It is the RS232 connection - make sure you are using the right one - from memory there are two serial ports on 808.3, one marked “maintenance” by itself on a panel in the middle of the player, and one marked “RS232” which is on the same card as all the comms and trigger connections. You want this one, on the comms card (800 C00), marked RS232.

What USB-Serial adapter are you using - if you use anything but the recommended Tripp Lite Keyspan USA-19HS you will often run into issues. You will also need a good null modem cable such as Tripp Lite Null Modem Gold Cable DB9F/F.

Are you getting any error messages?

HC: 861v4/ID40/LPS, DSP5000, DSP5000VC, DSP3100, SW1600, HD621, Oppo BDP-105, Sony VPL-HW55ES
2-Channel: Oppo BDP-105, TEAC VRDS-7/Trichord Clock 3/PSU, 551, Quad 303, Harbeth Compact 7ES-3, Kimber cables, Russ Andrews mains and supports.
Previous Meridian: 5000C, 5200SL, 7200.2, 6000, 562V.2, 568.2mm, 800v4, 808.2i, 818v2/v3, 861v8, 502