UPGRADING REPAIRING PCs

to your computer (primarily devices for data acquisition), you must
use a 25-pin serial port, not a 9-pin port. Adapters cannot be used
because the current-loop pins are not present in the 9-pin port.
UARTs
The heart of any serial port is the Universal Asynchronous
Receiver/Transmitter (UART) chip. This chip completely controls the
process of breaking the native parallel data within the PC into serial
format and later converting serial data back into the parallel format.
A few modem models lack a true UART and use the resources of the
computer and operating system for communications in place of a
UART. These so-called Winmodems are less expensive than ordinary
modems, but are slower and not compatible with non-Windows
operating systems, such as Linux.
UART Types
UART chips have been improved many times over the years, and
it’s important to know which UART your serial port(s) uses, especially
under the following circumstances:
• You want to attach a modem to the serial port.
• You plan to transfer data between machines via the serial port.
• You want to ensure reliable multitasking while using
Windows with your modem.
UARTs 169
Table 6.4 summarizes the characteristics of the major UART chips
(and equivalents) found in PCs. For more information about
UARTs, see Chapter 18 of Upgrading and Repairing PCs, 12th Edition,
from Que.
Table 6.4 Overview of UART Chip Types
Maximum Typical
UART Type Speed Buffer System Notes
8250 Up to 9600bps No 8088 Original UART;
replaced by
8250B
8250A Up to 9600bps No 8088 Not recommended
because
incompatible
with 8250
8250B Up to 9600bps No 8088/286 Debugged
version of 8250
16450 Up to 19200 No 386/486bps Minimum UART
(19.2Kbps) for OS/2