AB PLC5 Analog signals

scottsull

Member
Join Date
Jan 2004
Posts
42
Just a quick question.

AB PLC5/20 processor. During a "normal scan" when are the
analog inputs read and analog outputs written?

In other words in the "read inputs-execute program-set outputs"
scheme of things when are the AI's and AO's read in and written out?

I assume that when they are read/written they are done so
a whole module at a time. Is this correct?

As usual thanks in advance.......Great board!
 
In a PLC-5 the analog modules are read from and written to by the use of instructions in the logic. These instructions are called Block Transfers.

So the short answer is that these Block Transfers are triggered during the "execute program" stage.

Others viewing this thread will likely give you a more detailed description replete with information on queuing and interrupts.

OG
 
Thanks for the quick reply.

When I saw that the transfer was software driven I did not know if it occured in say LAD26 if you had it programmed that way or if the BTW and BTR were automatically done during the input scan portion of the scan regardless.
 
Greetings scottsull,



As my distinguished colleague Operaghost has already mentioned, with the PLC-5 processor family, the transfer of analog data between the processor and the analog modules, is taken care of by Block Transfer instructions which you must program into the ladder logic.



Let’s look at the Block Transfer Read for a minute. The same concepts apply to the Block Transfer Write also.

The sad truth is, when the processor scans the BTR with true logic, the actual execution of the BTR is not NECESSARILY going to happen BEFORE the scan continues on to the next rung. Think about this: We’re literally talking about TRANSFERRING (moving) a big BLOCK of data. Actually accomplishing that transfer of data might conceivably take up a considerable amount of time - time which would tend to slow down our processor as he tries to continue scanning the ladder logic program. And so - at least in the PLC-5 family - the processor puts this “request” for a transfer of a block of data into a “queue” or a “buffer” - or in other words, into a line-up of “communication-type” requests. These requests will then be accomplished as soon as practical by a “communications handler” inside the processor - but NOT necessarily before the scan moves on to the next rung.




So the truth is that we do NOT know exactly when the actual transfer of analog data will take place. It’s a totally random thing. The transfer MIGHT happen quickly - while the processor is right there on the Block Transfer rung. Then again, it MIGHT happen several rungs later during the same ladder scan sequence. Then again, it MIGHT happen at some random point during a subsequent scan sequence - and even while the processor is executing a completely separate ladder (subroutine) file. Your guess is as good as mine.



If you want to further research this effect, the keyword “asynchronous” should help get you started.
 
Ron Beaufort said:
Greetings scottsull,



As my distinguished colleague Operaghost has already mentioned, with the PLC-5 processor family, the transfer of analog data between the processor and the analog modules, is taken care of by Block Transfer instructions which you must program into the ladder logic.



Let’s look at the Block Transfer Read for a minute. The same concepts apply to the Block Transfer Write also.

The sad truth is, when the processor scans the BTR with true logic, the actual execution of the BTR is not NECESSARILY going to happen BEFORE the scan continues on to the next rung. Think about this: We’re literally talking about TRANSFERRING (moving) a big BLOCK of data. Actually accomplishing that transfer of data might conceivably take up a considerable amount of time - time which would tend to slow down our processor as he tries to continue scanning the ladder logic program. And so - at least in the PLC-5 family - the processor puts this “request” for a transfer of a block of data into a “queue” or a “buffer” - or in other words, into a line-up of “communication-type” requests. These requests will then be accomplished as soon as practical by a “communications handler” inside the processor - but NOT necessarily before the scan moves on to the next rung.




So the truth is that we do NOT know exactly when the actual transfer of analog data will take place. It’s a totally random thing. The transfer MIGHT happen quickly - while the processor is right there on the Block Transfer rung. Then again, it MIGHT happen several rungs later during the same ladder scan sequence. Then again, it MIGHT happen at some random point during a subsequent scan sequence - and even while the processor is executing a completely separate ladder (subroutine) file. Your guess is as good as mine.



If you want to further research this effect, the keyword “asynchronous” should help get you started.

Is this also true of Micro Logix and SLC PLC's???
 
Good job Ron,

It's funny, in my original response. My message said that if Ron Beaufort was listening to this thread he will probably respond with much more detail. That's why I underlined "triggered". But then I realized I didn't want to force you into replying (in detail) or dissuade others from replying so I made my comment more general.


OG
 

Similar Topics

Hi: I recently tried connecting 2 new single-ended 4-20mA analog inputs to an existing 1771-IFE. The IFE is in a RIO chassis. The values coming...
Replies
4
Views
1,366
I've got a 4-20ma pressure transducer connected to an analog card on an AB PLC5. Under the setup for the card, I have to set a raw minimum and...
Replies
12
Views
7,316
In our PLC5 processor we have the last analog value being overwritten on a couple of cards. The values from the next card down are being...
Replies
4
Views
1,849
My goal is to get an analog signal from a SLC-500 remote rack back to a PLC-5 main rack. Is this possible? If so, I may need some help. I've set...
Replies
4
Views
1,743
I know there have been a couple different posts on this subject but I was still unable to get the final information needed. In my facility we...
Replies
44
Views
8,692
Back
Top Bottom