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Hacking the Fluke 117 DMM

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ZGoode:

--- Quote from: squadchannel on November 07, 2024, 11:02:18 am ---from the trace, the P2 appears to be USART.
there maybe similar models that are PC communicable. command list may be close.

--- End quote ---

That was my first thought too.  But one of the pins on P2 is ground, another is battery +, another appears to be the backlight mosfet control, and the last one I have not determined yet.  Although my meter could just be behaving weird.

Everett_Tom:
P2 is a serial port, but it's not "safe" to use when the DMM is measuring; there isn't any isolation between the ground and the DMM input ground. SO - any high voltage you means may end up on that port - not great for a PC or a non-isolated serial connection.  I saw somewhere a setup where a board used the Batt and Gnd to power an optocoupler, and the other two pins are RX and TX—using commands like the "ID/r" or "QM/r." I think the backlight mosfet is for the FCTY mode ( unit off -> hold the "HOLD" button, and then switch the knob unit you see "FCTY", release the knob - you'll see the backlight flicker as it sends data out". That probably means that the line you have with the mosfet is the TX line.

ZGoode:
That makes sense.  I tried sending commands over the pin not connected to the backlight MosFET using a variety of different USART settings at 3V (since that is what the MCU appears to be running at), but had no luck in getting any kind of response.  Any chance you are able to find the link to where you saw that?
FWIW I was planning on adding a wireless module here to keep the isolation, definitely don't want to fry my PC.  I wasn't planning on doing any live measurements without any kind of isolation, both for my safety and my equipment's safety.

Everett_Tom:
I don't remember, sorry, it's been awhile. You might be able to go into the "FCTY" mode and see if you can get data off the RX pin - if they are the same line. That might give you a hint on serial port settings.

I think I have a Fluke 115 somewhere, and I'm sure I also have one of those FTDI serial pins to USB cables. I'll try with mine over the break if I get a chance.

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