Phil Storrs PC Hardware book

The PCs Serial Ports:

PC Hardware these days comes equiped with two Serial Ports. IBM originally called these Communications ports and so you will find them more often referred to as COM ports.

Serial Ports can be used for:

UART (Universal Asynchronus Transmitter Receiver)

This is the heart of the Serial Port with this device performing the parallel to serial and serial to parallel conversions and providing the Hand Shaking between the two devices connected together.

The history of the UART chip
Over the years, since the introduction of the DOS computer, three types of UARTS have been used in this hardware. The first was the 8250 chip, this was followed by the 16450 chip, and then the 16550 chip.
UART type Max. Data Rate
8250 9600 bits/second
16450 30K bits/second
16550 >100K bits/second

The 8250 chip was used in the Serial Ports of PC or XT computers, and the 16450 in the Serial Ports of 286 (AT) and then 386, and 486 machines, until early 1995. Over the years the maximum data rate provided by devices connected to the Serial Ports has been steadily rising. Back in 1987 a 2.4Kbits/second Telephone Modem was considered fast. The most cost effective Telephone Modems today are transferring data at as fast as 56Kbits/sec with 33.6Kbits/sec modems being phased out rapidly. The Serial Ports must keep up with the modem and therefore the UART must be faster than the modem.

The main job of a UART is to convert the computer's parallel data from the bus into a serial flow for transmission and when information is being received, the UART collects it into bytes (8 bits) and passes those bytes onto the bus. The UART provides the shift registers for parallel to serial and for serial to parallel conversions and all the Flow Control (hand shaking) required to control the flow of data to and from the computer and some other device.

What does the UART chip look like ?
The 16550 is otherwise identical to its predecessors and so it can be used as a 16450 or 8250 replacement. The older I/O cards found in DOS computer hardware had a 40 pin DIL UART chip mounted on a socket and so it would be simple to upgrade by just replacing the chip. A 40 pin 16550 chip usually costs more than a new I/O card and so it is not economical to replace the UART chip on old I/O cards. Some SPG cards had a 16450 chip soldered in, making it almost impossible to replace it. Another chip you will sometimes find on old I/O cards is the 16451. This chip is a 16450 with a Parallel Interface as well as the UART.

Starting with the SPG and IDE/FDC-SPG cards used in 386 and 486 hardware, the UARTS were in a chip called an ASICS chip. This was a custom VLSI chip that contained 2 UARTs, the Parallel Port, Games Port, and often the Floppy Disk Drive Controller.

Modern PC hardware has the UARTS built into the System Boards chip-set and all these provide 16550 type UARTS, capable of data rates in excess of 100Kbits/second. Diagnostic software is available that will detect the type of UARTs fitted.

The TTL logic levels from the UART device require Line Driver chips to convert the output signals to RS232 levels, and other Line Driver chips to convert the input signal RS232, levels back to TTL logic levels. Today these drivers are often built into the ASICS chip or the chip-set but the 1488 and 1489 line driver devices were used for many years. The line drivers require + and - 12 to 15 Volts supplies. Alternative Line Driver Chips are available that generate the + and - 12 Volts inside the chip and these are used in note-book type computers. The Line Driver Chips often fail due to near lightning strikes and ground potential faults.

How many wires do we need for a serial connection ?

RS232 connection can be made with as few as 2 wires and a ground wire, but a full implementation of the standard uses 9 wires. The Serial Ports on the back of the DOS computer use either a DB9P or a DB25P plug and maximum cable length in unshielded cables is about 30 metres. If you need to put a printer some distance from the computer it is being used on it is neccessary to use a Serial Interface rather than a Parallel Interface. The serial interface is not as prone to electrical interferance from power wiring.

The I/O assignments used for the Serial Ports:

The Serial Port requires a small range of I/O addresses and an IRQ line. The original DOS assignments were like this.
COM Port I/O Address IRQ
COM 1 3F8 to 3FF IRQ 4
COM 2 2F8 to 2FF IRQ 3

With the introduction of DOS vesion 3.1 provision was made to have two more Serial Ports and the resources assigned to these were:
COM Port I/O Address IRQ
COM 3 3E8 to 3EF IRQ 4
COM 4 2E8 to 2EF IRQ 3

The problem with the extra two Serial Ports is that they do not have unique IRQ lines assigned to them and some hardware and/or software is not good at sharing such resources. Specialised Serial Interface Cards are available that provide four or eight Serial Ports and these are intended for use in UNIX systems and they may not have driver software for DOS systems. These cards are often used for "point of sale" computers in installations like Service Stations and Supermarkets.

Using internal modems

Modern PC hardware has 2 Serial Ports, and these can be configured as either of the four available COM Ports. If you wish to use an Internal Modem on either COM 1 or COM 2 you must turn off the corresponding COM Port on the System Board (or I/O card with older hardware), or configure it as one of the other COM ports. An Internal Modem has a Serial Interface built into the Modem card and must be configured so it does not clash with other hardware in the system. You can often configure an External Modem as COM 3 or COM 4 but watch out for IRQ 4 and IRQ 3 clashes. Some ports and software will allow two COM ports to share the same IRQ line. Note: Modern PC System Boards provide control over the "on board" Parallel and Serial Ports via the CMOS setup routines.

Using a printer with a Serial Interface on a PC

Most application software configures the Serial Ports UART for Data Rate, Word Size and Parity. If you wish to use a Serial Port at the DOS level, you must use the DOS external command called MODE to do this.

If you wished to use a printer with a serial interface instead of one with a parallel interface, When running DOS applications, you would have to add these 2 lines to the AUTOEXEC.BAT file. This is not required when using a MSWindows Operating System.

MODE COM1: 9600,N,8,1,P
MODE PRN = COM1: (The alternative to this line is MODE LPT1: = COM1:)

The actual values in the first line above will depend on the parameters required by the printer. This information is obtained from the printer handbook and will be similar to the listing below.

Bit rates with the latest UART devices used in PC computers can be far higher than 9600, with speeds of 19.2K, 28.8K, 33.6K and 56K being used today.

The "P" tells the Serial Port Service Routine to wait for the device on the other end, do not time-out after a predetermined time. This is necessary if a slow device like a printer is connected.

Providing more the two Serial Ports

The problem
While each of the four Serial Ports defined here has it's own unique I/O addresses, only two IRQ's are assigned. PC hardware and software is not good at sharing IRQ lines and so the above assignments may lead to problems with devices interacting with one another in a way that hinders the proper operation of one or both devices, sharing the same IRQ line. This problem should be overcome with the full introduction of Plug and Play technology but until that happens, to both the hardware and the operating systems, we will have possible trouble with providing more than two Serial (Communication) Ports.
COM Port I/O address (hex) IRQ
1 3F8 IRQ4
2 2F8 IRQ3
3 3E8 IRQ4
4 2E8 IRQ3

While each of the four Serial Ports defined above has it's own unique I/O addresses, only two IRQ's are assigned. PC hardware and software is not good at sharing IRQ lines and so the above assignments may lead to problems with devices interacting with one another in a way that hinders the proper operation of one or both devices, sharing the same IRQ line. This problem should be overcome with the full introduction of Plug and Play technology but until that happens, to both the hardware and the operating systems, we will have possible trouble with providing more than two Serial (Communication) Ports.

Why would you want more than two Serial Ports ?
Serial Ports are used for a wide range of I/O functions and computers used in CAD/CAM installations for example may have a Mouse, a Digitizer, a Plotter and a Modem fitted. The Mouse can make use of the PS/2 Mouse port fitted to most modern System Boards, but this uses one of the Available IRQ's and still leaves us with three Serial Ports required.

Another situiations where more than two Serial Ports may be required is where multiple modems are required or devices like Bar Code Readers are in use. This is common if a PC is used as a "point of sale" terminal, and with the cost of PC being so low, they are often the most cost efective way of providing these facilities.

Overcoming the problem
The easiest way to overcome the lack of IRQ assignments is to change either COM1 or COM3 and COM2 or COM4 to alternative IRQ lines. Modern PC Computer hardware has the Serial Ports provided by the Chipset built into the System Board. You can change the I/O addresses and IRQs assigned to these ports from the CMOS setup routines. Many older SPG and FDC/IDE-SPG cards had jumpers to select the I/O addresses and IRQ lines for each port and to turn each I/O function off. You could provide extra Serial Ports, selecting alternative I/O addresses and IRQs, using one of these cards. Only one problem, remember most of these cards were fitted with 16450 UARTs, too slow for modern Telephone Modems. You would have to use these other Serial Ports for devices like the Mouse, and a Digitzer.

Serial Port cards are available that provide one or two extra Serial Ports at all possible I/O addresses and IRQs via jumpers or some form of Soft-setup, but these are quite expensive for what they are.

Possible available IRQ assignments

The IRQ lines are not usually required by the Parallel Ports and will usually be available for other uses. IRQ5 was used for the Hard Disk Controller in an XT type (8 bit bus) computer but is available in modern PC Computers. This means IRQ5 and IRQ7 may be available.

Common default assignments to look out for

Which IRQ lines may be available ?
From what I have said above it is clear you may have IRQ9, IRQ5 and/or IRQ7 available for use with COM3 and COM4. Watch out for what IRQ9 is actually called, IRQ9 in a 16 bit bus DOS computer is wired to the IRQ2 Bus pin, the real IRQ2 is used to cascade a second interrupt controller device. The IRQ9 input on this second device is wired to IRQ2's place on the ISA bus. Windows wants to call this hardware interrupt IRQ9 but some software is quite happy if it is called IRQ2.

In the past IRQ10, IRQ11, IRQ12 and IRQ15 have been available but recent advances in PC technology have lead to these being assigned to standard uses. IRQ10 is often used by Sound Cards, IRQ11 by Network Interface cards, and IRQ12 is used if the PS/2 Mouse Port fitted to most System Boards, is in use.

With the introduction of the second IDE interface channel these days, usually used for interfacing to a CDROM Drive, IRQ15 is assigned to this channel and is no longer available.

How to make use of these extra COM ports
Modern GUI Operating Systems have support for almost any combination of I/O address and IRQ built in and some DOS software packages have a facility to configure the COM ports for I/O address and IRQ line.

Back to the PC and its I/O Ports Back to the opening index Book two index

The common I/O port assignments The PCs Parallel ports The Games (Joystick) port

Copyright © Phil. Storr, last updated 26th December 1998