PLC Remote IOs

Hi Guys,

I don't know much about plcs, but I have worked with machines equipped with single unit PLC. I am just curious about those big plc unit with some remote IOs on different sections of machine. Could anyone please give me an insight on how is the process of setting them up before you can start writing program. Thanks...

Hi Francois.

Since you dont say which platform, I will respond in general.

Irrespective of if the i/o is in a local rack or in a remote rack, each i/o point will get an absolute address, like I10.4 or Q20.3.
When programming the user program, it is highly recommended to not use the absolute addresses but in stead setup symbolic addresses. In this way you will use the sensor named "=HA1-B3" to active the valve named "=HA1-Q4" in stead of "I10.4" and "Q20.3".

This makes if faster to write the program while keeping it more readable, and if you change the absolute addressing associated with the symbols, you dont have to manually correct the program - it will be done automatically.

In other words, writing the program is not directly dependent on that the i/o is 100% setup in advance.

JesperMP said:

Hi Francois.

Since you dont say which platform, I will respond in general.

Irrespective of if the i/o is in a local rack or in a remote rack, each i/o point will get an absolute address, like I10.4 or Q20.3.
When programming the user program, it is highly recommended to not use the absolute addresses but in stead setup symbolic addresses. In this way you will use the sensor named "=HA1-B3" to active the valve named "=HA1-Q4" in stead of "I10.4" and "Q20.3".

This makes if faster to write the program while keeping it more readable, and if you change the absolute addressing associated with the symbols, you dont have to manually correct the program - it will be done automatically.

In other words, writing the program is not directly dependent on that the i/o is 100% setup in advance.

Click to expand...

I appreciate this but could you please give me same sort of scene but specifically Siemens platform. We just had a new machine set-up and ready for commissioning and it is equipped with siemens step7-300 with several remote IO on some section. Do you mean wiring the remote IOs back to the main PLC and that's all needed before someone can start writing program. It is only the addressing of the remote IOs that I don't have a clear idea how to be done...cheers

franchois13 said:

I appreciate this but could you please give me same sort of scene but specifically Siemens platform. We just had a new machine set-up and ready for commissioning and it is equipped with siemens step7-300 with several remote IO on some section. Do you mean wiring the remote IOs back to the main PLC and that's all needed before someone can start writing program. It is only the addressing of the remote IOs that I don't have a clear idea how to be done...cheers

Click to expand...

Hello Franchois;
Simply wiring the I/Os, either central or remote, to your CPU will not be enough. You must first tell the CPU what to expect at each module location when the I/Os are wired in.

The tool for that configuration step for Siemens S7-300 and S7-400 CPUs (Step 7 programming software) is the HWConfig editor. There you will create a virtual configuration for your PLC modules, racks, CPUs, Profibus-DP network, that will match exactly the hardware you will be using in your project. This configuration needs to be compiled (as SDBs: System Data Blocks)and downloaded to the CPU before you can process any logic.

To get more information, download the Configuring Hardware and Communication Connections manual from Siemens' support site:
http://support.automation.siemens.com/WW/view/en/18652631
Hope this helps,
Daniel Chartier

Do you mean wiring the remote IOs back to the main PLC and that's all needed before someone can start writing program.

Click to expand...

No, if you read my post, you can see that writing the software (the code) and setting up the hardware are not dependant on each other. By using symbolic addressing, you can wait until the last minute to set the i/o addressing to match with the hardware.

Setting up a system with remote i/o can be described very shortly in this way:
1. Setup one or more DP or PN networks to be handled by the CPU as "DP master" or "PN IO Controller" (typically by integral DP or PN ports, but via CP's is also a possibility).
2. Setup one or more "interface modules" (aka IMs) as "DP slaves" or "PN IO Devices" connected to the DP or PN network handled by the CPU.
3. Adding i/o modules to DP or PN interface modules in exactly the same way as adding them to a local rack.

edit: Rephrased a bit to make it more clear.

Thank you guys, I know it's a long way for me to get there but it ease my confusion. thanks a lot...

franchois13 said:

Thank you guys, I know it's a long way for me to get there but it ease my confusion. thanks a lot...

Click to expand...

Hello Franchois;
Why don't you start with these tutorials prepared by Siemens for the S7-300 platform? They may help clarify many basic elements and clear the confusion:
https://www.automation.siemens.com/...download_training_material/Pages/Default.aspx

Note that different sections of the tutorial can be se;lected on thecolumn on the right of the webpage. Note also that most of the documentation can be downloaded in French, if that can help.

Hope this helps,
Daniel Chartier

dchartier said:

Hello Franchois;
Why don't you start with these tutorials prepared by Siemens for the S7-300 platform? They may help clarify many basic elements and clear the confusion:
https://www.automation.siemens.com/...download_training_material/Pages/Default.aspx

Note that different sections of the tutorial can be se;lected on thecolumn on the right of the webpage. Note also that most of the documentation can be downloaded in French, if that can help.

Hope this helps,
Daniel Chartier

Click to expand...

Thanks a lot, this is good. Something to start with. cheers