Have you got basic comms to the 596 - ie can you use hyperterm and get a connection ? You may try searching about making 596/8 region free and see if you can get the factory password to respond.

When you say "I tried a USB to RS232 null modem cable" do you mean a USB to serial converter and a RS232 null modem cable? Likewise, when you say "serial port and use a serial cable instead" do you mean a null modem cable rather than a standard serial cable.

The null modem cable is a requirement with both USB/Serial converters and standard RS232 ports.

Meridian owner since 1992
Prime & PSU, Focal Elear headphones, Roon (ROCK 8Gi5 NUC), Explorer 1 & 2, F80. 200/203, various Sonos (via Roon).