I/O card addressing and configuration

russrmartin

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Join Date
Aug 2002
Location
Eastman, Wisconsin
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Hey guys. Last nite a coworker asked me to help him configure a new card that he is installing on a PLC 5. The PLC is linked to 6 different racks. The rack he is installing in is rack 5, and the group number will be 6, slot 0. My initial response was to go to IO config, select the correct chassis, and add the card from the list. However, upon doing this we found that none of the other cards in this rack are done this way. Under the I/O config for that rack, not a single card is listed. However, on the data table pulldown on the left side of the screen, under the data files, there is a section for force files. The I/O for these cards are in there. My question is, how did they get there, and what is the benefit of doing it this way, rather than simply listing the card under I/O config and going about it that way. I did notice that all the other racks are done this way, and only rack 5 is different. I did look under the help file for RS5 and it says nothing about "force files" Even some direction on where to find out more about these would be helpful. As always, thanks in advance.

Russ
 
I/O Configuration tables for the PLC-5

first things first: there are MAJOR differences between the Allen-Bradley SLC-500 family of processors and the PLC-5 family of processors when it comes to the way the I/O is assigned ... the discussion below is concerned primarily with the PLC-5 family since that’s the one you’re having questions about ...

when a brand new PLC-5 processor is taken right out of the box, it already has a certain number of Input bits and a certain number of Output bits ... you can see all of these bits by simply looking at the input data file and the output data file ... or by looking at the force table files ... the arrangement of the data table files will precisely match the arrangement of the force table files ...

now to answer your specific question ... with the PLC-5 family, it is NOT necessary to actually list any of the input or output modules on the I/O Configuration tables ... all of the modules will work perfectly whether they are listed on this table or not ... (Note: this rule is DIFFERENT for the SLC-500 family ... with that platform, ALL of the modules MUST be listed in the I/O Configuration tables in order for the system to work properly) ...

so now the natural question: if we don’t HAVE TO insert the modules into the I/O Configuration tables, WHY would we ever want to? ... answer: because some of the modules (specifically the types of modules which use Block Transfers to communicate with the processor) require certain “setup” or “configuration” information that must be “written” from the processor over-and-into the module in order to “configure” the module for operation ... a very common example: the 1771-IFE analog input module requires that its scaling data, and its data format, and other such information be “written” into the module at “power up” time ... without this “configuration” information, the module basically “wakes up” as a totally stupid device that cannot give us meaningful information about its analog input signals ... now where do you think that the “configuration” data (actually a series of about 20 to 39 integer words) actually comes from? ... answer: the programmer inserts the module into the I/O Configuration table and then proceeds to fill out certain “Module Configuration” screens which pop up when he clicks on the “Display Module” selection ... the RSLogix5 software uses the programmer’s entries to determine the proper “code numbers” which are then stored in the processor’s integer files ... the programmer must also make sure that a set of Block Transfer Read and Block Transfer Write rungs are inserted into the ladder logic to allow the processor to actually communicate with the module ...

Major Idea: IF we already (somehow) knew the configuration “code numbers” that the module requires ... and we already had the Block Transfer rungs properly set up ... would we STILL have to insert the module into the I/O Configuration table in order for it to work? ... answer: NO ... at least not for the PLC-5 family ... with this platform, the I/O Configuration table is more-or-less just a handy “tool” which helps us make use of the RSLogix5 software to set up any modules which require “configuration” ...

Almost done ...

where the RSLogix5 software and the PLC-5 family of processors are concerned, you can think of the I/O Configuration tables as “just documentation” ... much the same as the “instruction comments” and the “rung comments” which we type into our program in order to “document” the program’s operation ... specifically, even if the “instruction comments” and the “rung comments” were completely left out of the program, the processor could still execute the ladder logic perfectly ... so too with the I/O Configuration tables ... they’re not really REQUIRED in order for the system to work, but they sure do come in handy when it’s time to set up a Block Transfer-type module ...

finally, since the I/O Configuration is “just documentation” then we could also say that the PLC-5 processor never actually “sees” this table ... specifically, the table itself is stored only on your programming computer’s hard drive as part of the .RSP program file ... it is NOT actually downloaded to the processor ... that’s why if you ever go “online” with a PLC-5 processor and do not have the .RSP file available, you will NOT see the “instruction comments” (that’s to be expected) ... and you will also NOT see the I/O Configuration tables ... but there’s the processor (bless its little heart) still chugging along happily - even though the I/O Configuration table is completely blank ... the processor has its “module configuration” data stored in its integer files ... and so all is well ...

and really finally ... most PLC-5 programmers NEVER take the time to list their regular-old discrete-type modules on the I/O Configuration tables ... you CAN ... but it’s NOT necessary ...

hope this helps ...

PS edit ... did you have specific questions about the I/O Force files? ... please post again if I haven't covered those in enough detail ...
 
Last edited:
I/o config

Ron,

I have never used nor paid attention to the force files, therefore I was somewhat under the impression that they had been created. I was unaware that they are already part of the processor. Your explanation was more than adequate. I think all of us here have a better understanding of how this works now. I am much more familiar with SLC's, and I was pretty sure that they had to be configured under the I/O config, but my first instinct was that 5 would be the same way. Thanks again for the help.

Russ
 
Excellent post. I want to thank you for this informative read, I really appreciate sharing this great post. Keep up your work. Thanks for this very useful
 

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