K8063 RPi Serial

I’m trying to drive four linked K8063s from a Raspberry Pi 3. The display goes through it’s powerup sequence ok, but does not respond to the RS232 commands from the RPi. The display works perfectly with a Parallax Basic Stamp. I have verified that the RPi RS232 commands are valid (including checksum) by connecting it to a PC via a USB-Serial converter. NB: I am sending bytes from the RPi in Python3 (e.g. serial.write(bytes([13,0,82,0,161])). I have tried using a 3V to 5V converter in case it was a voltage level problem, but that doesn’t work either. I’ve also noticed that two of my Serial-USB converters don’t work for the RPI-PC connection, but one does, could this be the same issue?