plc ladder diagram meaning

[westwind80 0]

Hi all,

  Can someone help me to understand the ladder diagram below? I am a newbie in plc programming. I'm try desperately to learn

[panic mode 246]

get programming manual for your PLC. it will explain each instruction, addressing etc,

each PLC also has own instruction manual that is product specific. so you really really need to know what exact hardware you are working with and get correct manuals.

in general CPU can access some memory directly. that includes memory location M, D etc. one can also directly access regular I/O (X and Y).

but when there is an "intelligent" device or I/O module things get a bit more complicated. intelligent module is practically EVERY module except the simple digital I/Os.

that includes products that have analog inputs, analog outputs, servo control, communication interface etc.

so each of those intelligent devices will also have own manual, explaining functionality as well as how to access the parameters. part of memory of the module that is accessible from "outside" (by PLC CPU) is called buffer. using TO and FROM instructions one can read or write the buffer locations. 

note that it also matters how the PLC is configured - module that is in a different location will have different base address, which is what you need to know in order to access buffer locations in that module. for example in some particular configuration, you may have several identical analog input cards. but you need to parametrize and get values from each separately. 

your program sample seem to do same kind of thing. Z0 is special register that can be used to add offset. K0 is decimal representation of 0. so K0Z0 allow you to use same set of instructions to access different module simply by changing Z0 value.

the first line says something like:

when M1090 is true, access module with base address K0Z0, then in buffer location zero (K0) put value from D458. in this case it is only copying one value (K1). quite often you can transfer several values at once by changing K1 to something larger.

 

next two lines are reading from the intelligent module. 

one is reading from buffer location 10 (K10) and placing value into D453. Actually here we are transferring 4 consecutive buffer location (K4 at the end of the instructions).

this means buffer 10 is copied to D453, buffer 11 is copied to D454, buffer 12 is copied to D455 and finally buffer 13 is copied to D456.

 

last instruction does the same, it copies buffer 29 (K29) which is a single value (K1) and that is a 16-bit, to a group of M bits starting with M932.

K4M932 means size of target is 4 nibbles (K4). One nibble is 4-bit or half a byte so K4 means 16 bits of entire 16-bit value is transferred and stored into M932, M933...M947.

transferring to block of M location instead of single D register indicates that they want to check the value bit by bit so in this case using M bits is more practical than working with D register.

 

 

 

[westwind80 0]

Hi PanicMode,

  Really appreciate your detailed explaination. Below are the plc diagram of the connection I have.  Can the 'K0Z0' be referring to special device of 'fx2nc-4ad' in the diagram below? the 1st 'fx2nc-4ad' ?

[panic mode 246]

Exactly....

 

K0 is initial value that gets offset by whatever value is in Z0. So you may want to look for instructions writing to Z0