Category: User Exchange
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
