I/O to binary/bit files for ladder logix

[David B]

Can anyone tell me why a programmer would move all the inputs and outputs on the PLC to the B3:xx/xx files the first thing in the program and the make all the addresses in the logic B3:xx/xx? The drawings all reference input and output numbers i.e. I:xx/xx but then one must figure out that input in the ladder logic.I cannot see the use in wasting time and processor memory to make the program harder to follow. To make matters worse, I cannot get the version of the program with descriptors to go onlline so I'm left monitoring the operation with a program with no descriptors. I am not a programmer so I may well be missing something. The processor is a Micrologix 1500, using RSLogix 500.
Thank you in advance
David Butler

 

[TConnolly]

It's not an unusual practice. Sometimes the program is written before the programmer knows the actual io addresses. Some programmers do it so that if an address needs to be changed he only needs to make one change. I'm not a fan of the practice but I see it often.

In a ControlLogix PLC io updates are asynchronous to the scam so it's done to buffer the I/O.

 

[David B]

Thank you for the reply. When using RSLogix 500, is there not a way where you can edit and change all the same bit addresses from just one, some menu item along the lines of "change all"? I come at this from a maintenance standpoint rather than a programmers, and I know that "points" on a card can go bad, usually outputs but an input could go bad I'd say, and one moves the input or output to an unused point. In that case I can see if there is not the above "change all like addresses" I can see where using the binary B3:xx/xx format could be useful. Again thank you.

 

[rdrast]

Thank you for the reply. When using RSLogix 500, is there not a way where you can edit and change all the same bit addresses from just one, some menu item along the lines of "change all"? I come at this from a maintenance standpoint rather than a programmers, and I know that "points" on a card can go bad, usually outputs but an input could go bad I'd say, and one moves the input or output to an unused point. In that case I can see if there is not the above "change all like addresses" I can see where using the binary B3:xx/xx format could be useful. Again thank you.

Never believe that "Change All" will in fact change all. For one thing, it won't change things addressed in a different way (by word instead of by bit, for example). It also won't affect any indirect addresses.

Another benefit of buffering the I/O (if done with any thought) is that you disable the I/O copies by not scanning them, and execute the program manually for testing.

 

[Mark Snodgrass]

Thank you for the reply. When using RSLogix 500, is there not a way where you can edit and change all the same bit addresses from just one, some menu item along the lines of "change all"? I come at this from a maintenance standpoint rather than a programmers, and I know that "points" on a card can go bad, usually outputs but an input could go bad I'd say, and one moves the input or output to an unused point. In that case I can see if there is not the above "change all like addresses" I can see where using the binary B3:xx/xx format could be useful. Again thank you.

That is a function allowed only when offline, and requires you to download your changes to the program. If your process cant be stopped then you have to find all the instances manually. By changing an input/output map you can change one instruction address online and have minimal effect on your process.

 

[bce123]

To make matters worse, I cannot get the version of the program with descriptors to go online so I'm left monitoring the operation with a program with no descriptors

It sounds like the path your going on-line with is set for the original PC it was programed on. when you try to go on-line using the Com's tab does a pop up tell you that it cant find the file associated with the processor or something like that? if so try using the browse feature to find your program then upload and use file, this should fix it after you go on-line and do a save. be sure and ALWAYS back up your offline file first. I like to always upload from the processor when going on line to make sure i don't overwrite the program in the processor accidentally.

 

[The Plc Kid]

I always buffer I / O in any platform makes io changes easier to make. And from a maintenance point of view it is much easier for bubba to troubleshoot with.

 

[David B]

Thank you to everyone who took the time to respond, by revealing my ignorance I learned a lot, without being humiliated. I also talked to the programming engineer at the plant, Carriage House- Buckner, Kentucky, now a part of Cargill, and he quckly illuminated the arguments for buffering, such as easily moving a card in a rack or adding to an existing basic program without changing a large number of addresses. He also mentioned that it allows one to write the program without knowing the exact type or layout of the PLC that will be used, as TConnelly mentioned. Again , thanks to all.
David Butler

 

[Mickey]

And from a maintenance point of view it is much easier for bubba to troubleshoot with.

I will politely disagree with that one, Bubba wants to see the real I/O's.
I know, I'm Bubba.

But I do understand the other reasons.