MITS 88-2 SIO Board
Crude by today's standards but this was the S-100 board that introduced many of us to the concept of serial IO.

The board was designed around an 6850 ACIA's (IC's D & E).  Baud rate and address selection was selected by jumper wires on the board.  Each ACIA utilized two I/O ports.  The board was often used in those early days with Teletype Corporation's ASR33 "Teletype". Later versions of the ASR33 had an RS-232 hookup option but earlier machines relied on a 20mA current loop connection.

The manual for this board (with code examples in octal!) can be obtained here.