Hello,

I understand how to use/read indirect addressing and I find that all programs in our machines use indirect addressing.

but I don't find a good answer why and when we use this type of addressing ? and what'is the advantage of using indirect addressing?

Hello,

with direct addressing your program is "static" (at every plc cycle the same address is processed).

with indirect addressing you can create "dynamic" program (at every plc cycle address can change).

Best Regards

mindware

Hello,

with direct addressing your program is "static" (at every plc cycle the same address is processed).

with indirect addressing you can create "dynamic" program (at every plc cycle address can change).

Best Regards

 Thank you for your reply, so what's the benefit of dynamic program? Maybe be with static program or dynamic program we have the same result? 

Is there any effect on time cycle or memory or ,...?

Indirect addressing is good to repeat a program code for several times but with different values. Then you can create a step counter and the counter value will be the base of the indirect address to load different values. You have an axis e.g. which has to be moved to different positions in a sequence. The counter will be increased, if the actual position is reached. Then you repeat the last step. With this you can move the axis to several positions in a sequence with less program code, Of course you can use an array instead, but this is also an indirect addressing format.

Regards, Scorp

a good explanation is here 

Best Regards

Med_selmi

and what'is the advantage of using indirect addressing?

Using indirect addressing has only drawbacks 

It's error-prone, quite complicated, can create some serious problems when used improperly.

But in the case of S7-300/400, this is the only way to access arbitrary data.

Hello Med_selmi;

Think of the control systems that the PLC replaced, historically. Plants would have hard-wired relay panels, where the address of each input and output was completely known at "programming" time, and the logic was connected by wires. The relay logic  included contacts, switches, times, counters... that needed to be used in a known physical position (from the electrical plans of the control panel) and connected through electrical signals.This is what our friend mindware had in mind with his description of a "static" program: changing a control logic meant disconnecting and connecting the control relays, and could not be done on the fly.

https://en.wikipedia.org/wiki/Relay_logic

When PLCs replaced the relay logic panels, they were based on microcontrolers, and added to the relay logic (which you can understand has been retained in Ladder Logic programs) the programming capacities of digital computers. This included higher-level programming, and the use of pointers.

https://en.wikipedia.org/wiki/Pointer_(computer_programming)

As you can see in the Wikipedia definition, a pointer is used in a computer program to store the address of a memory location, instead of the value contained at that memory location. This allows the programmer to choose a specific memory register where he needs to start an operation, and then use following memory locations designated by an offset. A special register (AR1, for example) is used to point to a special memory location which was not known at the time the program was designed, and it is loaded with the pointer to that memory address. Then by using special instructions (known collectively as indirect addressing), it can be programmed to point at a lower or higher address, and used to execute operations there.

Pointers are used in many different programming contexts. FIFO tables are a very useful application: before you choose tit you do not know where in memory the first register of the table is situated; you use a pointer to set it at execution time. Then you want to shift all the adresses of the table one memory address lower so you can copy the new value of the FIFO table in the start position. Very difficult to program with relay logic, especially since you have no idea where the table begins before you program it, on a computer it is easy to understand and develop.

Loops, memory and data copy and many other operations depend on pointers and indirect addressing in PLCs.

You can find more and better information on the subject from this article frpm plcdev.com:

http://www.plcdev.com/siemens_s7_indirect_addressing

Hope this helps,

Daniel Chartier

Maybe if you try to explain by giving small examples it will be better to understand for who has a little programmer knowledge like me.

Thanks for everyone which wants to help.

Mert Turgut

Maybe if you try to explain by giving small examples it will be better to understand for who has a little programmer knowledge like me.

Thanks for everyone which wants to help.

 The last link that Mr Chartier supplied would help somewhat. I think the topic starter has had his question resolved. If you have more questions of your own please feel welcome to state them in a new topic.

In this way the forum can focus on the theme that your question puts forward.

NOTE - These are suggestions as from a friend. These are no orders.

Dear Med_selmi, please allow me some thoughts on the topic.

One aspect of indirect addressing was not mentioned:
Some use it to conceal code / methods / data. Sometimes as a security method and other times as job-security ("keep me employed, I am the only person who can work with this code").

Indirect addressing can be impossible to track if it is not expected. Even if it is expected, some "sniffing" code hacking is needed to expose the true nature of the indirect addressing.

Modern PLCs we aim to avoid indirect addressing. Mostly that is possible when utilizing the refined workings of arrays.

Dynamic functionality do remain a problem. Functionality like a technician being able to configure which inputs and outputs are linked to the code via an HMI.

BR