Re: Re: 3 axis controller

[Ray <rehenry-at-up.net>]

John

Forgive me for jumping in here but I think I know just a little about both
questions below.

On Fri, 06 Apr 2001, john s wrote:

>I believe I would use the same
> PLC with the emc.  What I don't know is how powerful the emc is compared
> to the Mektronix for all of the miscellaneous functions necessary in a
> clean retrofit.  It is obvious that you are familiar with the Mektronix
> control.  If you could give me some additional insight, I would
> appreciate it.  

Fred Proctor at NIST said in a recent post that if someone held a gun to
his head that he would write a PLC in Tcl/Tk and run it directly from the
computer running EMC.  In fact he has done just that with two files called
tkio and iosh.  It is not possible right now to do the usual visual ladder
logic with this approach but it will work to do most everything that ladder
will do.

The problem with using the same proprietary PLC that you would use otherwise
is that you will have to figure out how to make the EMC's NML communicate
with that PLC.  It might be possible to write a serial program that watches
the NML and tells the external PLC what to do next.  Someone else would
have to respond here.

There are also other "soft" or PC based Programmable Logic Controllers out
there.  (I wrote that the way I did because at least one company slapped my
hand for writing soft and plc in the same word.  They believe that they
copyrighted that word.)  I have seen several efforts in this direction
including the PuffinPLC project that currently is able to run a DIO-48 card
and has plans available for industrially hardened interface to that card.

www.linuxplc.com 
www.puffinplc.com

None of these projects have directly interfaced into the EMC at present.

>Also would you use the servo-to-go board for the motion
> board.

STG is a good choice.  Everything is in place to use it.  Jon E. and Matt
have been working on a system that wires up to a machine in a cleaner
way than an STG system.  The STG card uses 4 50 pin flat cables to bring
out all of the points on an eight axis system.  These exit the PC box and
travel to a set of screw interface terminal boards or some such.  Three of
them could be made internal to the PC, the fourth looped back in but any
way you look at it you have a pile of wiring into and out of the PC.

The system worked up by Jon and Matt, (perhaps they will send pictures. 
Perhaps you will see it at NAMES.) uses a single parallel port cable to
carry all of the signals between an EPP and an auxiliary card cage to which
all of the machine wiring can be attached.  The PC could be a laptop or SBC
built into the machine's control station and the single 25 wire cable can
be snaked from it into the control cabnet.

If you look at some of the photos on Matt's site you can see how he rigged
an STG card to a couple of systems.

http://users.erols.com/mshaver/index.htm

You may also understand why he has been pushing the parport alternative.

Hope this helps

Ray