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In Siemens PLCs the Status Word is an internal CPU register used to keep track of the state of the instructions as they are being processed. In order to use STL more effectively it is important to understand the Status Word and its functions.
Each bit in the Status Word has a specific function to keep track of bit logic (RLO, STA), math (OV, OS), comparison operations (CC0, CC1) and whether the logic should continue, be nested or start new (/FC, OR, BR). only the first 9 of the 16 bits are used.
Bit Positions
8 |
7 |
6 |
5 |
4 |
3 |
2 |
1 |
0 |
BR |
CC0 |
CC1 |
OV |
OS |
OR |
STA |
RLO |
/FC |
Each instruction may do the following to each bit in the status word.
- |
No read or write |
* |
Read |
x |
May write "1" or "0" |
0 |
Reset to "0" |
1 |
Set to "1" |
The status word can be seen by displaying the STATUS column while monitoring in STL view. The RLO (bit 1) and the STA (bit 2) are also displayed in the RLO and STA column.
The Most important Status Word Bits
/FC – First Check (bit 0)
If the /FC bit is a 0 then the instruction is considered to be the first instruction being processed. If the /FC is a 1 then the instruction being scanned will use the logic from the previous instruction. Certain instructions like =, S and R will set the /FC bit to 0 thus starting new logic after it. Other instructions like A or O will set the /FC bit to 1 signalling to combine the logic with the next instruction.
