Advantest R6581 VFD replacement

[Mickle T.]

As is well known, the Advantest R6581 DMM has two main problems, the second of which is a custom VFD display that is not available commercially.
As a result of reverse engineering the multimeter firmware and analyzing the circuit diagram, I was able to find at least two possible ways to replace the VFD that has become unusable:
1) A difficult to implement, but flexible and interesting way: installing a microcontroller module that "spy" on reading the external dual-port display buffer from the Hitachi H8 controller on the front panel board. The resulting ASCII character codes and their attributes can be rendered on any graphic display.
2) A simple and cheap to implement way: installing a microcontroller module that "spy" on writing to the shift registers on the front panel board of the multimeter. The resulting bit image of characters can be output only to a limited number of displays with a resolution of at least 256x48 pixels.

If you choose the second way (which is what I did), then replacing the display will cost about $20 and half an hour of your time. As a basis, I took the most common STM32 Blue Pill (STM32F103C8T6) and 3.12" OLED with SSD1322 controller. Attached is the schematic diagram and the project archive in STM32CubeIDE.

[TheDefpom]

That is excellent, thank you for providing this !

Now I just need to figure out how to use the STM32 IDE and get a blue pill board and ST-link.

I've never used an STM32 so a bit of STM32 learning is in my near future, which is probably a good thing anyway !

[dietert1]

In another thread a 4.58" TFT bar display was proposed as a possible replacement. In comparison to original VFD it might look like this. Don't know yet how it fits inside the R6581.
The controller uses a STM32H730 and its LTDC peripheral. It fits to the back of the TFT. At 300 MHz the MCU needs about 5 msec to paint the image in its video memory. TFT refresh rate came out as 41 Hz.

Regards, Dieter

[dietert1]

Thanks to Mickle T.s great reengineering it was fairly easy to connect the TFT display to one of our R6581. Also i unmounted the R6581 display board to check available space. The TFT and its controller can become one module and fit very similar to the original VFD.
The firmware reads all display data packets and paints each one of them, so there is no visible delay in comparison to the VFD. I also added kind of OCR to the firmware. Right now it only recognizes that strange "DISPLAY OFF" message and turns the TFT backlight off. Maybe one can substitute the 5x7 matrix by a higher resolution font.

Regards, Dieter

[dietert1]

In order to train the OCR i would like to post various text messages to the R6581 display via GPIB. Many GPIB devices support some kind of message display but i couldn't find the proper GPIB command in the R6581 manual. Who can help with hint?

Regards, Dieter

[Mickle T.]

The original, unmodified firmware does not contain commands for text output and complete blanking of the display.

[dietert1]

Unsoldering the original VFD display was quite some exercise. Meanwhile some brackets (3D printed) were glued to the boards as support for the 4.58" TFT display and its graphics card. A narrow fit!
By the way: The VFD cathodes need about 4 W for heating, while the TFT needs about 20 mA x 12 V = 240 mW for its backlight plus 150 mW for the graphics card. Roughly a 10x reduction.

Regards, Dieter

[dietert1]

First image shows the +12 V backlight supply. It has a voltage doubler and a LM7812.
Meanwhile i finished two R6581T with the replacement TFT display.

The interesting number in the image is the upper one with 15.57 uV. This is with an 18 MOhm resistor and a 1 uF cap across input, so input offset current < 1 pA. That R6581T got a mosfet input mux. The other one got a mains preregulator similar to the Keithley 2002. So there is more work to do..

Regards, Dieter

[kjk24]

Thank you 👍 verry much