California Computer Systems - 2830 Serial IO Board
This was a state of the then current art Serial IO board utilizing the powerful Zilog DART UART chips for IO.

The CCS Model 2830 Six Channel Serial I/O Board employed three Z-80 DARTs (Dual Asynchronous Receiver/Transmitters) and a Z-80 CTC to provide six programmable serial ports with programmable baud rates. Designed to conform to the IEEE 696/D2 specifications for the S-100 bus, the 2830 also featured the unique Z-80 Mode 2 Interrupt Daisy Chain look-ahead scheme which significantly reduced the ripple time for long daisy chains, alleviating the need for wait states during Interrupt Acknowledge cycles. The base address of the CTC and the three DARTs were hardware-selectable at any multiple of 16 between 0 and 255. The serial port interfaces, configured for DCE but easily reconfigured for DTE, meet EIA RS-232-C specifications.
 
The manual for this board can be obtained here.