Enhanced Serial Ports
The proliferation of high-speed external serial devices, notably serial ISDN terminal adapters, made it obvious that something faster than the standard 16550 UART was needed. The standard serial ports on most PCs top out at 115 Kb/s or (rarely) 230 Kb/s. That’s not fast enough to support the full throughput of a dual B-channel ISDN terminal adapter, which operates at 128 Kb/s natively and up to four times that fast when data compression is enabled.
To address that need, various manufacturers supply enhanced serial ports, which are add-on cards that include one or two serial ports. These use 16650, 16750, 16850, or 16C950 UARTs, which are essentially 16550 UARTs with additional buffering added and support for higher data rates. Some cards also include an embedded processor, typically an 80186 or 80286, which offloads interrupt handling from the main processor.
These adapters are overkill for normal serial port applications, may provide some benefit for 56K dial-up modems, and often provide a noticeable performance increase for ISDN applications. If you buy such a card, make sure that it includes any necessary drivers for your operating system.