HP 34401a 6½ digit multimeter OLED display mod
About this project
Many of the old meters in 34401a series are still in great working condition, however their aging VFD displays are failing in various ways. Low contrast, leaking current in segment drivers, shattered glass are frequent cause of these excellent devices being written off. While replacement VFDs are still sold on some markets, they are not cheap and getting them intact in mail is a lottery. Also replacing the VFD itself does not always resolve the issue since in some cases it's the driver IC that is at fault. On older issue 34401a meters (fw version 06-04-01 and prior) have a custom HP designed micro controller with integrated high voltage VFD driver which is not obtainable, unless bought as a whole front panel assembly, which again is not cheap.
After acquiring one of such devices off ebay and encountering the leaking driver issue which lead to artifacts on the screen, a decision was made to rectify this situation. This project is result of many evenings of carefully reverse engineering the protocol between the CPU of the meter and it's front panel controller IC and designing a sniffing device that would extract the data sent to the front panel and display it on a more modern display instead.
After the protocol was reverse engineered a simple sniffing device based on everyone's favorite stm32 f1 series micro was designed. First prototype used a bluepill board and an Arduino compatible 3.5" TFT screen.
After the prototype showed promise it was quickly upgraded to a screen that would work well inside the meter and a custom board was designed. In the process a useful fast bargraph feature was added.
And this is what final product looks like:
For protocol details see
For compilation and flashing instructions see
License and credits
Firmware source is distributed under MIT license. See LICENSE for more info.