A host of EMC newbie questions - sorry!

[webgeek]

Originally Posted by andy55 realtime is not about throughput, it's about latency.

True enough, the latency of those high throughput technologies is variable (though coming down dramatically with some tweaks to USB) BUT that is no reason for us to assume that signals need to be generated directly from the PC. Gecko's use of a signal generator tied to the PC makes perfect sense. Let the PC do what it does best and let low-level hardware handle generating signals. I'd love to see something like the Gecko controller as an I/O board I can just drop into my computer.

[unterhaus]

The point Tom Brown and webgeek are missing is that with a 66 MHz pc and a parallel port I can successfully run a milling machine including a pretty decent GUI. Of course, I better firewall it pretty good because it will be rooted in about 10 minutes. Now they've increased the processing power 50 times, and I need to run a cable from the back of my 4GHz computer to another processor that has roughly the same power as a 66MHz Pc. So I'm replicating my PC only with a proprietary device. It will be so much easier for the entire software/hardware industry when those pesky device drivers are all gone. What's happening is that there are now tons of proprietary designs like the Intel Stargate and the gumstix that are popping up where a pc alone could have gone. I really don't know why the parallel port is still around, but the serial port is incredibly useful for a lot of things, so I expect to see the Tiawanese Mobo manufacturers keep that around for a while longer. A realtime system I built was synchronized with 115kbaud serial, and it works extremely well, but my method is pretty specialized to the one problem. The canbus that makes new cars work is just a fast serial port, and that is very useful. Too bad it wasn't invented by IBM for the orignial pc.

As far as latency goes, the ISA bus still rules the world of serial and parallel ports. They just don't have the ISA bus connectors. But that latency is deterministic. Ethernet can be made deterministic, but USB can't. Firewire also is fairly deterministic, but IMHO, it's a goner. There is a $500 ethernet card for gamers which is low latency.

I'm not a big fan of step/direction control, but it works ok for a stepper system. I wouldn't run my giant VNC with it though.

Actually, connecting to the USB bus is trivial if you have $14. Or you can have remote intelligence and usb for $20-$30. It just doesn't do real-time unless you need tons of data at a 1Hz rate. I'm sure there are problems like that, but most people don't have the guts to sample that slow.

[andy55]

moving all of the RT of motion-control into dedicated hardware is not as easy as it seems. If you look at the RT part of EMC you will see that it includes trajectory control (acceleration/jerk limited velocity profiles), blending in G64 mode etc.
Those require a lot of processing and not just a simple step/dir generator...

[unterhaus]

Originally Posted by andy55 moving all of the RT of motion-control into dedicated hardware is not as easy as it seems.

And it's silly when the computer can do the work. The only reason the work is being forced off of the computer is because Microsoft doesn't want to do real-time processing. The generic Linux kernel is actually moving towards real-time capabilities, so it makes sense to keep it on the cheap high-performance hardware. I can buy a low-end computer for what a GRex costs. I don't know which is going to be obsolete first, a cheap computer I buy today or a GRex, but I'm guessing the computer is going to be around much longer.

[webgeek]

Ha ha, my comments started more discussion then I intended. I didn't mean to be argumentative I just don't like that parallel ports are the interface of choice for CNC yet becoming increasingly less common on PCs. My latest PC doesn't even have one and this is starting to be happen more and more on high-end motherboards. Sadly, none of my laptops do either. So now I have to purchase a parallel port expansion board (and burn a rare PCI slot) for my latest machine just to control an external device when I already have "newer" external ports (USB, Firewire, eSATA, ethernet, etc) coming out of my ears. Now I agree with you that parallel ports appears to be the ideal way to control CNC equipment, but a guy can hope can't he?

The point Tom Brown and webgeek are missing is that with a 66 MHz pc and a parallel port I can successfully run a milling machine including a pretty decent GUI.

Is that TurboCNC? I didn't know of anything that could run on a machine that slow. Heck, I can't get a 500MHz machine to properly run any controller software as it's border-line for EMC according to the docs.

Thanks!

[unterhaus]

I didn't really want to be argumentative either, but I know it came across that way. The whole idea of EMC and realtime linuxes is that you can develop a realtime system without going into embedded systems.

My point about the 66MHz computer is that you can go back to old versions of linux and EMC and run those. It would take work at this time, but the hardware is capable of doing the job. It would be silly too, because you can have a modern linux on a much faster cheap computer for very little money.

One thing people have done is run EMC over the network, with one computer just doing the realtime stuff, and the display working on another computer. You could even do the display on a windows computer. I'm basically doing the same thing right now, there is a big cluster of linux computers in the back room doing calculations and I'm running them from this windows computer.

[webgeek]

One thing people have done is run EMC over the network, with one computer just doing the realtime stuff, and the display working on another computer. You could even do the display on a windows computer. I'm basically doing the same thing right now, there is a big cluster of linux computers in the back room doing calculations and I'm running them from this windows computer.

I'm very interested in this. My latest machine will be running Linux in a multi-boot scenario already so this might be a good option. How do you run EMC remotely? I probably missed it in the docs somewhere. Thanks!

[unterhaus]

I've never run EMC remotely. It may be as simple as logging in using ssh with x enabled, and then exporting your display export DISPLAY=xxx.xxx.xxx.xxx:0 where xxx.xxx.xxx.xxx is the ip address where you want the display
and then running EMC the normal way. I don't know why that wouldn't work.