JPH
A V.F.O. might be a better approach for Your 0 - 24 volt reading.  Buy reading just Freq. from the VFO it would use only one I/O pin as well.

You'll have to find a VFO with a TTL level output of coarse.

This brings up a good point that I failed to point out. On the enclosure there will be a headphone jack style connection for each of the voltage options so that the user can connect this system to anyone of the other interfaces. so unless I get in to displays and user interfaces having a single I/O doesn't seam like it would work.. But maybe I am thinking inside a box..

JHP
I thought You were looking for voltage readings. This is what I got from Your first post. I see now that Your trying a form of MULTIPLEXING with the SX28. Since Your using only one headphone jack,I don't see the need to tie up 13 I/O pins.But its Your project, So 13  window comparators, each set a volt higher then the previous, should work.

I personally would do this with software, But maybe You have found a new way to do this.

If You could add some code/hardware info, this would help Your post.

I will start with a better end application example....

The user will have a pigtailed line that can be placed on one of 4 different systems (but we see the need to have growth room for other control voltages that is why I have 13 different comparisons) when these systems send a voltage higher than the control or base voltage then we need a relay to trip and activate another system...

I really hope this helps clear things up.  NOTE: End user will not have a technical mind so it needs to be a simple "ok due when you plug it in to the red box set the switch to the red dot, when using the blue system set the switch to the blue dot...."
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Ok what if I did this...

Headphone jack -------------------------> ADC0831 8-bit A/D converter ------------> 13 or more position switch ------------> I/O pin 1-13 on SX28AC/DP-G
I may still be thinking inside the box, but to be truthful I don't fully understand your suggestions so I am doing a little Google research today to better understand them...

Code: I don't know the code yet, I will have to learn the SX assembly language or use a compiler but the logic would look like this....

Each pin would then have the question
If (the 8-bit reading converted to a numeric value) is greater than (the control voltage) THEN send OUTPUT pin HIGH
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I don't know what a window comparator is so I may change my thinking once I do my homework.  Without a doubt I want the easiest and cleanest way to do this, so I am open to all ideas I don't have "a way I want to do this" you will never suggest something that I will argue with based on pride or and existing plan....  
and lastly I can't thank you enough for taking the time to help me.

AM I MAKING THIS TO HARD???

Copied from http://www.parallax.com/Store/Microcontrollers/SXChips/tabid/138/ProductID/355/List/1/Default.aspx?SortField=ProductName,ProductName
"The on-chip functions of the SX28AC/DP-G include a general-purpose 8-bit timer, an analog comparator, watchdog timer, a power-save mode with wakeup capability, a configurable internal oscillator and high-current outputs."
The key question being the "analog comparator"  can I use this organic ability of the chip to do this with nothing but a switch and software?

Headphone jack -------------------------> 13 or more position switch ------------> I/O pin 1-13 on SX28AC/DP-G
Or in reality because of the 12vdc and 24vdc needs this...

Headphone jack ------>  13 or more position switch -------> 1-11 to I/O pins 1-11 on SX28AC/DP-G and 12-13 both going to 12vdc and 24vdc relays that will then send I/O pins on the SX28AC/DP-G to 5vdc when selected?

JHP
The SX28 does have a comparator. But only one, and its limited to 5.25volts MAX input!, This is pins RB0,RB1, and RB2.

You have Me confused with the 13 pos. switch and 1-11 I/O pins?

Can You draw out Your schematic in windows "Paint" and then save it as a JPEG and attach it to a post reply?
This will help Me and others to see what Your after.

Ok here is my latest idea bases on my limited understanding of the capabilities of these components.

A little disclaimer my ideas seams like the worse idea i have ever had.  My problem is that I just don't know what tools I should use solve this problem... Therefore I am trying to make round things fit in square holes...

Nonetheless here it is:

JHP
I kinda see what your after and I have a few ideas. I'll have to think about this a Bit, and see which idea uses the least components.

Maybe I'm missing something but it seems to me that if you want to detect the incoming voltage reliably you should run it through a divider to limit it to a safe level and then read the voltage with an ADC.

As I said in my first post that was my first thought, but I am learning different ways to do this everyday...

JPH
Hang in there.

I'm really tied up now, But I'll get some free time soon.

Keep searching these threads and You might find what You need.

Thank-you so much, no rush, I am just thankful for the help, I have e-mail alerts set up so when you get time to post your ideas I will get a heads up....

As for the rest of this forum I am looking but haven't found anything yet.... I will keep at it.

JPH
Did You get Your project going?
Or do You still need some with it?

Yea I still need some help, I haven't found anything worth mentioning.  So any ideas you have would be welcome.

JPH
I think Your Idea of using the ADC0831 is the way to go. Like JonnyMac suggested, A simple voltage divider in front of the input to
the ADC0831 would do the trick.( reduce 12volts down to 5volt TTL)
The ADC0831 has some really good resolution, So scaling 12volts down to 5volts shouldn't be a problem.

A simple 10kpot with a current limiting resister will divide the 12volts down to 5 volts (by adjusting the pot) in Your test setup to the ADC. This could be replaced with simple fixed resisters after You have tweaked the circuit, for a production run.

I would use another voltage divider for the 24volt signal and use the comparator pins on the SX28.( pins RB.0,RB.1 and RB.2 )
I would also make sure that all voltage dividers DO NOT exceed 5volts on their outputs before I hooked them to any component!, along with
5 volt zener diodes in My test bed to help insure I don't ZAP anything.

I know this might sound complex but its just for the test and setup mode.Once a setup has been debugged the components can greatly be reduced.
The specs for the ADC0831 can be found on the Parallax home page/store. its a PDF.

Ok things are getting crazy at work and it may be 2-3 weeks before I sit down and start bread-boarding this.  If I PM you and post any question I have would that be considered a good way to get in touch with you?

JHP
I know the crazy thing at work, just about to start up a 4th boiler, with its own Turbine-Gen. set.

I know I didn't cover any software in My last post reply, But feel free to PM me.

I have an old tractor that I'm working on, And charging voltage is something I need to monitor using the SX28.
I'll use a ADC0831 to read this voltage along with a voltage divider in front of the ADC.

Hi All;
But why would you use an external A/D when the SX can do this all internally so easily..... that is if I understand the problem correctly..... I think this can all be easily done with some push buttons and a handful of resistors.

JPH, I think you are making this WAY too hard. But please, to be sure, can you explain in different words exactly what you are trying to accomplish... there is still some guessing going on here on the readers' parts.

Think outside the box; don't presume certain implementations such as multiple input phone jacks, particular relays etc., unless such are a specific requirement, and  then explain why so we can get a  better understanding. Describe what needs to be achieved, but not how to implement it.

I suspect what is required here is bordering on trivial, but we need a better desciption.