2 Feb 1995
There are several components to serial communications. If all of the parts are sold separately, there are four important terms that should be defined. In certain packages, such as an internal modem, some of these components may be combined or disappear:
UART - This is the computer chip that forms the heart of the COM port. The letters stand for Universal Asynchronous Receiver/Transmitter (for those who care about such things). The UART receives a byte of data from the I/O bus at computer speed, then at a much slower speed it sends a start bit, eight bits of data, and a stop bit over one of the wires of an external interface. In the other direction, it receives data one bit at a time until the entire byte is assembled. It then generates an interrupt to the PC so the system will read the new data from the chip and store it in RAM memory. A UART chip was used in terminals and minicomputers for a decade before the first IBM PC came out. There are many different types of UART chips, but the PC COM port is based on chips compatible with the National Semiconductor 8250.
Modem - a device that converts data bits to sound, so the data can be transmitted through an ordinary phone system. Modern modems also provide data compression, error recovery, and some telephone services (dialing and perhaps eventually Caller ID).
COM - the "COM1" and "COM2" names are assigned by DOS to the first two serial ports. Every PC device has a characteristic address and interrupt level. COM1 uses address 3F8 and interrupt level 4, while COM2 uses address 2F8 and interrupt level 3. The UART chip does all the work for the COM port, but it must be connected on one "end" to the I/O bus of the PC and on the other end to the modem interface plug. So a COM port includes a UART, plus some chips to handle the address and interrupt part of the PC I/O bus, plus some chips to convert signals from the 5 volts or less typically used inside a PC to the 12 volts used on cables that run to external devices such as modems.
EIA RS-232 - An ancient standard for the connection of teletypes
and computers to modems. The RS-232 standard specifies the conventions used to send data
as a sequence of voltage changes signifying 0 or 1. It also provides signals to control a modem
through operations such as "hang up the phone." There are many EIA standards, but this is the
most famous. When the "EIA interface" is discussed without any particular standard being
referenced, then the RS-232 interface is almost certainly the topic.
Although most computer systems have all of these individual components, technology has advanced to the point where there is good reason to combine them. An "internal modem" combines the COM port, UART, and modem on a single card. On such devices, the UART and modem functions may be combined. This gets rid of the RS-232 interface, saves a lot of bother, and provides the opportunity for much better performance and lower cost.
Copyright 1995 PCLT -- The Storm Before the COM -- H. Gilbert
