I've done a separate "project page" for uCom (RS232 add-on card for the Z80-MBC2) here.

The uCom board, as the uTerm VT100 board, has a "transparent" USB-serial adapter connector, so you can upload firmware to the Z80-MBC2 (using Arduino IDE) or load an Intel-Hex file (with iLoad) or use XMODEM to exchange files with a PC (running a terminal emulator that supports XMODEM file transfer) while the uCom is in use.

Both the "mixed" power supply scenarios (USB-serial adapter not powered from USB but Z80-MBC2 powered and vice-versa) are managed by the HW, so you don't need to worry about it.

uCorm can be mounted horizontally or vertically to the Z80-MBC2.

The 3D printed custom angled brackets .STL files are the same of the uTerm.

Here connected with a "vintage" RS232 terminal (Ampex 210 relabeled Kyber):