cdavis
Member
Is it possible to have a FIFO of strings in Siemens? I see that the FIFO instruction (FC85) manipulates words. Also, on a side note, are there comparison instructions for symbols that have been designated as words instead of ints?
Siemens S7 FIFO
- Thread starter cdavis
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I'm not at all sure about the FC85 FIFO handling STRINGs. I'd suspect not. I think this is one where you'll have to roll your own using a pointer.
Siemens are rather weird about the parts of the IEC61131-3 spec they observe faithfully and the other parts they implement to suit themselves. Strictly speaking, according to IEC61131-3, INTEGERs are decimal values occupying a 16-bit piece of data, and WORDs are bit patterns occupying a 16-bit piece of data. Doing a comparison between two decimal values does make sense, hence the IEC instruction set caters for that. However, what is the equivalent when you try to compare two binary patterns? Testing for equality is an XOR with a zero result, but the concept of greater than or less than doesn't really exist. Unless you start treating each of the bits as representing a value in which case you're back to redefining the WORD to be an INT. So in this case there is no easy way to define the data as a bit pattern and then ask "Is this pattern more than another pattern?"
Ken
Siemens are rather weird about the parts of the IEC61131-3 spec they observe faithfully and the other parts they implement to suit themselves. Strictly speaking, according to IEC61131-3, INTEGERs are decimal values occupying a 16-bit piece of data, and WORDs are bit patterns occupying a 16-bit piece of data. Doing a comparison between two decimal values does make sense, hence the IEC instruction set caters for that. However, what is the equivalent when you try to compare two binary patterns? Testing for equality is an XOR with a zero result, but the concept of greater than or less than doesn't really exist. Unless you start treating each of the bits as representing a value in which case you're back to redefining the WORD to be an INT. So in this case there is no easy way to define the data as a bit pattern and then ask "Is this pattern more than another pattern?"
Ken
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