Electronics > Repair

Advantest R6581 VFD replacement

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dietert1:
In my project (see archive posted above) character recognition is implemented and the "DISPLAY OFF" message that appears after the "Display Off" GPIB command turns off the TFT backlight.
My board was designed for that purpose with its size matching the size of the TFT, so it "disappears" when mounting the TFT on those brackets i showed above. The intention was reducing EMI, as all high frequency signals are covered by very close metal surfaces: The metal back of the TFT and the ground planes of the display board on one side and the ground plane of the R6581 front PCB on the other side, both with less than 3 mm distance.

Regards, Dieter

gamalot:

--- Quote from: dietert1 on November 24, 2024, 12:05:34 pm ---In my project (see archive posted above) character recognition is implemented and the "DISPLAY OFF" message that appears after the "Display Off" GPIB command turns off the TFT backlight.
My board was designed for that purpose with its size matching the size of the TFT, so it "disappears" when mounting the TFT on those brackets i showed above. The intention was reducing EMI, as all high frequency signals are covered by very close metal surfaces: The metal back of the TFT and the ground planes of the display board on one side and the ground plane of the R6581 front PCB on the other side, both with less than 3 mm distance.

Regards, Dieter

--- End quote ---

What you are looking at is a multi-purpose PCB, which can be a replacement screen for R6581, an alarm clock with weather information, or a lithium battery charger. Every time I order PCBs from JLC, I only use one or two of them, and the rest are all wasted, so I decided to try to make them have different uses.

I will remove the components that are not relevant to this project in the next version (if there is one), change the PCB size to the same height as the TFT display or even smaller, and shield them with copper foil tape if necessary (actually I don't think there will be EMI problems).

dietert1:
Yes, i understood that you designed your own board, too. I also saw a Adafruit board that drives the TFT, once more somewhat larger than mine. I tried to focus on getting the display replacement.
Concerning EMI: My graphics board produces digital video at 21 MHz, while the original VFD probably multiplexes at two KHz or so. And once more i didn't want to study that in all detail, but find a good solution that doesn't add any negative impact on the 8.5 digit meter.

Regards, Dieter

gamalot:

--- Quote from: dietert1 on November 24, 2024, 01:57:00 pm ---Yes, i understood that you designed your own board, too. I also saw a Adafruit board that drives the TFT, once more somewhat larger than mine. I tried to focus on getting the display replacement.
Concerning EMI: My graphics board produces digital video at 21 MHz, while the original VFD probably multiplexes at two KHz or so. And once more i didn't want to study that in all detail, but find a good solution that doesn't add any negative impact on the 8.5 digit meter.

Regards, Dieter

--- End quote ---

Speaking of frequency, I found something strange. Mikle's circuit diagram marked the negative pulse width of CK4 as 2us, but I actually measured the frequency to be about 1MHz on my meter. Have you ever noticed the frequency of the clock signal on your R6581?

dietert1:
In my notes if found that the interrupt signal ("start of transmission") is a positive pulse of 4.73 usec every 9 msec.
SPI clock is 1 MHz, 8 negative pulses every 12 usec. During the 4 usec pauses the clock idles high. The data signal changes its value with the falling edge of the clock.
One packet of 5 bytes = 40 bits = 1 digit takes 184 usec in total.

I estimated 9 msec => 110 Hz refresh rate, times 18 mux positions gives about 2 KHz. Except the lower line has more positions, so it may be more like 3 KHz.

Regards, Dieter

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