Filename: COMPORTS.TXT WWW URL: http://www.firmware.com/support/bios/comports.htm FTP URL: ftp://ftp.firmware.com/texts/comports.txt Revision: 03/24/97 TLS Micro Firmware Technical Support Summary: Notes on configuring COM ports - IO addresses, IRQs, etc.
COM port is short for communications port. COM ports are also called serial ports and on PCs are sometimes called asynchronous communications ports. PCs can normally have up to 4 COM ports. Most PCs have 2 COM ports either built in to the motherboard or on an add-in I/O card. COM ports built in to a motherboard are usually configured with options in CMOS setup but may be configured with jumpers on the motherboard. COM ports on add-in cards are normally configured with jumpers on the card.
External modems plug into an existing COM port on the back of the system. Internal modems have their own COM ports built-in. The COM port on an internal modem is normally configured with jumpers or DIP switches on the modem. Some modems may be software configurable, including Plug-and-Play modems.
COM ports are configured to use a particular I/O port address and a particular IRQ (Interrupt Request Line).
All COM ports in a system must be configured so that their I/O addresses and IRQs do not conflict.
Here are some standard address and IRQ settings:
The addresses above for COM1 and COM2 are standard. There are several alternate addresses that can be used for COM3 and COM4. If these are hard-wired into a board or card they will normally be paired as shown. If they are individually selectable, it is desirable to use them in the pairs as shown.
COM1 - 3F8 IRQ4
COM2 - 2F8 IRQ3
COM3 - 3E8 IRQ4
COM4 - 2E8 IRQ3
Note that the usual address for COM4 (2E8) will conflict with any SVGA video cards that are 8514 compatible. This includes some ATI cards. These cards use I/O addresses from 2E0-2EF, which would conflict with 2E0 or 2E8.
COM3 - 3E8 338 3E8 2E8 220
COM4 - 2E8 238 2E8 2E0 228
Also most sound cards use 220 as a base I/O address which means that the addresses 220 or 228 should not be used if a sound card is set to 220.
Other cards such as network adapters may also be using one of the above port addresses.
Some systems will allow two COM ports at different addresses to share the same IRQ while other systems will not allow sharing of IRQs.
Other devices such as sound cards, scanners, CD-ROM drives, etc. will have IRQs assigned to them. It is advisable to keep a list of all devices in a system to keep track of configuration settings.
IRQ4 and IRQ3 are normally assigned to COM1 and COM2 respectively. Alternate IRQs used for COM3 or COM4 include IRQ2, IRQ5, IRQ9, IRQ10, and IRQ11. Most other IRQs are reserved. The IRQ numbers correspond to physical wires on the ISA bus. Here is a list of IRQs:
NMI Parity Check
0 Timer
1 Keyboard
2 Cascaded Interrupts from 2nd 8259 (IRQ 8-15)
3 Serial Port 2
4 Serial Port 1
5 Parallel Port 2 (LPT2) or may be assigned to COM ports, etc.
6 Floppy Disk Controller
7 Parallel Port 1 (LPT1)
8 Real-time Clock
9 Redirected as IRQ2 (should be Available)
10 Available
11 Available
12 PS/2 Mouse (may be available if no PS/2 mouse is present)
13 Math Coprocessor
14 Hard Disk Controller
15 Available (Normally used by second HD controller if present)
To detect COM ports, type DEBUG at the DOS prompt and then type D 40:0 at the dash prompt. Interpret the top line of hex values as shown below.
Note - 2-digit HEX bytes for port addresses are reversed in above DEBUG output - 03F8 appears as F8 03, 02F8 as F8 02, etc. The output of DEBUG above shows that only one COM port is present in this system and also that no LPT ports are present.
C:\>DEBUG
-D 40:0
0040:0000 F8 03 00 00 00 00 00 00-00 00 00 00 00 00 00 00
COM1 COM2 COM3 COM4 LPT1 LPT2 LPT3 EBDA
The summary screen that displays on bootup lists COM ports and LPT ports by port address in the lower right corner. All ports on the system, including those on modems and/or other add-in cards, are detected and listed. This can be useful in troubleshooting.
The Micronics DXLB (M4GS25), Micronics JX30 series (M4GS45/G/GP, M4HS45G/G/GP) and the Anigma motherboards do not allow sharing IRQs.
Many Gateway 2000 systems are supplied with ATI video cards, which means that COM4 can not be used unless it can be set to an alternate port address besides 2E8. All 8514-compatible SVGA video cards (such as the ATI Mach 32) use a port address that conflicts with the standard addresses for COM4.
If two devices are set to the same IRQ with a Micro Firmware Phoenix 4.03 or 4.05 BIOS upgrade installed, an error message should appear on bootup indicating what COM port is conflicting. No error message is given for IRQ conflicts.