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Old January 8th, 2019, 10:02 AM   #1
sparkytex
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1769 IO Rack capability

Hey Guys,

Quick question, How many modules can you have on a 1769 compact logix rack before you need to install two of those end caps to start a new rack. As it stands right now There's 8 modules including the processor and power supply currently in the rack and I need to install 3 more input modules.

thanks,

- Tex
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Old January 8th, 2019, 10:16 AM   #2
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Depends on the CPU

Page 4 https://literature.rockwellautomatio...d006_-en-p.pdf
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Old January 8th, 2019, 10:32 AM   #3
LoganB
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Does it also depend on the IO itself? Analog vs digital? Or is that just for the power supply size/location.
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Old January 8th, 2019, 10:45 AM   #4
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Quote:
Originally Posted by LoganB View Post
Does it also depend on the IO itself? Analog vs digital? Or is that just for the power supply size/location.
I have never ran across that, just limited by a number... so if you have a x processor it can handle x number of modules, see the link above and look at page 4, larger the CPU (process power) the more number of I/O's it can support, a 35 can support 30 modules and a 24 can only support 4
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Old January 8th, 2019, 10:51 AM   #5
MikeyN
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Please, correct me if I am wrong, but I think the power supplies can only handle 8 modules. If the processor can handle 16 modules, this would need to be broken up into two banks of 8 with a power supply for each bank. As long as OP's processor can handle the number of modules, he will still need to break it up into two banks.
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Old January 8th, 2019, 10:54 AM   #6
LoganB
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Originally Posted by MikeyN View Post
Please, correct me if I am wrong, but I think the power supplies can only handle 8 modules. If the processor can handle 16 modules, this would need to be broken up into two banks of 8 with a power supply for each bank. As long as OP's processor can handle the number of modules, he will still need to break it up into two banks.
This is probably what I was thinking of.
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Old January 8th, 2019, 11:25 AM   #7
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Quote:
Originally Posted by MikeyN View Post
Please, correct me if I am wrong, but I think the power supplies can only handle 8 modules. If the processor can handle 16 modules, this would need to be broken up into two banks of 8 with a power supply for each bank. As long as OP's processor can handle the number of modules, he will still need to break it up into two banks.
Depends on the power supply and current draw of the cards/CPU

The maximum amount of current the system supports in both directions is:
1769-PA2, 1769-PA2K, 1769-PB2, 1769-PB2K: 2 amps at 5V DC; 1 amp at 24V DC
1769-PA4, 1769-PA4K, 1769-PB4, 1769-PB4K: 4 amps at 5V DC; 2 amps at 24V DC
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Old January 8th, 2019, 11:51 AM   #8
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I was thinking of the distance rating: https://literature.rockwellautomatio...d008_-en-p.pdf

The distance rating of the PA2, PB2, PA4, and PB4 is 8 on each side of the power supply for a maximum of 16 modules. I've never considered it, but can the power supply be mounted in the middle of your IO? For example: COMMS|CPU|IN|IN|OUT|PS|OUT|OUT|AI|AI|AO? The only setup we have here at our facility that is anything like this is setup like this: communications module, power supply, processor, I/O.
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Old January 8th, 2019, 12:06 PM   #9
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You can also have multiple power supplies in the same rack if you are unable to break up the rack with the cables. I'm sure it's not recommended by rockwell but I've seen it done enough times with no issues to keep it in the back pocket.
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Old January 8th, 2019, 12:15 PM   #10
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Quote:
Originally Posted by MikeyN View Post
I was thinking of the distance rating: https://literature.rockwellautomatio...d008_-en-p.pdf

The distance rating of the PA2, PB2, PA4, and PB4 is 8 on each side of the power supply for a maximum of 16 modules. I've never considered it, but can the power supply be mounted in the middle of your IO? For example: COMMS|CPU|IN|IN|OUT|PS|OUT|OUT|AI|AI|AO? The only setup we have here at our facility that is anything like this is setup like this: communications module, power supply, processor, I/O.
Yep. People locate the power supply in the middle of the block sometimes, I've seen that. I always try to do PSU>CPU>COMMS>IO but there are times (especially with conversions) where you want IO to be in a particular slot #, and that sometimes means the PSU needs to be located further down the block.
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Old January 8th, 2019, 12:20 PM   #11
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Quote:
Originally Posted by LoganB View Post
People locate the power supply in the middle of the block sometimes, I've seen that..
The reason I have seen it use is for arc flash, 24v on one side and 120 and up on the other, the cabinet was double doors and the power supply was in the middle... so in theory you could open the left side of the cabinet and not need to get dressed up in all the arc flash PPE, if you opened the right side you needed to be dressed, not sure if it passed the code inspection but that was the thought behind the ones I saw
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Old January 8th, 2019, 03:41 PM   #12
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Quote:
Originally Posted by MikeyN View Post
I was thinking of the distance rating: https://literature.rockwellautomatio...d008_-en-p.pdf

The distance rating of the PA2, PB2, PA4, and PB4 is 8 on each side of the power supply for a maximum of 16 modules. I've never considered it, but can the power supply be mounted in the middle of your IO? For example: COMMS|CPU|IN|IN|OUT|PS|OUT|OUT|AI|AI|AO? The only setup we have here at our facility that is anything like this is setup like this: communications module, power supply, processor, I/O.

The way you describe is the best practice way to do it, as far as maximising your module count.

As others have mentioned, there are two factors that limit the size of a Compact Logix chassis: the processor type and the power supply distance rating.

The processor type hard-limits the number of I/O modules on your local chassis. For example, the 1769-L30ER can handle 8 modules and only 8 modules. Though this isn't strictly speaking related to the original question, because if you hit this limit, then just adding an extension cable with another bank of modules and another power supply won't help you - the L30ER only supports 8 local modules, period. If you needed to add more than 8 modules, you'd have to use a "remote" rack, which could still be physically in the same location, but would need a fieldbus adaptor like the 1769-AENTR, as well as a power supply.

The power supply distance rating is a somewhat tricky one, because when you're defining local I/O, you don't tell the PLC if or where it's split into a separate bank of modules (using the extension cable). The PLC doesn't know, so technically you can exceed the distance rating, and nobody will stop you. Sometimes you'll get away with it, sometimes you won't, and if you don't the problems you encounter may be quite sporadic, misleading, and hard to pin down. A case of "just because you can, doesn't mean you should".

So, boiling all of that down to practical numbers: assuming your CPU supports a large enough number of local I/O modules, the largest number of modules (excluding the CPU) on your first local rack is generally 11, and on all subsequent racks is generally 16. This is because the CPU has a distance rating of 4, meaning you can put 3 modules on the left of the PS, between CPU and PS. Then most other modules have a distance rating of 8, meaning you can put 8 modules to the right of the CPU. Total 11 modules. On the extension racks you can put 8 modules to both the left and the right of the PS, so total 16.

Of course, if some of your modules have a lower distance rating, you'll either have to arrange your rack to keep them close to the PS, or shorten your rack. I mentioned in the last paragraph that "most" modules have a distance rating of 8 - but the 1769-OB32 has a distance rating of 6, and some specialty modules have a rating of 6 or even less. So if you've got OB32's or specialty modules in the picture, you'd best dig a little deeper to confirm.

Of course, all of this is academic if you use Rockwell's IAB software, which will work all this out for you and tell you exactly what you need. Though it's still useful to know, because in some cases IAB might tell you that you need another rack with an expansion cable and PS for a particular configuration, but if you understand the limitations, you might be able to rearrange your rack to put the low distance rating modules closer to the PS and get it to work with a single rack.
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Old January 8th, 2019, 03:44 PM   #13
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The processor type hard-limits the number of I/O modules on your local chassis. For example, the 1769-L30ER can handle 8 modules and only 8 modules. Though this isn't strictly speaking related to the original question, because if you hit this limit, then just adding an extension cable with another bank of modules and another power supply won't help you - the L30ER only supports 8 local modules, period.

And just because George will be along shortly to add this little piece of information if I don't do it for him: The 1769-L30ER also only supports one bank of modules - i.e. even though it supports 8 modules, you can't put 4 modules on one bank and 4 on another, it's 8 modules all on one bank. Other controllers have similar restrictions, so this should be considered when working out your maximum expandability (as well as module placement and physical positioning in the cabinet)
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Old January 8th, 2019, 04:28 PM   #14
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Quote:
Originally Posted by sparkytex
...How many modules can you have on a 1769 compact logix rack before you need to install two of those end caps to start a new rack. As it stands right now There's 8 modules including the processor and power supply currently in the rack and I need to install 3 more...
Pretty much all of the good info ASF has provided you is also available here for anyone who can access it...

33181 - CompactLogix 1769 I/O and Bus Guidelines
Access Level: TechConnect

I will add or remind you of two things...

Don't forget that "if" you do here, or ever have to in the future on any application, have to add additional 1769 Local I/O Banks, then as well as having to "install two of those end caps to start a new rack", you will also require a separate 1769 PSU in each additional Bank. It's just your statement reads as though you only think you would require the end caps and the Bank 0 PSU will power the additional Bank(s) via the expansion cable. That's a negative. For a valid configuration, all Banks must have their own 1769 PSU and there must be only one 1769 PSU in each Bank.

Secondly, and as ASF has outlined - if your controller and module types do support you adding up to the general limit of 11 I/O modules in Bank 0, which would fit nicely with your 8+3 requirements, you may have to reorganize your existing layout to facilitate the PSU distance ratings. The reason I say this is because your current layout could be as follows...

Controller|PSU|8 I/O modules

Here you would need to move the PSU to the maximum distance permitted for the controller of 4 modules away, as ASF also pointed out. This then permits 3 modules in between them. The remaining 8 modules then added after the PSU for a total of 11 I/O modules.

If you already have 3 of the existing 8 modules in between the controller and PSU then you can simply add the 3 new modules after the 5 on the other side of the PSU.

This all of course still depending on you using all standard I/O modules and not speciality modules which may reduce that maximum of 11 modules down somewhat.

And of double course all depending on your controller model supporting more than the existing 8 modules in total.

I also posted this before which is somewhat similar to this thread...

Number of local expansion modules, max - AB

Regards,
George
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Last edited by Geospark; January 8th, 2019 at 04:31 PM.
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Old January 9th, 2019, 05:36 AM   #15
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And just because George will be along shortly...
And in under an hour, good guess ASF considering the time difference
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