Boolean operations in STL(Simatic S7)

[neo_gunner]

Hello,
I am new to PLC programming.
I have a very basic doubt about STL programming.

As per Siemens manual 'Statement List (STL) for S7-300 and S7-400 Programming',
" 'A' (i.e. A I0.0)checks whether the state of the addressed bit is "1", and ANDs the test result with the RLO".

We require 2 values to carry AND operation and as per manual, "A" ANDs the test result with the RLO after checking whether state of addressed bit is 1. I know that RLO is "Result of logic operation' which is a part of Status Word.

My doubt is what's the value of RLO that 'A' ANDs with?How do we know RLO value?

Can you please explain the following operation step-by-step?
A I0.0 (NO contact)
AN I0.1 (NC contact)
= Q 4.0

Thanks.

 

[L D[AR2P#0.0]]

The first check bit is used so the first A (for example) conditions the RLO, subsequent logic is then combined with the RLO. Siemens help below:


/FC First Check Bit (Status Word, Bit 0)

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.