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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:
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 UART-Hardware'' shows how the maximum serial port speed is limited by the capabilities of the UART hardware.
Serial Port Speeds and UART-Hardware
| - | |
|---|---|
| Port Speed (Bps) | Limits of UART-Hardware |
| - | |
| 300 | |
| 1200 | |
| 2400 | |
| 4800 | |
| 9600 | Highest speed for 8250 |
| 19200 |
Highest spped 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.