High Density IO

TimeFluxCap

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Join Date
Nov 2002
Location
Australia
Posts
321
Hi all

99% of the PLCs I have seen have used 16 point input or output digital cards, in some cases many slots such as 8 cards to make up 128 inputs. Why not use high density IO 32 point cards so only four slots are taken up? Is there a sound engineering reason behind this?

I am looking after a vendor installing our first Control Logix on site and they have selected to use 32 point cards and Im happy to do so.
 
With Omron CJ1 I use 32 and 64 bit cards all the time. Make my own leads also - no room for bulky manufacturer terminal blocks. Make the leads long enough and they get to the door and regular terminal blocks direct.

With Omron CS1 (slightly smaller than SLC) I use up to 96 bit cards.

Can think of no reason not to use them except customer specification, too lazy to make up leads, manufacturer leads are too expensive, no room for manufacturers terminal blocks/relay blocks.
 
A lot is personal preference or company standard or company inventory.

I like 16 point input cards, and 8 point relay output cards. I like relays, becuase they are universal (ac or dc), and typically have higher outputs. Plus, while I try to avoid it at all costs, you can mix an ac and dc output on the same relay output card. It is good engineering (and safety) practice to keep ac and dc outputs on seperate cards.

When dealing with wires coming from the cards, 32 I's or O's is a lot of wires.

32 point is more expensive, and hopefully it's terminal strip will plug into the card, rather then having to rewire in an emergency with lots of people standing around watching.

Once again, I get to post Eric's 8" x 12" panel, note the number of wires coming from the 16 point input card, and 8 point output card. "LINK"

I tried to just post the picture, but it filled my whole montor, so I just posted the link. You can see, if you increase the numbers, it could get difficult. Some of the linking cables that some manufacturers have to remote terminal blocks are a great idea, speciall with the 32 point stuph.

Visit the Control Panel thread I started to see pictures of panels with cards of varying number of points and amounts of wires to fight with.

"LINK"

anyway...

regards.....casey
 
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If the i/o cards have cage clamp terminals with wire entry from the front, then i recommend using 32-point i/o cards.

If on the other hand the terminals are screw type, and maybe even arranged in two overlapping layers (like the ones in PLC5 32-point cards - they are AWFUL, HORRIBLE) then I dont recommend it.

For my larger systems I use 32 point cards with cage clamp terminals.
 
I agree with JesperMP, it depends on the physical layout of the card.
I've used the 32 point cards for the SLC and PLC-5 once . The wiring is difficult unless you buy prefabbed (expensive)swing arms.

But, I have dozens of TI-505 32 point cards and they are no more difficult to wire than the 16 point cards. I would use them without hesitation.

So...I think the deciding factor is ease of installation and maintenance.
 
/agree with most of the above.

High density cards are nice, but as PLC's get smaller, they become MUCH MORE difficult to wire. Just because of our usual specs, we often have to wire all digital I/O with 18AWG wire. 16 points isn't bad, but 32 becomes a nightmare.

The other potential issue, is that higher-density I/O modules as a general rule, can handle less current per point and per module, and can heat up more.
 
I always use 32 point input and output cards in my SLC 5/03, 5/04, 5/05. I also use the AB IFM to make my wiring neat, remote, and easy. Just like Casey states, company standard. We pretty much always use 32 point DC cards in all our systems. That keeps spare parts inventory low. The IFM might be more expensive than most want to spend, but it is much easier than wiring on the front of the card. Just my opinion.
 
If you are looking for a sound engineering reason I think rdast hit it. As the point count goes up either the output current per point goes down or the module heating goes up. So you need to look at your loads and see if the high density modules can drive the majority of your loads. I don't see the use of going with high density output modules and then buffering everything with solid state relays.

Alot of companies still like to use 120VAC for their control voltage. That limits both input and output point count in a given package due to required clearances for voltage withstand.

Keith
 
There are certain laws that just don't change, regardless of technology advancements.

One of them is current driving. You can't have less copper and be able to drive more amps. It just doesn't work.

With that said, when you have 32 point output configurations for modular PLCs, your output rating goes way down and your additional hardware cost goes up

For instance:

The DL 205 series DC output modules:

D208TD1 - 8 DC outputs (300 mA / output; 2.4 amps per module)

D216TD1 - 16 DC outputs (100 mA / output; 1.6 amps per module)

D232TD2 - 32 DC outputs (100 mA / output; 3.2 amps per module)

You are looking at 100 mA / output. Many real world signals are above this rating, thus you are adding additional relays to drive your outputs, thus adding cost. You may think you are saving panel space, but with the additional components, you are only increasing your need for space.

With regards to the 32 point input module, you are now using the Zip Link hardware and terminals...again additional cost when with other modules, you can wire directly to the PLC.

With regards to relay outputs, you will not see a high cluster output module, due mainly to space. The smaller the relay, the smaller the current driving capability.

I was asked by a potential customer...why are there more inputs than outputs in PLCs. The simple answer is space. You can incorporate more inputs into the same space requirement than you can outputs, due to the compoents used.

Potential customers ask us...why are your boards so much bigger than other PLCs. Our answers are, a base unit contains all components. With boxed micros, they usually comprise several circuit boards ribboned together. Our other response relates to our output ratings and built in protection. Our relay outputs are designed to handle 7-8 amps / output and our solid state can handle a 2 amp continuous rating with 8-10 amp inrush with fusing. The 32 point modules do not include this, and in order to do so will require additional panel space.

FYI - I only used AD as an example for this response because their catalog was handy.

God Bless,
 
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As PLC's got to be smaller it simply has become impossible to put all the functionality wanted onto such compact cards (ie 96 Relays!!). (For exactly the reaons Stephen has outlined above). In response the IO card itself has simply been extended out via a pre-made component to the terminal strip.


1. The panel does not have to be significantly larger. Usually it finishes up MUCH better laid out and tidier. Prebuilt wiring systems have been pre-tested and reduce errors. Sure they cost some to buy but this is usually offset by the cost of hand wiring your own and testing and fixing the mistakes.

2. The the remote terminal modules can be specified with a useful range of features such as LED indicators that show the field side status (as distinction from the logic side status on the PLC modules), fuses, monitoring points, and so on. Examples


Until about 5 years ago in this part of the world there was a lot of resistance to using premade wiring systems, but the pendulum has swung a lot and I would venture to say the majority of new systems here use 32 (or >) point IO and wiring systems.

In the NZ dairy industry we exclusively use premade wiring arms and terminal blocks. They are required by spec. Major grumbles occur if you don't.

Forget using a 32 point modules and wiring directly to them; major source of nightmares. (Unless you are truly driven by considerations of space and purchase cost as BobB outlines above.) But with the usual feeding chain four or five people may have worked on your panel before you get to fire it up, and premade wiring systems are just one less thing to go wrong.
 
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Interesting comments so far. Consider the following:

1) I always use high density cards, if available.
2) Space is a major issue but so is cost.
3) A 16 point relay output card costs about $350AU.
4) A 64 bit transistor output card costs about $700AU.
5) I wire all outputs to relays or direct to LED indication lights that are fitted with diodes.
6) It is an absolute pain to replace a relay on a PC board but easy to replace a plug in relay outside the PLC.
7) Input current load is not a heat issue, the input only draws 7ma.
8) The output current load is not an issue as the relays I use only draw about 14ma and multichip LEDs draw not much more.
9) If I do use a relay output card i still use external relays due to point 6).
10) I wire my own plugs so that I can have leads up to 5 or 6 metres to reach everything on the door, or the panel next door. The wire I use is 7/0.20 - do not know how that relates to AWG. Some manufacturers leads use solid conductors - no thank you.
11) Using manufacturers leads means that you have extra terminal blocks or very expensive relay blocks. No thank you.
12) I have jigs to make the leads. I have compared the cost of rolling my own to manufacturers leads and special relay blocks and have found the cost to usually be triple or more. I am then still stuck with extra terminal blocks, extra wiring, special relay blocks that take up too much space etc.

Yes, it is often a personal choice. I have made the decision to roll my own.
 
brucechase said:
I always use 32 point input and output cards in my SLC 5/03, 5/04, 5/05. I also use the AB IFM to make my wiring neat, remote, and easy.

Thats what i always try to use 32 I/O with IFMs on 24vdc if i need to i may use the 16 output relay cards as well
 
1) I always use 24VDC high density cards if needed since 1978.
2) 110-230AC is for the lighting, not for the control since 1978.
7) 110-230AC is zero 100-120 times/second
5) I don't use relays, but field- and ASi-busses for VFD's eg.
3) We can ctrl 24VDC-Solenoids for Pneumatic directly without relays.
4) I don't use hydraulic, its is dirty.
9) In principle, Relay is electro-mechanical problem-waste.
0) Use 0.5mm2 wires or les (awg 18) and small circuit breakers, currents are mA's .
8) Wiring problems are inside of the brain.
 
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Stephen Luft said...
"I was asked by a potential customer...why are there more inputs than outputs in PLCs. The simple answer is space."

I assume you are referring to a basic single CPU Module with inputs and outputs.

While it is certainly true that...

"You can incorporate more inputs into the same space requirement than you can outputs, due to the compoents used."

...there is also the issue of "How many Inputs does it take to Control an Output?"

A simple motor starter requires three inputs to control one output; Stop, Start and Aux Contact. That's three inputs per output.

A simple light circuit might be as simple as one switch and one light bulb. That's one input per output.

The PLC manufacturer has no idea what a particular process might look like. But he can be sure of one thing... if there are motors involved then, at least for the motor circuits, there must be more inputs than outputs.

The question is... How many more?

How many motor-type circuits might there be?
How many inputs can actually be incorporated in the space available?
What is the ratio of Inputs to Outputs?
Does that ratio seem reasonable for most cases? (whatever that means)
What is the cost per input?
How does the number of inputs jive with the addressing scheme? (is that an issue?)

"Smart Blocks" can be a pain in the a$$ with four outputs and only four inputs.

Sure, it's possible to have the four inputs control four motors. But that would mean developing a normal hard-wired circuit for each motor, but then, instead of having the circuit drive the motor mag, have it drive a control relay which provides the aux contact for sealing the circuit and a signal to the input.

There would have to be additional wiring to provide overload protection from the motor mag back to the hard-wired control ciruit... the "Smart Block" becomes nothing more than an un-necessary extra expense.


So... I think the "simple answer" is...
For a given number of outputs, the odds are that you will need more than one input per output.

Is it reasonable to assume that all outputs will require three inputs? No. That would probably be too many... maybe.

Is it possible that one input might control several outputs? Sure.

However, would it be reasonable to assume that each output will require only one, or less, input? No. That would surely be too few in most cases.

So, it would appear that a reasonable ratio of inputs to outputs is somewhere between 3-to-1 and 1-to-1. That averages to 2-to-1.

However, we rarely see a CPU module that has twice as many inputs as outputs. It seems that manufacturers have settled on a ratio somewhere between 2-to-1 and 1-to-1. 10-inputs to 8-outputs is 1.25-to-1. It appears that manufacturers are guessing that there are likely be more 1-to-1 cases than 3-to-1 cases. Similar modules provide approximately the same ratio.

The real-estate required for an input circuit is considerably smaller than that required for an output circuit.

If it is simply a matter of real-estate, then why aren't there three inputs per output? That would surely cover most of the worst case situations. However, unless every single output is a motor, that would probably result in a higher percentage of unused inputs.

It's a game of balancing a reasonable expectation of user requirements with the cost of manufacturing. The name of the game is profit margin.
 

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