What is a Flag in CX-Programmer?

Question

During the reading of PLC documentation (Omron CP1L PLC and CX-Programmer), there are some missing explanation. For example, it defines "Flag" as "a bit that serves as an interface between in*struction", does that mean flag is some sort of conditional Power Flow?

It gets more confusing with the terms "Differential Up/Down", "Carry Flag"? What are flags and what do they do in the ladder logic? Are they a simple or instruction to use or just a concept that I don't really need to program in ladder?

[EDITED]

Where to add / modify / delete the flag in an instruction? I open up the edit but flag isn't there.

Answer 1

Ok, this is a better question.

PLCs are like any program - data is stored as different types. Think of flags as interchangeable with the term "bit", "boolean", etc. They are very important.

If you have CX-Programmer, a much better place to get information is the Instruction Reference (Help --> Instruction Reference --> yourPLC). These show time diagrams of most of the instructions and how each of the parameters and flags operate.

For example, a basic timer (TIM) works by assigning it a value. If you use a BCD type 100ms timer and assign its SV (setpoint value) a BCD value of 300 then you have created a timer with a 30 second limit (300 x 100ms). When the timer turns on it will begin counting and the PV (process value) will start from 300 and count down. When the value reaches zero the timer's flag turns ON to indicate that it has expired. If the timer's number is, say T100 then you can use T100 as a contact in another rung of logic - it will be true when the timer's execution conditions are TRUE and the timer has expired.

Differentials (UP/DOWN) are special flags which are true for only one PLC scan (ie: they are true for one execution cycle only) when their input conditions change from FALSE to TRUE (ie:OFF to ON) for UP differentials, and TRUE to FALSE (ie:ON to OFF) for DOWN differentials. You would use differentials in cases where you wanted to perform an action the moment a given condition changes.

Flags can be used for almost anything. You can use them as general booleans in your own programs, they can be parts of certain operations (ie: the CY (carry) flag is used on arithmetic operations which result in a carry - other flags are used to indicate overflows or div/0 errors, etc).

Edit again : (to answer extended question).

A basic timer's completion flag is a contact with its number. Say I have a 100ms timer, T100, which turns on when contact 10.00 is on:

     10.00                                       ___    
|-----| |---------------------------------------|TIM|
                                                |100|
                                                |   |
                                                |#20|
                                                |___| 

Now, once 10.00 has been ON for two seconds, the timer will elapse and the flag for timer 100, T100, will turn ON. If I had another rung where

|    T100                                 W15.00
|-----| |-----------------------------------( )

Then work bit W15.00 would be turned on when the timer elapsed and would remain on so long as the timer's input condition remained satisfied (ie: so long as 10.00 remains ON). Flags work in different ways for different things, however. Each operation can use them in different ways.

The example from the Omron Instruction Reference (Help -> Instruction Reference -> [select PLC]) looks like this :

Answer 2

Well very good explanation with example and the flags value can be found in memory area it is pure binary either 0 or 1, as i was read the documentation work-bit memory location changes as per timer type e.g TIM/TIMX or TIMH or TIMHX, both are BCD timers but unit for the timer changes.