I have an extremely niche problem relating to connecting a DEC VT520 terminal to my mac. Specifically I get occasional garbage characters, with increasing frequency as I increase baud rate. Running "screen" gets rid of the vast majority of the garbage - this is mainly what I don't understand. Can anyone shed any light? I have looked at the UART with a logic analyser and the decoder is reporting occasional frame errors, and there are significantly fewer of these when running screen. I realise there are a multitude of things to tweak, I have tried most of the communication settings on the terminal, it's set to 8 bit word, no parity, 1 stop bit, XON/XOFF flow control. Could my dongle stack be the problem? The USB-RS232 adapter is ok I think but it's going through an apple USB C - USB A to connect to the laptop. I just don't understand why running screen seems to solve a lot of the issues.
The second photo shows what I get running at 115.2K baud. It is significantly less awful at lower baud rates. The first photo is also at 115.2K baud but running screen.