figured it out

Within the Double Word, the individual bytes are addressed in descending order. The bits within the bytes are addressed in ascending order.

So for a double word with address 0 (0.0-3.7), the least significant bit is at 3.0. The most significant bit is at 0.7. A bit strange to this newbie.

 

[MartyW]

Thanks Jeebs. Your explanation is a clearer.