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'[OT] Would this be a useful feature in a micro PLC'
2007\03\25@175141 by William Couture

face picon face
Well, at work we're adding PLC functionality to our temperature
control units, doing (at least for now) ladder logic.

We have a 16-bit ADC with support circuitry, and can do a 24-bit ADC
if we need to.  However, this is a fairly slow ADC (as low as 8 samples
per second).

However, I'm saying that, if we are going to allow users PLC-type
programmability, we should also give them some kind of access
to the on-board ADC.  While it is only 10-bits (and figure 8 bits
with noise and whatnot), they can have kilohertz update frequences.

The EE says that it would be "useless" unless the surrounding
circuity was designed for a specific application.  I'm saying that
we don't know what they will want to do, so we should give them
maximum flexibility, even if that means that they have to add an
external opamp (beyond our onboard opamp / buffer).

Your thoughts?

Thanks,
  Bill

--
Psst...  Hey, you... Buddy...  Want a kitten?  straycatblues.petfinder.org

2007\03\26@093103 by M. Adam Davis

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Well, 25% or more of the bits will be useless unless the surrounding
circuitry was designed with the application in mind or you very
carefully designed the ADC circuitry on the inside so that one could
easily buffer it externally with no added internal noise.

The EE is correct if by "useless" he means that the A/D is losing
significant accuracy and can't be used for many fast PID type tasks.

However, if your customers only need to know whether the temperature
of a process is within 10 degrees of optimum, or the motor current is
not excessive, etc then they may only need a solid 4 or 5 bits of A/D.

But I think the real answers are going to come from your customers.
>From my perspective you're going to be spending a lot more time and
energy supporting such a feature (and explaining it's drawbacks) than
you'll spend implementing it.  Make sure you have it very well
documented, including the important bit of schematic so another EE
using your product can design a decent front end for it.

-Adam

On 3/25/07, William Couture <spam_OUTbcoutureTakeThisOuTspamgmail.com> wrote:
{Quote hidden}

> -

2007\03\26@120601 by Aaron

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Absolutely provide access to the onboard ADC!  I've been doing PLC
programming 6+ years and can count on one hand the number of PLC
applications without analog that I've completed.

Aaron

William Couture wrote:

{Quote hidden}

2007\03\29@142322 by William Couture

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Hi!

A follow-up to your response:

You do realize that I'm talking about adding the onchip
high-speed-but-low-resolution ADC in addition to the external
low-speed-but-high-resolution ADC of the base system, right?

Thanks,
  Bill

On 3/26/07, Aaron <.....aaron.groupsKILLspamspam@spam@gmail.com> wrote:
{Quote hidden}

> -

2007\03\30@122329 by Aaron

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William Couture wrote:

{Quote hidden}

Sorry, I did misunderstand you.

I did wonder why you were talking about 16 or 24-bit A/D, though...

Aaron

2007\03\30@125455 by William Couture

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On 3/30/07, Aaron <aaron.groupsspamKILLspamgmail.com> wrote:

> >A follow-up to your response:
> >
> >You do realize that I'm talking about adding the onchip
> >high-speed-but-low-resolution ADC in addition to the external
> >low-speed-but-high-resolution ADC of the base system, right?
>
> Sorry, I did misunderstand you.
>
> I did wonder why you were talking about 16 or 24-bit A/D, though...

But, still...

Do you think a high-speed-but-low-resolution ADC would be a
useful feature?

Thanks,
  Bill

--
Psst...  Hey, you... Buddy...  Want a kitten?  straycatblues.petfinder.org

2007\03\30@130304 by Dario Greggio

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William Couture wrote:


> Do you think a high-speed-but-low-resolution ADC would be a
> useful feature?

Only for capturing video signals ... !


--
Ciao, Dario


'[OT] Would this be a useful feature in a micro PLC'
2007\04\02@170403 by Aaron
picon face


William Couture wrote:

>Do you think a high-speed-but-low-resolution ADC would be a
>useful feature?
>

Yes.  I've done stuff where low-resolution ADCs are suitable.  Believe
it or not, we are currently rebuilding an 80-year old hydraulic press
for one of our customers.  We are using one of these:
http://www.ab.com/programmablecontrol/plc/micrologix1100/controller.html
and monitoring hydraulic pressure with the onboard 10-bit ADC.  In this
particular application, 8 bits would likely be sufficient.

An expansion module with a 15-bit ADC is doing the critical task of
measuring position from some LVDTs.

Aaron

2007\04\02@222419 by Bob Axtell

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Aaron wrote:
> William Couture wrote:
>
>  
>> Do you think a high-speed-but-low-resolution ADC would be a
>> useful feature?
>>
>>    
>
>  
Yes, because the harsh environment we are in does not allow practical
12-15bit A/D conversions.
10-bit or 8-bit is fine, but a 0-5V input would be very helpful.
Normally PLCs have a 0-10V input,
and we have to shift our 0-5V to 0-10V before it is useable.

--Bob

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