Mill Retrofit - Mesa 7i29-7i37TA n00b needs help!

I've been working on bringing an old Lagun Republic FTV-3 back to life with some Rutex R2010 drives, R2210 encoder interfaces and R21110 motherboard, but having spent a fair bit of time tuning servos and cursing at SPI faults and Rutex's lousy drive software I'm realizing more and more that I should be using a closed loop system back to the control PC.

This mill had no existing servo drives when I got it, and I got a good deal on the Rutex drives (used) but want some input on my next hardware purchase before I buy.

Aside from the current Rutex hardware this mill has Electrocraft servos on all 3 axis, Heds 5500 500 CPR encoders on all axis, and I've put together a 1.5 kVA 75 VDC power supply for the servos and a 24 VDC supply for the other stuff. Eventually I'd like to put a 4th axis (rotab) and VFD on the mill but want the axis' all working first. The control PC is a Dell desktop which I plan on building an enclosure for to keep chip/coolant free, and I've got a nice swivel display panel with keyboard and mouse I scored off ebay that hangs off to the right side of the mill head. I've put a bunch of pics and a bit of a build log on my website at www.inrustitrust.com

I'm not going to be doing anything real crazy with the mill and its unlikely to ever be a "production" machine, I expect to mostly do odd stuff and one-offs with it.

I think I'd like to run a couple of Mesa 7i29's to get my encoder inputs and servo control, and a 7i37TA for my limit switches, spindle brake, coolant pump, etc.

1) I'm up in the air between the 5i20 and the 7i43-P. As I understand it from reading the specs, the 5i20 is twice as fast (100 Mhz vs 50) at a bit over twice the cost ($199 vs $79) with a more reliable bus (PCI vs. parallel port). The additional money doesn't bother me if its less headache, but I'm having a bit of confusion as to how (if I buy it) I should run the cabling. The AIO cable Mesa sells would appear to just be flat cable with header ends, and as it is currently the control PC is 5-6 feet away from the box I'd like to use for the servo drives. I'm concerned about EMI messing things up over that long of an unshielded cable, as well as being unable to (easily) keep chips and coolant away from the AIO cable without running a HUGE flexible conduit between servo drive cabinet and PC cabinet, which doesn't seem right?

Is there something wrong with making up a board to convert a 2x25 pin header to a DB50 connector (and vise-versa) so I can use a round shielded cable to connect the control card in the PC to the servo drives and I/O breakout board I'd want to use in a separate cabinet? Or am I better off just trying to find a tiny PC with a PCI slot I can fit in a 24x24x10" cabinet?

2) Conversely, the 7i43 I could just run a DB25 cable to the cabinet with the 7i43 in it with the 7i29's and 7i37TA cards, is this correct?

3) For a basic 4 axis mill config, am I likely to need more than a 200k FPGA? Searching hasn't given me a good feel on what a mill retrofit takes up for space on the FPGA configuration

4) The 7i29's manual says two 7i29's can share a single 50 conductor cable, is two ends on one cable just an option I would ask for when I have the cable made, do the 7i29's stack together or am I missing something?

5) Do the 7i29's require differential encoders or 3-channel (A,B,I) encoders, or will they work OK with just 2-channel non-differential encoders? Would it be beneficial to use something like the Rutex 2210 encoder interface to change a HEDS encoder output into differential for noise/error reduction?

Thanks for the help!

andypugh wrote:

greenbuggy wrote:

1) I'm up in the air between the 5i20 and the 7i43-P. As I understand it from reading the specs, the 5i20 is twice as fast (100 Mhz vs 50) at a bit over twice the cost ($199 vs $79) with a more reliable bus (PCI vs. parallel port). The additional money doesn't bother me if its less headache, but I'm having a bit of confusion as to how (if I buy it) I should run the cabling. The AIO cable Mesa sells would appear to just be flat cable with header ends, and as it is currently the control PC is 5-6 feet away from the box I'd like to use for the servo drives.

Have you looked at the 5i25 boards? Normally you would use a 7i77 for analogue control / encoder feedback servo systems, or the 7i76 for stepper systems.
You have step/dir drives and encoder feedback, so the 5i25/7i76 ($199) or 5i25/7i77($239) kits don't fit perfectly. However the combination of small PCI card + 25-pin S-sub cable to the boards sounds like it would fit your application.

Perhaps your best combination would be the 7i76 plug-and-go kit plus a 7i85 encoder board on the second header. (total cost $268). That gives you a set of VFD control pins and lots of general IO, along with step generators and encoder counters.
It is possible that the rs422 ports on the 7i85 can control the Rutex drives digitally. However step/dir is not a bad solution if you use velocity mode stepgens to make it behave like an analog speed/direction command format.

I'm ditching the Rutex drives, and want to replace them with a pair of 7i29's, so I need a 50-pin FPGA mesa board to do so which makes the 5125 unsuitable.

Searching didn't turn up much on the 7i80HD - and I talked to Mesa yesterday they said its got "beta" support but the person I spoke with said outright that I shouldn't just expect to plug in and run with it. On the flip side this project has already taken longer than I want it to and cost more (surprise!) and Mesa said that in a month or two there would likely be much better 7i80 support for linuxCNC.

That said, an ethernet solution is a preferred solution to having a long signal cable stretching from where my PC is to where the drives and I/O breakout boards are. At least ethernet has some resilience to signal/packet loss built in. If I go with a PCI breakout board its looking like other option at this point is to get a tiny ITX computer inside the control cabinet and run Linuxcnc on it instead of the desktop I have now.

I don't know much about Linux kernels, is there a downside to running Xenomai instead of RTAI?

I would go for the miniITX. you can get them brand new for ~$75USD. If you have the other bits like ram power supply and HDD, then you are all set and only have 1 "box" to deal with.

I run them with a 4G sata DOM, 4Gram, and a $30 power supply.

FYI - I ended up buying a WTM-D28 Intel Atom 1.8 board w/2 GB ram, wireless mini card and a PCI slot for $79.99 shipped.

I've ordered a Mesa 5i20, 2x 7i29, AIO cable and mounting hardware, should ship today. I found a guy on CNCzone who is selling me a used 7i37TA to handle my I/O for coolant, spindle brake, etc

I'm still curious as to what the difference between Xenomai and RTAI is and why (or why isn't) it a good thing, even though for now it looks like I won't be using the 7i80HD.