RPi 4B w/LinuxCNC 2.9.3, Mesa 7C81, Isolation Card, PWM Card, Relay Card, More

For sale as a group. $500 US shipping included.

MESA 7C81 is FPGA motherboard host for a Raspberry Pi
1.5" RPI GPIO cable for 7C80 and 7C81
DIN Tray 107MM X 102MMFor 7C81

Raspberry Pi 4 Model B (8 GB).
Includes SanDisk Ultra 32GB microSD card with RT PREEMPT 6.6 linux kernel version of Raspberry PI OS. See this thread regarding installation.
Pi heatsinks, power cord, fans.

KBSI-240D (9431) Signal Isolator. DIN tray for this card.

2 DPDT Signal Relay Module Board, DC 24V . DIN tray for this card and PWM card.

Ideal Power's 56YSD15S 24v DC 15W DIN Rail Mount power supply

3.3V PWM to 0 to 10 V Voltage Converter . DIN tray for this card and Relay card.

Three signal isolation cards I designed and fabricated. DIN tray for this card. Ribbon cables from 7C81 to signal isolation board. Connectors and pins for the connectors that mate to the signal isolation board.

Cards are described in detail in the next post.

 
 
 
 
 
 
 
 


 

I was planning to use this controller setup for a Precision Matthews 728v-t mill using Teknic ClearPath SD servos. I was not keen on interfacing to the servos and mill spindle directly from the FPGA card, so I designed a 3v - 5v level shifting buffer / isolation card. I did the design in KiCad and all KiCad schematic and PCB files are included on the microSD card. Here is the schematic of the card.

 

Here is how the card was going to be used in this controller setup. The 7C81 had been flashed as a 5ABOBX3D.  There are three cards because the minimum board order was three. I decided to build out all three in case there was an issue with one, and to have spares. The boards have been powered up and tested. They are fully functional.

Obviously given HAL you don't have to use the board exactly as I intended.

 

Here is the how the controller power was planned.

 

As noted the MicroSD card has the OS and LinuxCNC. It has the KiCad files. It also has the data sheets and documents for boards and components used on the boards I made.
 

Here is a screenshot of the system running. The Pi is mounted on the 7C81.
 

Here are screen shots of the directories with the included files mentioned above.
 
 

If you have any questions you can email me directly at This email address is being protected from spambots. You need JavaScript enabled to view it.

Best

Depending on the way you jumper the 7c81, 3v - 5v translation isn't required as the 7c81 provides level shifting, hence the reason $5 chinese BoBs can plug directly into the 7c81, the 7i92 series are the same as well.

I am aware that the default jumper is for 5v tolerant. My preference was for setting it to native 3.3v and adding level shifting drivers between the 7C81 and the other components.

Out of interest, apart from the speed increase on the 7c81 side that would be mitigated by the off board level shifting, what was the the thoughts in that decision. I just went with on board level shifting on my "roll my own" 7c81 board, tho it can run other configs and not just hostmot2 firmware.

My thoughts regarding my decision to create my own Break Out Board were influenced by my background. I've very recently retired from a 40 year career in hardware and software development. The largest chunk was in integrated circuit design. Where I don't have any experience is in milling and CNC, and I've been finding the learning curve on that to be more than a bit frustrating.

Going more or less chronologically:

I had decided that I wanted to be able to both control the speed of the spindle and read the "actual" spindle rpm. I wanted these lines to be electrically isolated from the mill's electronics. To isolate the spindle rpm signal I wanted to use an opto-isolator. So now I needed a board of some type whether purchased or made by me. Initially I bought a SparkFun Opto-isolator Breakout for this purpose. Spindle RPM control on the PM-728VT is a 0 - 5v analog input. So I searched for a PWM input to analog out board to purchase, because that's circuitry I'm not comfortable designing. I could not find a  board that accepted 5v input, nor could I find one that was 0-5v output. That's not saying no such thing exists, but I couldn't find one. So I went with the board listed above that's 3.3v digital and can output 0-10v. Five volt switching levels are nice in a noisy environment, but from a purely digital design standpoint they are archaic. Which might be why I had a hard time finding a PWM DAC with a 5v input. So if I ran the 7C81 in 5v tolerant mode I'd need to level shift that input. Not a big deal. Initially I was planning to simply put a 138 ohm resistor in series and call it a day. But I started to wonder if between it being a 5v tolerant output, and not a true 5v driver, and the series resistor, would the PWM waveform be sharp or distorted. So I thought the better solution for this particular signal was a true 3.3v signal. I went with the KBSI-240D Signal Isolator board between the PWM DAC and the PM-728vt circuit board because someone on another forum with a PM-718vt used one and was very positive about it.

I wanted a BOB to convert the 26 pin "ribbon cable" connectors on the 7C81 to some type of more robust cabling from the controller box to the mill. I didn't find any boards with three 26 pin "ribbon cable" connectors on one end. What I was seeing were a lot of boards that converted a single connector of some number of lines to terminal blocks, or to 25 pin connectors. I did not want to run a 25 strand cable to the mill and split it at the mill to the individual servos. Also, I personally don't like handling a large number of signal lines via terminal blocks. I also started to think about buffering the 7C81 from the servo and mill electronics. On the off chance that those electronics went south, I didn't like the thought of it possibly toasting the FPGA. Especially since the FPGA is connected directly to the RPi that I spent a lot of time setting up with Linux and LinuxCNC.

So at this point I wanted to make my own BOB that was space efficient at converting the three 26 pin "ribbon connectors" to lower pincount connectors where I could run low wire count cables directly to  each individual servo, and a couple others for other purposes such as limit switches etc. Since I was going to the effort of making a BOB, it seemed only logical to include the opto-isolator rather than having an additional board for that purpose. Figured I might as well put a voltage divider on my BOB that would make the 0-10v out of the PWM DAC 0-5v to feed into the KBSI-240D. Since the BOB now needed power for the opto-isolator anyway, and I felt why not buffer the signals from/to the 7C81 and improve robustness, So I decided to put a couple signal driver ICs on the BOB. Transceiver ICs that level shift are common. Since the  PWM DAC input was 3.3v, and the mill and servos are 5v signals, running the 7C81 at 3.3v and using level shifting transceiver ICs was a logical choice.. Two 74ALVC164245 bus transceivers handled the number of signals I needed and have good specs. At the time designing the BOB was a fun diversion. Amazing what one can do with the FOSS available today. I'm very impressed with KiCAD. My BOB is 2.5" x 3.5" which is pretty compact for the functionality it contains, and I'm rather proud of it.

Best, Gene

Mesa makes a board that does pwm to analogue.
store.mesanet.com/index.php?route=produc...oduct&product_id=205

Yeah I went that whole route when I was using machine kit and a beagle bone black.
It's a shame that the BBB hasn't had a upgraded CPU with the PRU and more memory. Was a great little platform, actually still is but a little underpowered.

Will agree that there are some great tools out there. I still have a copy of the DOS version of EasyTrax I was given by a very surly PCB maker, top bloke but didn't suffer fools.

I edited a post above to indicate that the three BOBs have now been fully tested. Previously I had checked all the paths with an ohm meter, powered up one of the boards and checked some signals.

 

All three boards have been powered up and tested. All inputs were connected to 3.3v or 5v. The corresponding output was checked for a high (5v or 3.3v). The input of the line being tested was then connected to ground, and the corresponding output checked for a low. The input was again connected to 3.3v or 5v, and the corresponding output checked for a high. Every input and output was verified to work correctly.

I have 100% confidence in the BOBs. If anyone purchases all of these components for $500, and the BOBs don't function properly, I'll refund $100.

Best, Gene
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