Universal Asynchronous Receiver Transmitter (UART)

A chip which controls the RS-232 serial port in PCs. The 8250B is used in older PC or XT-class systems, the 16450 or 16550A is used in AT-class systems (standard since 486). The 8250 UART allows a maximum rate of 9600bps, which is too slow for many modern Internet Service Providers (ISPs). UARTs are found in modems, parallel port controllers, hard disk controllers anywhere where communication is taking place.

The More Info button in Modem Diagnostics provides the identity of the UART used by the modem.

The Properties button on the general page of Modem Properties gives access to four pages of properties, the Port Settings button on the Connection page refers to the FIFO (First In-First Out) buffers.


Enable/Disable the FIFO Buffer

To disable the FIFO buffer for a 16550 UART chip, click the "Use FIFO buffers (requires 16550 compatible UART)" check box to clear it.

Receive Buffer

The Receive Buffer slider supports the values 1 (low), 4, 8, and 14 (high).

The default value of 8 should be sufficient in most cases. If you decrease this value, more interrupts are sent to the processor, slowing bytes into the UART. If you increase this value from 8 to 14, characters may overrun the FIFO buffer.

Transmit Buffer

The Transmit Buffer slider supports the values 1 (low), 6, 11, and 16 (high).

If you experience problems uploading files, decrease the default value of 16. If you decrease the buffer from 16, more interrupts are sent to the processor, slowing down the rate at which bytes are sent into the UART Transmit buffer, reducing serial overruns. The higher the number, the fewer interrupts are sent, increasing system performance during uploads.