UARTs

A serial port is controlled by a Universal Asynchronous Receiver and Transmitter (UART) chip. The UART converts parallel data (for example, 8-bit bytes) into a serial data stream which can be transmitted over a serial line. (It also converts received serial data to parallel data for further processing by the CPU.) Other functions of the UART are:

• Adding start and stop bits to the transmitted data.

• Generating a hardware interrupt (IRQ) when a character is received and when the UART is ready for another character. The trigger level refers to the number of characters required to be in the UART's receive buffer before an interrupt is generated.

• Performing flow control.

A data overrun occurs when the CPU fails to retrieve the data that is in the receive buffer, before some of it is lost due to new incoming data. The 8250 and 16450 UARTs incorporate a one-byte receive buffer; the 16550 has a 16-byte buffer and supports adjustable trigger levels; the 16650 has a 32-byte buffer; the 16750 has a 64-byte buffer. The larger buffers of the later chips and the adjustable trigger levels can be used to alleviate, but not eliminate, data overruns. You can set the trigger level using the Serial Manager.

The serial drivers in UnixWare support the 8250, 16450, 16550, 16650 and 16750 UART chips. Earlier systems may have 8250 or 16450 UARTs. In these cases, if you want to use high-speed modems, then you should consider upgrading your UARTs.

``Serial port speeds and UARTs'' shows how the maximum serial port speed is limited by the capabilities of the UART hardware.

Serial port speeds and UARTs

Port speed (bps)	Limitations of UARTs
300
1200
2400
4800
9600	Highest speed for 8250
19200	Highest speed for 16450 when used with fast modems
38400
57600
115200	Highest speed for 16550

 Port speed (bps)   Limitations of UARTs
 300
 1200
 2400
 4800
 9600               Highest speed for
                    8250
 19200              Highest speed for
                    16450
                    when used with fast
                    modems
 38400
 57600
 115200             Highest speed for
                    16550

The table shows the highest speeds in bits per second (bps) that can be used with ports controlled by 8250, 16450, and 16550 UARTs on normally loaded systems.

The 16450 and later chips in the series can achieve higher speeds than the 8250 chips. The specification for the 16450 chip allows it to run at speeds of up to 115200bps when used with terminals and printers. It is unreliable at speeds over 19200bps when used with high-speed modems because it does not have a receive buffer. High-speed modems, with speeds up to 115200bps require 16550 UARTs.