Difference between Rack and Chassis of a PLC

I am confused due to use of these two terms. In some books the term rack is used and in other books the term chassis is used. Are these two terms same?

They can be the same.
A Chassis is normally a full frame to support the CPU/I/O.
A Rack can be the same But it may also be pluggable components
limited in length only by the maximum I/O of the PLC
this is also Manufacturer dependant

I'll assume (gosh I hate that word) that you want the "technically correct" answer ...

let's go back to the days of Allen-Bradley PLC-5 (and incidentally there are still a LOT of those older systems out there – still faithfully chugging along) ... we could probably go back even further – but that would be before I personally started working with PLCs, so I tend to draw a line at the PLC-5 systems) ...

here goes ...

a CHASSIS is a piece of hardware - which has SLOTS into which you can insert different types of MODULES ... if you look at the label on the end of the CHASSIS, you will see that it is clearly marked/labeled as a "CHASSIS" ...

specifically, (and technically) the piece of hardware which has the SLOTS should NOT be called a "RACK" ... but ... (and here's what makes it tricky) many (most?) people DO indeed call the thing a "rack" ... that's unfortunate ...

because ...

the word "RACK" actually DOES have a definition ... the word "RACK" technically means:

EIGHT words of IN-puts – and – EIGHT words of OUT-puts ...

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so when you say "RACK" you're talking about sixteen WORDS of data – and there are sixteen bits (boxes) in each of those WORDS ...

so ...

technically speaking, a RACK is a "unit measure" ... it's like an ACRE of land, a TON of steel, a GALLON of milk, a DOZEN eggs ...

more specifically, a "rack" is NOT a piece of hardware – with slots to hold modules ...

once you understand that "secret handshake" - then it becomes easier to understand how what some people would call a RACK* can actually contain TWO (or more) RACKS ... (*hint: here they're mistakenly calling a "chassis" a "rack") ...

the problem/confusion arises because some (many? – most?) people throw the word "RACK" around where it really doesn't belong ... so ... you'll hear them try to explain things this way:

Using one-slot addressing with a 16-slot rack – you'll have two LOGICAL racks within the one PHYSICAL rack.

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do you see what's happening? ... they insist on using the single word "RACK" – but they're giving it TWO separate meanings ... that's why they have to differentiate between what they call the LOGICAL RACK (the unit measure) and the PHYSICAL RACK (the hardware) ...

let's paraphrase the statement and get it right ...

Using one-slot addressing with a 16-slot CHASSIS – you'll have two RACKS within the CHASSIS.

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now see if the chart below makes sense – and post again if you have any questions ... we still haven't covered TWO-SLOT or HALF-SLOT addressing – but hopefully the difference between a CHASSIS and a RACK is a little more clear now ...

note: with SLC and (other Allen-Bradley systems) the rules change ... this discussion has been based on the PLC-5 ...

.

An actual Rack -


is this early enough Ron

well, for some students the idea of torture definitely applies to this particular subject matter ...

Thanks for the proper exclamation Ron.
I think a lot of us think of a chassis as being a empty rack. Once you add all the components and power supplies you then refer to it as a rack. Even in the control logic platform we still refer to the assembled components as a rack even though there is no chassis.

Mark Buskell said:

Thanks for the proper exclamation Ron.
I think a lot of us think of a chassis as being a empty rack. Once you add all the components and power supplies you then refer to it as a rack. Even in the control logic platform we still refer to the assembled components as a rack even though there is no chassis.

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Were you thinking of CompactLogix, Mark ?

That definitely does not have a chassis, the modules simply "butt" together, and the assembly can be DIN-rail, or panel mounted. You address the modules by their "slot" number, even though there is no physical "slot", and of course you can't have "empty" slots

ControlLogix, however, does have a chassis, it's the card cage that you attach a power supply module, and plug the Controller, I/O, and Comms modules into. ControlLogix chassis come with 4, 7, 10, 13, or 17 slots for modules. They can be left empty.

iant said:

is this early enough Ron

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I prefer this sort of Rack...

I hate those senior moments what I get confused. On second thought maybe this was a test to see who was paying attention.
Good example Daba.
I could think of one more though.
She has a nice chassis with an amazing rack.

and to Quote Monty Python
"Huge Tracts of Land"

Mark Buskell said:

She has a nice chassis with an amazing rack.

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Amazing car

http://www.youtube.com/watch?v=FHYja3xEYOc&feature=relmfu

L D[AR2 said:

Amazing car

http://www.youtube.com/watch?v=FHYja3xEYOc&feature=relmfu

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Needed a bit more acceleration I think....

L D[AR2 said:

Amazing car

http://www.youtube.com/watch?v=FHYja3xEYOc&feature=relmfu

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You've got to love the strategically placed seatbelt

I agree with Ron