California Computer Systems - 2718 Serial IO Board
This was an extremely well designed board. Very useful. Unfortunately I cannot locate a good image. If you have one please let me know.

The CCS Model 2718 was a highly flexible I/O interface board featuring two serial ports and two parallel ports, with full handshaking for each port. Of the serial ports, one is asynchronous (a UART), while the other can be programmed to be synchronous or asynchronous (a USART). Both provide considerable format selectability, the UART through hardware, the USART through software. The interface for each serial port conformed to the EIA RS-232-C specifications, while each parallel port included a full four-line handshaking scheme, with jumpers allowing selection of the sense polarity of the individual lines. The base addresses of the serial ports, the parallel ports, and an optional (user-supplied) 2K ROM were jumper-selectable.  
 
The well written manual for this board can be obtained here.
 
The schematic for the board can be obtained here.