The 561 supports both firmware update and remote control via its serial port, I can confirm that I've done both. The latest firmware is 1.77.

If a bit is stuck 'high', then it could either be a problem with the computer's serial port or the serial port in the 561.

I normally connect at 9600,N,8,1 (9600 baud, No parity, 8 bits, 1 stop bit), without any problems from an old laptop with dedicated (i.e. real) serial port. I try to stick clear of USB to Serial converters if possible after having a bad experience with one whilst attempting a firmware upgrade, although they're generally OK for configuration.

I think you need to either try a different computer or another piece of Meridian equipment and see if you can control it using the Meridian Remote Control application. If that works, then configuration should be fine.

Good luck,

Steve

Main: 861v6 (LPS), 808v6, UHD722, DSP8000SE Anniversaries, DSP5200HC, DSP7200.2, SW1600, Sky Q, Cambridge Audio BD, Roon, M80
Office: 861v4 (LPS), 818v3, 800v4, HD621, M&K S150P, DSP5500HC, M&K MPS2810 sub x2
Son's system: 861v4, HD621, DSP6000.2, DSP5k.2C, DSP33, F80
Spare: MCD, 100 Series, 200 Series, 500 Series, C15s.