Attached is the waveforms when pots (R3 and R6) are set in a position so RX is most readable. Still many garbled characters, but the best I could do.
CH1 is taken directly from phototransistor leg (going to LM393 non-inverting input) and CH2 from RXD line (LM393 output).
Quite slow rise on LM393 output (CH2). Perhaps reduce R3 for faster pull?
Could it also be that the phototransistor doesn't react fast enough? Maybe I need a more sensitive one?
Update: I found a multi-turn 4.7k pot, so I managed to finely tune R6. Found messages to be quite consistent and readable between 1.478-1.556k. Anything outside that would mess up more and more characters the further off I went. But it wasn't completely consistent. Seemed the ack or termination character would print either on the end of the message or at the beginning of the next.
So I replaced the pot with a 1.5k 1% resistor at R6. But that didn't help at all. Now it was completely garbled again. (Yes, I measured ~1.5k at the end of the potentiometer wires, to account for the wire etc. - what R6 pads actually sees). When
I also tapped in to the Fluke 28x firmware update, and this is what the program sends (to try to initiate comms):
0x10 0x02 0x03 0x10 0x03 0xA2 0x8E (plus carriage return i think: 0x0D 0x0A).
And when I send this manually, I get this response:
.....√Ì
(so it must be a valid message I'm sending, since no other messages gives response (except the ones in the spec document)).
Perhaps the updater switches to a lower, more safe/noise resistant baudrate(?).
I tried the firmware update when I tuned R6 to nearly perfect (20-30 in a row) responses from command QDDA (dumps readings present in the meter LCD), with no luck.