I've run into an interesting issue with my NVEM and would be keen to get your perspective on this. I've been trying to get serial output from the device on the four pin XH header with no luck. Would be good to see the output here as I build up my config and troubleshoot future issues. It would have helped in the issue I had recently where I was accidentally uploading incorrect config files.
I've probed the signal coming in TTL (Pin 10, yellow signal on oscilloscope) from the MCU, and also out from the MAX232 after conversion to RS232 (Pin 7, shown as the noisy signal in blue on the scope).
To compare, I built up my own circuit with a known working MAX232 chip with the charge capacitors as recommended in the data sheet. Probing my own chip allows me to measure the signal in and out at both TTL and RS232 levels cleanly on the scope so that tells me it's not my 10X probes loading the signal down. This was my initial theory looking at the long rise times. My own circuit was running at 5V so this could be the difference.
I'm not sure why signal coming out of the chip is so noisy. I can see on the board that the chip is getting 3V3 (Pin 16). The board seems to be missing one of the charge pump capacitors (C4 in the datasheet on V-, pin 6) and I wonder if this is the issue? The V+ and V- (pins 2 and 6) shows 4V across them which tells me it's doing something to multiply the voltage needed for RS232 transmission.
I could desolder the chip and see if it behaves properly with all of the recommended capacitors per the datasheet but at that point I'll probably just bypass it and run TTL out to an FTDI module. I see the NVEM manual says that the XH header has "undefined" behaviour so I was surprised to see the MAX232 chip there if they didn't intend to use it for a documented feature.
The PCB says it is a V6 dated 2022. Has anyone else experienced this issue? What do you typically see on the TX Line on the XH header?
