Beginer's question about SLC 500

rqiu

Member
Join Date
Feb 2006
Location
Ontario
Posts
3
Hello,

This is a beginer's question. In Siemens S7-300, A, AN, O, and ON are used to organize the logical relationship of the relay inputs, but I can not find the similar instructions from AB's SLC 500 instruction set. The examples in their manual doesn not contain any source code in text. All the design is represented as ladder diagram.

I wonder if SLC 500 only use software RSLogix 500 to translate the ladder diagram input by the user to the machine code automatically, so user doesn't need to know the text version of the instructions?

Robert
 
The SLC 500 programs only in ladder, so thats why all the examples are in ladder. There are mneumonic equivilants though, (A)=XIC (Normally open series contact), (AN)=XIO (Normally Closed series contact)... For (O) and (ON), you need to use a branch around another part of the logic, using BST (Branch Start), XIO/XIC/other logic, NXB (Next Branch) and BND (Branch End).

Pasting this logic into a rung (in the text edit bar) will give a simple start/stop circuit:
XIC I:1.0 BST XIC I:1.1 NXB XIC O:2.0 BND OTE O:2.0
 
Using Ladder Diagram is better

Thanks for the reply! It seems that using ladder diagram is much more efficient.

I had used Java/C++ during the past 5 years. PLC programming reminds me of the assembly language I used 12 years ago. For me, rather than using the I:3/6, or O:2/7 all over the ladder diagram, I would like to assign a name to the bits, and use the name. Just like the concept of 'variable' in the C++. I believe this will make the ladder diagram much more clear.

For instance, instead of:

LD I:1/1
OR O:0/1
AN I:1/2
= O:0/1

I prefer this way:

Define startButton as I:1/1
Define stopButton as I:1/2
Define runLED as O:0/1

LD startButton
OR runLED
AN stopButton
= runLED

I believe this provides a big flexibility when dealing with complex projects with dozens of I/O points.

Is this a good idea?

Robert
 

---| |---- is equivalent to A


---|/|---- is equivalent to AN


---| |---+--- is equivalent to O
|
|
---| |---+ << this being the OR


---| |---+--- is equivalent to ON
|
|
---|/|---+ << this being the OR-NOT

 
rqiu said:
...For me, rather than using the I:3/6, or O:2/7 all over the ladder diagram, I would like to assign a name to the bits, and use the name. Just like the concept of 'variable' in the C++. I believe this will make the ladder diagram much more clear.

For instance, instead of:

LD I:1/1
OR O:0/1
AN I:1/2
= O:0/1

I prefer this way:

Define startButton as I:1/1
Define stopButton as I:1/2
Define runLED as O:0/1

LD startButton
OR runLED
AN stopButton
= runLED

I believe this provides a big flexibility when dealing with complex projects with dozens of I/O points.

Is this a good idea?

Robert

In the RSLogix you can assign a tagname to each of the bits (Or words for that matter) that you have declared for the program. It is MUCH easier to read a ladder program that has names for the bits and inputs and outputs. I'm not sure about 'flexibility', but clarity is VERY important when someone else has to look at he program. (Or even your self.)

It is easy enough to rename the bits (Words) but


In the Automation Direct Software there is an option to replace bits (words) in the program but leave the tagname in place on the ladder. I would think that there is a way to do that in RSLogix.
 
Thanks for the help!

Thanks for all the help!

The only reason for these strange questions is that I do not have any real hands on experience on a PLC system. All the information I collected is from books & manuals.

It seems I'll have to invest some money to get a PLC training with practice.

Robert
 

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