AVR-Chat

On Tue, 2005-08-16 at 19:09 +0000, riza1pk wrote:
> hi
> this is my first mail to group

   welcome.

> i am working on my final year project can i want to replace the analog
> module of plc in my project therefore i have to design a 
> miicrocontroller interface card(the graphic display is to be viewed on
> pc for this i am using RS232

  While it can be done, of course, sending graphics over an RS-232C link
is probably not optimal.  Not much bandwidth there, things will be very
slow to display and update.  Now, of course, if the graphcis are all
being generated on the PC by a contorl application, and all that is
being sent is process data, that is a different story.  That will work
just fine.

>  but i have to send the status signal from computer to interface card

   What "status signal" are you referring to>  A lot more specificity is
needed here.

>  and then to logo 

  Logo?  perhaps you meant to type "logon?"  Evenso, not sure I
understand what you mean by this.

> for this task i dont know what should i use please let me know thanks

  Nowhere near enough information for your question to even approach
being meaningful.  What is this plc going to be doing?  What kinds of
status informatino is to be displayed, at what data rate, and with what
degree of criticality in terms both of accurayc and in terms of timing?
What kinds of data rates do you anticipate?  What quantity of data needs
to be displayed?

  Are you asking for advice on what processor to use?  How to architect
your firmware? What algorithms to use for your ADC?  

   Essentialy, any processor in the AVR line COULD be used successfully
for this. Speed and data capcity (memory) are the two biggest factors
for you, I think.  A processor with on board ADC might be a good choice,
but that depends on your application.  How accurate and fast (and what
resoltuoion) does it need to be?  If you need more than 10 bits of
resolution, you'll have to use an external ADC and interface it through
the processor chip.  If 10 bits at relativelylow speeds (less than 15
Ksps) will suffice, and a little inaccuracy due to noise in the system
isn't a huge problem, the on board ADCs available in some AVRs will
suffice.  You can also opt to use the ADC Noise reduction sleep mode to
reduce the impact of switching noise onthe ADC conversion accuracy.

  AVRs are available that run on clock speeds from 1 Mhz through 20 Mhz,
which translates, roughly, to 1 MIPS through 20 MIPS throughputs.  The
speed to choose is predicated primarily on the requirements of your
application, with a secondary consideration being cost.  Don't choose a
processor substantially faster than what you really need, as it
unnecessarily drives up the cost.  On the other hand, if your processing
requirements are such that an 8 Mhz processor will be hard put to
complete processing one conversion before the next conversion is due,
you may need a faster processor (in the AVR line, that could be 10, 12,
16 or 20 Mhz). 

  Keep in mind that all AVR processors are 8 bit machines.  That doesn't
mean you have to use only 8 bit arthmetic, but that will certainly be
the fastest way to program.  I have applications runningon AVRs that use
32 bit integers.  A tad more complicated to program, unless you're
programming in C or some other high level language.

   For PLC applications, some AVRs have other peripheral options you may
want to consider as well.  For example, processors with PWM capabilities
can be used toprovide DAC as well as ADC capability.  They can also be
used for motor speed control, illumination controls (light dimming), and
other forms of proportional power control (with approrpiate power
interfascing circuitry).  Frankly, I believe that ther AVR is ideally
suited for PLC use, and that an exceellnt, full featured PLC could be
based onthe AVR chips at low cost and with truly spectacular performance
capabilities.  Most PLCs offer very little inthe way of programmability,
memory resources, or I/O capabilities.  Consider, for example, the
AT90CAN128.  A trulypowerful ittle 8 bit controller, with built in CAN
bus networkign capabilities.  Also offers several serial controllers on
board, making it ideally suited as the foundation for a powerful and
flexible PLC design.  Combined with all the digital GPIO capabilites,
and ADC and DAC capability, and networkign, how could you go wrong for a
few dollars?

Tom