/diy/ - Do It Yourself

i have a seemingly very strange problem that is making me insane. i have exhausted google so i hope that /diy/ autism can solve this mystery.

>teensy 4.1
>BK-SIM7600 modem breakout board, communicating over UART
>logic level converter

the problem is unreliable and corrupted messages coming from the SIM7600. i am sending simple AT commands from the teensy TX, which the modem responds "OK" on the teensy RX.

>around half of all received messages are totally or partial garbled - they are either nonsense, or something close to the correct command, with a few corrupted characters
>tx appears to be working fine, because i can interpret the garbled replies as being exactly as i'd expect, but just somewhat garbled

here's the really weird part. when i put my finger on the RX pin on the teensy - it suddenly works and 100% of messages are uncorrupted. as soon as i lift my finger, the problem returns. attaching my oscilloscope (or touching the RX pin with all kinds of inert but conductive objects like a screwdriver, peice of wire, paperclip etc.) also seems to "fix it". i've tried swapping the modem, swapping the teensy, swapping the board, and the same thing happens. what causes this kind of effect? is the wire/screwdriver/finger acting as some kind of inductor?

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