RE: [CAD_CAM_EDM_DRO] Re: I'm curious

Hi Dave,

Thanks for the very informative tutorial on PLCs. I've not used or
implemented one, I only remember ladder logic from the fire control
(missile) systems on the FBM sub I was on. That was a long time ago!

I liked the ladder logic diagrams we used, from what I remember, in
troubleshooting you could walk right through them, looking for where the
progress stopped, and determine what was missing from the "equation".

Humm, in my new company, we're doing "process control" of signals coming in
from an industrial machine (example, laundry), and controlling the
dispensing of materials in response to those signals. Some of the
"processing" involves logic that might lend it's self to a PLC language,
rather then a complicated set of "rules" written by endless if-then
statements. It sounds like it would make the process much easer to support
and understand.

I do see how some ladder logic or PLCs could aid in machine control (CNC).
And the more ways you know how to control something...

Perhaps I'll look up a simple example of a PLC language, and see how it
could be compiled (interpreted?) on a PIC. Who knows!

Thanks very much for the info!

Alan KM6VV

> -----Original Message-----
> From: CAD_CAM_EDM_DRO@yahoogroups.com
> [mailto:CAD_CAM_EDM_DRO@yahoogroups.com] On Behalf Of David A. Frantz
> Sent: Thursday, July 21, 2005 10:20 AM
> To: CAD_CAM_EDM_DRO@yahoogroups.com
> Subject: Re: [CAD_CAM_EDM_DRO] Re: I'm curious
>
> Hi alan;
>
> Another dave here. PLC's are an abstraction of older machine
> controls that where all relay based. In a sense the PLC environment
> emulates a relay controller. To do this an interpeter is often
> implemented on the target processor, though is also possible to
> implement a compiler to do translation into another form. So to run
> PLC code on a PIC you would need to implement an interpeter to execute
> PLC "code" and the monitor software to allow communications.
>
> Communicaitons on a PLC is a key element in the technologies acceptance
> in industry. This leads to the Ladder Logic displays on a laptop seen
> in factories all over the country. This provides for an "easy" to
> debug environment. In effect the PLC software does parse in a manner
> similar to Gcode but is optimized to solve logic equations instead of
> motion control commands. Many large CNC systems have a PLC function
> integrated into their contoller. The CNC subsection handles motion
> and the PLC handles the rest of the machines needs.
>
> As to your description of a PLC and its looping through equations that
> is very very very close to being right on the button. What you have
> to realize is that there are update operations that take place after
> looping through the equations. That is the PLC handles things
> roughly like this:
> 1. Read inputs and place them in a bit table for inputs.
> 2. Solve the logic equations.
> 3. Take the results setting in the output bit table and update the real
> world outputs.
> 4. Handle house keeping such as timer updates.
> 5. Loop back to the top and start over.
>
> The logic equations are often solved via a stack mechnism. Generally
> the elements of the equations look at the status of bits it the bit
> tables for Inputs Outputs and Booleans to derive their Boolean value.
> Timers and counter have to be supported also. At that point you have
> a PLC, but with modern controllers much more is added to them to enhance
> what can be done programmically.
>
> With the more advance controllers the enhanced functions allow for very
> easy programming of state machines but often you end up loosing osme of
> the advantages of a PLC. That would be the friendly debugging
> environment and almost self documentation. Aften you can end up in a
> situation where there is little advantage to a PLC over more
> conventional languages when the advance functions are used. At least
> from the programmers or technicians viewpoint. You still have the
> PLC's biggest advantage on the plant floor which is the hardware and its
> ability to snap in place various bits of hardened I/O.
>
> As to "ladder logic" there is actually an ISO standard for this.
> Unfortunately I do not know of many complete implementations and it does
> look like there was to much influence exerted by the big PLC
> manufactures. Worth looking up though as a base point. Better yet
> experience a PLC first hand by taking a training course at Allen Bradley.
>
> As far as implementing a PLC it doesn't take much, some PLC's didn't
> even have a microprocessor in them to speak of. If any body here
> remembers the 5TI and worked on them they can elaborate on just how
> limited a usefull PLC can be. The size of the required microprocessor
> depends upon what you wish to implement. A PIC could very well
> implement a simple PLC that has the primary function of being an
> enhancement to a PLC controller. It might not work to well as a
> stand alone PLC competeing against PLCs with 32 bit embedded
> hardware. I do not think that the PLC core on the PIC would be the
> biggest part of the software project though, user interface and
> communications would be a significant piece of work.
>
> In any event keep in touch with what you are doing. There is a long
> way to go with respect ot low cost CNC hardware and software. PLC
> support is probably wedged in there someplace as a future gota have.
> Ideally all CNC I/O not axis specific would be handled on a PLC module.
>
> Thanks
> dave
>
> Alan Marconett wrote:
>
> >Hi Dave,
> >
> >Do you actually implement ladder logic on the PICs?
> >
> >I have seen and worked with ladder logic hardware, (that was a long time
> >ago) and I'm quite familiar with PIC ASM and C programming. But I don't
> see
> >any direct way to code ladder logic. Implement a state machine? Or is
> >there a compiler for ladder logic? I can see that ladder logic could be
> >expressed as Boolean equations, perhaps one just keeps looping through a
> few
> >equations? Just curious.
> >
> >Or maybe you meant do Gcode parsing in a PIC. This I can understand, I'm
> >thinking of porting my CNC controller program STEP4 (the motion module)
> to a
> >PIC. OK, a BIG PIC!
> >
> >Alan KM6VV
> >