RSLogix 500 - Bit Addressing

[jmaty23]

I am a rookie programmer so this question may be very simple to answer.

I working with an RSLogix 500 program for a Micrologix 1500 PLC.

When I read through the logic, all of the inputs and outputs are controlled with B3 bits. It seems as if O0 and I1 data files are not used. How is this possible and how can I find out which input and output the B3 bits are tied to through RSLogix?

 

[chavak]

You may find a copy instruction [COP] copying B3 word to and forth the inputs & outputs. Do a search on the input or output word.....

Regards

 

[bernie_carlton]

While it would be unusual for a program to contain no references to physical input or output points it can be done. The PLC program can still operate on information supplied from networks or display panels which arrives in the PLC in other data areas.

Open either the input or output data table. Click on the 'Usage' button to see which item are actually referenced in the program. To locate any particular used item in the program right-click on it and select 'Find All'.

Only by then interpreting the program ladder relative to these inputs and outputs will you discover the logical link to 'B' register bits.

 

[andrewstar03]

You could do a Search with what ladders use those bits, the programmer might have used mirroring on that Program. you could click the search results to see on what Ladders used the Bit.

 

[jmaty23]

Chavak, I was able to find where they use [COP] to make the conversion. What is the point of doing this?

https://docs.google.com/file/d/0B9PKnB_WXZhOek5XZERnYjNlLUk/edit

 

[Helliana]

A lot of programmers use B bits for everything then move/copy the B bit to the I and O bits later. One of the main advantages is if you need to change a output to something else (because of a welded relay, etc), you only have to change it in one place in the program.

 

[jmaty23]

Sounds good. Thanks for your help!

 

[Eric Nelson]

In this particular case, it looks like the programmer is using the 'Simulate Bit' to disable the physical I/O. Very handy for testing. When that bit is ON, you don't need to force inputs, just turn the B bits on and off. Also, no physical outputs will fire. When that bit is turned back OFF, the physical inputs clean up your overrides. I may borrow this idea!...



-Eric