Why don’t standard asynchronous serial adapters support 9-bit protocols?

Applications that require 9-bit protocols are rare so the chips that support them are more expensive than standard 8-bit UARTs.

Most 8-bit UARTs can simulate 9-bit protocols by setting the parity bit to mark or space. This was an easy solution on the 16C450 UART and other 8-bit UARTs without FIFO buffers. Modern 8-bit UARTs like the 16C550 and 16C850 have FIFO buffers that make 9-bit support more complicated to implement.

The 16C950 UART and 16C954 quad-port UART solve this issue by including 9-bit data buffers in hardware. Most Sealevel asynchronous serial adapters can be upgraded with 16C950 or 16C954 UARTs for a nominal charge. Call for details.

Note: USB serial adapters use a different communications chipset and cannot support 9-bit data framing.