دورة تدريبية في الحاكمات المنطقية قابلة للبرمجة طراز Siemens S7

The /FC bit signal state controls a logic operation string.

The /FC bit is status word bit 0.

Each logic operation queries the /FC bit signal state and the addressed contact.

· If the /FC bit signal state equals ‘’1,’’ an instruction logically combines the result of its signal state check on its addressed contact with the RLO generated since the first check and stores the result in the RLO bit.

· If the /FC bit signal state equals ‘’0,’’ the logic string begins with a first check.

The logic string ends and the /FC bit is set to ‘’0’’ with the assignment of a value (S,R,=) or with a RLO-dependent jump instruction.

The RLO bit stores the result of a logic operation string or comparison instruction.

The RLO bit is status word bit 1.

The first instruction in a segment checks the contact signal state. The RLO is set to ‘’1’’ if the check is executed. The second instruction also checks the contact signal state. This check result is now combined with the value stored in the RLO bit according to the Boolean algebra rules and stored in the RLO bit. This logic string ends after an assignment or a conditional jump. Depending on the RLO bit value, an assignment or a conditional jump is executed.

The STA bit stores the value of an addressed bit.

The STA bit is bit 2 of the status word.

The status of a bit logic instruction that performs a read access to memory (A, AN, O, ON, X, or XN) is always the same as the value of the addressed bit. The status of a logic instruction that may perform a write access to the memory (R, S, or =) is the same as the value of the written bit or, if writing is not performed, the same as the value of the addressed bit. The status bit has no significance for bit instructions that do not access the memory. These instructions set STA to 1. The status bit is not read by instructions, it is interpreted for you when viewing the online status of program variables.

The OR bit is used for combining AND functions before OR functions.

The OR bit is status word bit 3.

The OR bit is set if the RLO of the AND logic operation is 1. This anticipates the result of the OR logic operation. Every other bit-processing instruction resets the OR bit.

The OS bit stores the OV bit if an error occurs during math instructions or comparison instructions with floating point numbers.

The OS bit is bit 4 of the status word.

The OS bit is set, together with the OV (Overflow) bit, in the event of a fault. It remains set after the fault has been eliminated. It thus stores the OV bit status and indicates whether or not a fault has occurred in one of the previously executed instructions.

The following commands reset the OS bit:

· JOS (Jump if OS=1)

· Block call instructions

· Block end instructions

The OV bit displays errors for math instructions or comparison instructions with floating point numbers.

The OV bit is bit 5 of the status word.

It is set by a math instruction with floating point numbers after a fault has occurred (overflow, illegal operation, comparison unordered). The OV bit is reset when the fault is eliminated.

The BR bit is bit 8 of the status word.

The BR bit transfers the results obtained from processing Statement List (STL) instructions on to the next instructions to be processed.

When writing a function block or function in STL that you want to call from LAD/FBD, you have to store the result of the logic operation (RLO) in the BR bit immediately before leaving the block in order to provide the enable output (ENO) for the LAD/FBD box. You do this using the SAVE, JCB, JNB instructions.

When you call a system function block (SFB) or a system function (SFC) in your program, the SFB or SFC indicates whether the CPU was able to execute the function with or without errors by providing the following information in the binary result bit:

· If an error occurred during execution, the BR bit is ‘’0”.

· If the function was executed with no error, the BR bit is ‘’1”.