1794-ASB Rack & Group?

Sorry for asking this dumb question. Can anybody pls explain to me what does the 1/4, 1/2, 3/4 and Full? Group 0,1,2,3..and Rack 0,1,2,3,4...etc. I would gladly appreciate any replies.

Rockwell Literature Library
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Legacy Rockwell PLC5 stuff related to the CPU memory mapping...

Group = An I/O addressing unit consisting of one input and one output word (16 bits each) of the data table.
Depending on the density of the I/O module and the addressing mode (2 Slot Addressing, 1 Slot Addressing or 1/2 Slot Addressing) used, some of the bits in a group may be unused.
The group number is included in I/O addresses in the position represented with a 'g' in the format: I:rrg/xx.


Rack = An I/O addressing unit that corresponds to eight input image table words and eight output image table words (8 groups).
A rack is a logical entity not to be confused with the physical chassis.
8 groups = 1 full rack, 6 groups = rack, 4 groups = rack, 2 groups = rack. The rack number is included in I/O addresses in the position represented with an "rr" in the format: I:rrg/xx.

Depending of the PLC class there are different numbers of available logical racks: 5/11s have one (3 octal), 5/20s have three (1-3 octal), 5/30s have seven (1-7 octal), 5/40s 1-17 (octal), 5/60s and 5/80s 1-27 (octal).

Some elements are carried over to the Universal RIO communications on the SLC family platforms.

here's the basic idea:

a "rack" consists of 8 words of input data – AND – 8 words of output data ...

specifically, the word "rack" should NOT be used to mean a "chassis" or a piece of hardware ... that's what's most confusing about this particular subject ... people throw the word "rack" around where it doesn't belong ...

so ... a "rack" is something like:

an acre of land ..
a ton of steel ...
a dozen eggs ...
a gallon of milk ...
etc. ...

specifically, the word "rack" is a UNIT MEASURE – not a piece of hardware ...

keep in mind that a "word" of data (in the context we're using here) is made up of 16 bits ... just like a "dozen" holds 12 eggs – but a "word" holds 16 bits ...

Allen-Bradley will not allow you to address anything less than a quarter of a rack ... (no eighths of a rack are allowed) ...

so ...

1/4 of a "rack" equals TWO words of input data – and TWO words of output data ...

1/2 of a "rack" equals FOUR words of input data – and FOUR words of output data ...

3/4 of a "rack" equals SIX words of input data – and SIX words of output data ...

a full "rack" equals EIGHT words of input data – and EIGHT words of output data ...

here's another Allen-Bradley rule ...

when you address something (a Flex-I/O communications for example) the rack address MUST start on an EVEN number ...

so ... considering just quarter rack devices for examples - you could have a quarter of a rack start at address:

0 (an even number) ... or ...
2 (an even number) ... or ...
4 (an even number) ... or ...
6 (an even number) ... or ...

the 0 assignment would "use up" words 0 and 1 (two words) ...
the 2 assignment would "use up" words 2 and 3 (two words) ...
the 4 assignment would "use up" words 4 and 5 (two words) ...
the 6 assignment would "use up" words 6 and 7 (two words) ...

here's an analogy: suppose that you're a land developer – and that you have two acres (two "racks") of empty land to work with ...

you could put a nice ranch-style house on 1/2 acre of land ... in a PLC this would be something like assigning a PanelView to use 4 words of input data – AND - 4 words of output data ...

for example: Rack 02 – Starting at Group 2 ...

then you could put a neat little Cape-Cod-style house on 1/4 acre of land ... in a PLC this would be something like assigning a Flex-I/O system to use 2 words of input data – AND - 2 words of output data ...

for example: Rack 03 – Starting at Group 0 ...

here's a figure from a PLC-5 system that might help ... if you're using some other type of processor, tell us what you're using and we'll try to give you some more detail ...

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I love you all Guys! Thanks for the assistance!

Ron Beaufort said:

here's a figure from a PLC-5 system that might help ... if you're using some other type of processor, tell us what you're using and we'll try to give you some more detail ...

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Click to expand...

I am actually using Control Logix Processor with RIO-scanner thru Flex Logix with two sets of 1794-ASB (in daisy chain) with IB16 and OB16.

I hope I can jump in here with a question along the same lines. Ron's explanation was great and opened up more questions.

My system uses a SLC 5/04 with a 1747-SN scanner module. This was done by someone else a long time ago. There is 3 logical racks set up.
Here is how they are set up:
Rack0: 8) Flex I/O modules - 4) IA16 and 1) OA16 3) Open
Rack1: 2) Flex I/O - 2) IF4I Analog modules
Rack2: 8) Flex I/O - 6) IA16 2) Open
The RSL G file is set up with all 3 racks being full racks.

OK now the questions:
1. Based on Ron's answers, I can have up to 8 words of inputs and 8 words of outputs per full rack. So does that mean that I can have 8 IA16 and 8 OA16 modules per rack? I know I would need to add another ASB adapter.
2. Is there any problem with setting up the G data as full racks even though I don't have a full rack on all the racks?
3. The rack with the Analog inputs has 2 modules. How many words does each IF4I use up? Is it more than 4?

Thanks for the help. I think I am finally getting how this works!!

One more question: Can I put 8 point modules on the same rack as 16 point modules?

Hi

It maybe better to start a new post, but I will try to answer some if your questions.

1. No you can Ony have 8 groups per rack so the how I look at this is I can only have 8 tb bases which is 8 groups. But as each group could be a input or output module both the input and output data files will have words for that group. Also some input cards use the output words to set them up and some of ye output cards use the input word for status words for that card

2:Not sure but in plc 5 this would not work but not sure about slc. But if it was me I would st it up as 1/4, or 1/2 or 3/4 or full only as the rack is done as you may want to use let's say the second half of the rack on another asb

3 again not sure but the data sheet will show you some cards have a status word and some input cards need outputs words to set them up.


Hope this helps


Donnchadh