Redesigned MKS 901P/902/971/972 Pressure Transducer Interface

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Philippe David
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Redesigned MKS 901P/902/971/972 Pressure Transducer Interface

Post by Philippe David »

TL;DR: I redesigned Finn Hammer's MKS 901P pressure transducer interface to be manufactured affordably, and it will very soon be available for sale from Joe Ballantyne!
IMG_7148.JPG
Hello everyone,

I wanted to share my recent work updating Finn Hammer's MKS 901P pressure transducer display and power source from 2017 for both myself and others on the forum. The impetus came from wanting to build my own version of Joe Ballantyne's Fusor V2 and wondering what the most cost-effective way would be to monitor the vacuum pressure. I had noticed that 901P Vacuum Transducers could be acquired (in questionable condition) from eBay for less than $50, which led me to Finn's thread. After that, I set out to make my own.

I used JLCPCB, a very affordable Chinese PCB prototype service, to order the last revision of the PCB along with the other parts and battery. All together, I think it came out to be about $50. After I got all the parts, several issues arose:
  • All the OLED displays seem to have swapped VCC and GND pins since Finn's original design - you can see the jump cables I used in the picture to address this.

    Programming the Pro Mini was a pain. The CP2102 USB 2.0 to UART TTL with a USB-C port also caused me much aggravation - it just wouldn't work on my Mac. A deep dive on the manufacturer's forum eventually led me to the answer that the chip would only work on Mac when connected through a USB-A port. I had to go from USB-C -> USB-A -> USB-C to get it working.
    Screenshot 2024-10-16 at 2.29.42 AM.png
    Screenshot 2024-10-16 at 2.29.42 AM.png (43.54 KiB) Viewed 4855 times
    It was incredibly difficult to reprogram the Pro Mini due to the serial being flooded by the usage of the only dedicated serial pins to communicate with the 901P. Programming it required holding the reset button until a precise moment (that could only be guessed) right after the initial communication from the computer to the Arduino - this issue would bite me again later...
I eventually was able to hack together a working copy!
Screenshot 2024-10-16 at 2.47.01 AM.png
Even before I put together the board, I had wanted to use JLCPCB's Assembly Capabilities to remove the need for soldering and soldering surface-mounted components on the front and back of the board. I initially came up with a design that brought all of the components to the top side of the board so that I could use JLCPCB's "Economic PCBA" service, which only supported single-sided placement for SMT/Through-hole. I also wanted to use USB-C. I came up with a design that removed the MAX3232 module and integrated it into the PCB, avoiding the need to solder this module and instead have it assembled by JLCPCB.
Screenshot_100 (2).png
After this, I was quite happy but wanted to gauge if there was any interest in the community, so I texted Joe Ballantyne to ask if he was interested in my work since he hadn't yet posted about how he was measuring pressure in his fusor. After getting an very positive response and a bunch of questions, we had a very productive call where I got a number of great suggestions for improving the design - the main one being that I should tear out the complicated battery circuit and just power the 901P directly from USB, as fusors are typically pretty stationary. Joe also offered to pay for the initial prototypes :)

His enthusiasm inspired me to go further. I had wanted to swap out the deprecated Pro Mini with the more readily available Arduino Nano, which had an onboard USB FTDI Interface for programming. Additionally, it was readily available with the 2.54mm headers pre-soldered, saving more soldering work. With both these changes, I could eliminate all soldering completely.

I added a TPS61170DRVR-based circuit to provide about 9.6V of power to the 901P and updated the design to include female headers for both the display and the Nano so that it would be plug-and-play.
76091433697280-Produce_DanZhi.SMT_Snapshot.Top.8321345A_Y2.SMT02409171950790 (1).png
I placed an order for the minimum 5 copies of the redesigned board. I also experimented with a number of displays - I purchased 0.96 inch (used by Finn originally), 1.3 inch, and 2.42 inch OLED displays. The 2.42 was so big I had to use a standoff header so that it would fit - it was very wobbly!
IMG_6614.jpg
Unfortunately, after I got the prototypes in the mail, I came across an issue that I should have foreseen - overloading of the Arduino's serial ports. It turned out that the FTDI chip just sharing the lines was enough to interfere with the 901P connection. In exasperation, I ripped out the USB/serial chip from one of the Nanos and resorted to programming it the same way as I had the Mini. Afterwards, I realized I could instead emulate serial communication on other digital pins of an Arduino board using software to replicate the functionality. The emulated serial connection, of course, lacks the performance and capabilities of the dedicated hardware-based serial communication, but the communication with the 901Ps is quite simple. For the rest of the boards, I was able to simply clip the two female header connectors that connected to the Nano's serial pins and solder two jumper wires behind the board to connect the Max3232 board to two unused digital pins on the back- you can see the green and white wires in the picture below:
IMG_7035.jpg
After this, I sent Joe the 4 remaining unharmed boards along with 6 different displays with a variety of sizes and colors to test. After getting back some feedback, I've updated the design to use a simpler on-off button that doesn't stick off to the side so much and changed the D-sub connector to one without hexagonal standoffs sticking out (turns out my 901P did not have threaded studs, but most do!). I've also updated the circuit to use an emulated serial port from the digital pins on the Arduino. I've ordered 5 more boards with this (hopefully) final design! I also updated the software - the buttons now simply iterate either forwards or backwards through the three pressure units (mbar, torr, and pascal). Using two buttons for this purpose is a bit excessive... but whatever :)

When purchased in decent volume, the new design is almost an order of magnitude cheaper than the old design! We decided on the white 1.3 inch OLED - if you want to use one of the other display sizes, you will have to reprogram the board, which should be easy now that they have an onboard USB connector. Soon they will be available for purchase paired with a tested 901P on Joe's store! If you already own a 901P then I'm sure he can sell you a board alone as well.

I want to thank Joe for his numerous suggestions and support and for helping fund the first two rounds of 5 prototype orders. I also want to thank my local electronic parts supply store - they saved me when I inadvertently ordered the wrong type of D-Sub connector from Digikey for my first order: Anchor Electronics, 2040 Walsh Ave, Santa Clara, California. Now, I need to finish my Cloud Chamber project!

In the future, one could use this to also recalibrate 901Ps: https://www.mks.com/mam/celum/celum_ass ... n-REVI.pdf

Theoretically, the board could be simplified further… the entire top row of buttons, switches, and the usb c port could be dropped. The board would then just be powered through the Arduino’s own usb c and pressure units could be fixed (changed by reprogramming). Power on and off would be controlled through unplugging it.
Last edited by Philippe David on Thu Oct 17, 2024 3:26 am, edited 7 times in total.
Philippe David
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Re: Redesigned MKS 901P Pressure Transducer Interface

Post by Philippe David »

I received some valuable feedback from "manshiro" on the "Vacuum Hax0rs" Discord regarding the MKS 902 series. For the next set of prototypes, I'll increase the voltage provided to the pressure transducer to <12V, allowing these boards to work with MKS 902s as well. If anyone has an MKS 902 they could lend me (or just stay on a call with me), I'd be happy to update the software to support both the 901P and 902 series. The same offer extends to the MKS 971 or 972 models. For those with a 9-pin sub-D connector, I can create an adapter connector upon request if there's sufficient demand. Similarly, if there's interest in a board that supports both RS-485 and RS-232 variants, I'm open to developing that as well.
Last edited by Philippe David on Thu Oct 17, 2024 3:27 am, edited 1 time in total.
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Richard Hull
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Re: Redesigned MKS 901P/902/971/972 Pressure Transducer Interface

Post by Richard Hull »

Fabulous report on the small upgrade. I feel your pain on linking stuff up via serial com among differing platforms. I find this kind of task to be a bit worse than having all your teeth extracted in an afternoon. Nice piece of work! A big thumbs up! Plus three or four atta boys.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
Philippe David
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Re: Redesigned MKS 901P/902/971/972 Pressure Transducer Interface

Post by Philippe David »

Thanks for your kind words, Richard!
JoeBallantyne
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Re: Redesigned MKS 901P/902/971/972 Pressure Transducer Interface

Post by JoeBallantyne »

Thanks Philippe for your excellent work updating Finn's original design, and thanks Finn for creating and publishing your design.

I will be making Philippe's boards available in my store in the For Sale section of this forum once the second set of boards arrives.

I have quite a number of MKS901p gauges, and plan on offering gauges with the board, in addition to stand alone boards.

I will update my store listing once the boards are available.

Joe.
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Finn Hammer
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Re: Redesigned MKS 901P/902/971/972 Pressure Transducer Interface

Post by Finn Hammer »

[ is with great satisfaction and pride, that I learn about this last iteration of the pressure gauge adaptation board.
Initially it started out as a battery powered unit, so that you could chuck it in your pockets and bring it along to test a pump in the field, if you wanted to buy it, great work Philippe and Joe.
I don't do fusors for the time being, having digressed back to head porting motorcycles, building Tesla coils, and Van de Graaf generators.

attachment=0]20240804_212633_009.jpg[/attachment]It


Best of luck to you all.
Finn Hammer
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Richard Hull
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Re: Redesigned MKS 901P/902/971/972 Pressure Transducer Interface

Post by Richard Hull »

Finn,

Nice to hear from one of our most successful fusioneers of the past. Glad to see you are monitoring fusor.net, still.

I assume your photo was taken with you on an insulated platform and have your hand on a Van de Graaf HV terminal or ball.

Richard Hull
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My biggest standard Tesla coil, &quot;Nemesis&quot;, before going onto magnifier (3 coil) Tesla systems.  Photo taken circa 1991.<br />The lab in the image was a 20 x 34 foot addition built onto my home in 1990 purely to support my Tesla coiling activities.  The back wall in the image is the current location of my fusor.
My biggest standard Tesla coil, "Nemesis", before going onto magnifier (3 coil) Tesla systems. Photo taken circa 1991.
The lab in the image was a 20 x 34 foot addition built onto my home in 1990 purely to support my Tesla coiling activities. The back wall in the image is the current location of my fusor.
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
Gerold-Schneider
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Re: Redesigned MKS 901P/902/971/972 Pressure Transducer Interface

Post by Gerold-Schneider »

Hello,

I am new to this forum...

I have bought an MKS 901P Loadblock sensor recently without realizing that you actually need a special cable
to connect to a PC. Therefore I have become aware of this forum and posts related to MKS 901P sensor.

I didn't browse this forum completely, but I believe that this thread is correct to find some feedback / help.

According to the docu for the sensor:
https://www.idealvac.com/files/manuals/ ... HYUWobLjH_
refer to page 9 - RS232 user interface

one should be able to connect the sensor from VGA 15pin to RS232 9pin according to specs (refer to picture)
by adjusting the pin configuration.

MKS delivers a software for free to display the data transmitted from the sensor...
https://www.mks.com/medias/sys_master/r ... ger141.zip

From my point of view no additional hardware is needed...?

Am I correct with this assumption? Thanks for the feedback!

regards
Gerold
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Philippe David
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Re: Redesigned MKS 901P/902/971/972 Pressure Transducer Interface

Post by Philippe David »

Hey,

A VGA port won’t work. If you have an old desktop then it will probably have a 9 pin RS232 port that can be used. Otherwise you’ll need a USB to serial port device (see AliExpress for cheap options) but you’ll also need to power the device with 9+ volts. Honestly, I think the most convenient option is the interface board described in this thread. You can get output on your computer using the Arduino.

I’ve found I can mail them in a letter to Europe for only 3$ shipping btw
Philippe David
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Re: Redesigned MKS 901P/902/971/972 Pressure Transducer Interface

Post by Philippe David »

IMG_2468.jpeg
Just wanted to provide a quick update on my work. The picture above shows a prototype version of a new low cost version of the board. The majority of the cost savings will come from using a (“10 cent”) Chinese RISC-V processor instead of the Ardunio (2.2$). With a sufficient order size the cost should have about half of the old design.

I will provide all of the design files and source code for the new design as soon as it’s ready.

I’ll soon also be providing the source code (including some small quality of life updates such as a nicer initialization screen) and instructions on how to flash/update the ardunio boards.
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Re: Redesigned MKS 901P/902/971/972 Pressure Transducer Interface

Post by Jerry Biehler »

That's better. Kind of silly to go through the trouble of making a board and not just put a microcontroller on the board and be done with it.

FWIW, we had a presentation at work and a new version of the sensor is coming out with a third pressure transducer, though it really won't affect any of us. (I work for a division of MKS)

And a note, putting company logos on the board is REALLY not a good idea.
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