Fluke 8050A Display Use and Construction Notes
So you want to build this yourself? And when you do, how do you use it? Here's some hints; this is a long way from the "Heathkit" experience, but ultimately, not that complex.
Using the modified Fluke 8050A DMM
There's not very much difference than the "stock" mode in how the multimeter is used.
The primary difference is that the display that you see is much enhanced from the stock experience. In addition to the primary display of the measured value, the display is annotated with additional information, such as the current measurement mode (Volts, Ohms, Amps), as well as an AC/DC indicator where appropriated.
In addition, a Maximum and Minimum measurement value is displayed. These values are reset whenever you change measurement mode, or the scale, or switch in or out of RELative mode. There's also a bar graph display.
There's one particular aspect to make note of. That's the state of the "REL" (Relative Display) button when the Fluke 8050A DMM is turned on. If it's in the standard, non-relative position, then the display is "normal".
If the REL button is depressed on power-on, then this invoked a debugging mode. At power-up, you'll see a count-down which pauses the main display process, showing you some version information. If a key is pressed on a serial terminal connected to to the STM32F103 microprocessor, then the user will have access to the interactive FORTH console to poke around and test things, debug the FORTH application code, etc.
Even if the REL button is not depressed and the multimeter starts directly in the normal display mode, its possible to send a character on the STM32F103 microprocessor's serial port, which will halt the display update and drop into the FORTH console. Most people won't use this.
Other debugging information
In addition to depressing the REL switch, its possible to set a debugging mode by depressing a push button one microprocessor board. On the Maple Mini clone device, there is a button connected to the PB8 GPIO pin. The normal state is that this button is pulled LOW. When the button is pressed, it's pulled to HIGH logic level. The start-up FORTH code will look for this button being actuated and enable a debugging mode.
This button (and the REL button behavior) was intended to capture control of the FORTH software system before the display update loop was started, which is where much of the complexity in the software lies. So it's possible to get interactive control via the FORTH console before the code manages to crash itself. This was very useful during developement; much less now that the code has been running unchanged for 4 years or more.. But it is something to be aware of.