My apologies for restarting this conversation - but as others have posted before me, I'm trying to utilize multiple of these Waveshare SC16IS752 cards for a project and am having some issues.
What I have found so far, all done within this last week, is that if I start with updating Buster completely, I am able to use one of the cards successfully. i2cdetect -y 1 displays both cards, (soldered for 0x48 and 0x49) but only prints UU for 49. 49 transmits and receives successfully while doing a loopback test. ls /dev/ttySC* also only shows SC0 and SC1.
If I update the kernel via rpi-update, all of a sudden I receive the additional serial ports (SC2 and SC3) and I can transmit successfully, but I can not receive. I wracked my brain for a bit before discovering that the IRQ pin on the chip would drop low as soon as I opened a terminal for the serial connection (like puTTY or miniterm). Watching it with a meter I can see the IRQ fall when the terminal is opened, rendering the port only able to Tx and rise again with the terminal was closed. I also noticed that I could still get it to momentarily work by applying a load (in this case a Sunfounder2004LCD) to the 5V bus which would momentarily cause the IRQ to go high and serial messages would appear in miniterm/puTTY. After perhaps a second it falls low again.
Any help would be greatly appreciated!