UPGRADING REPAIRING PCs

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Chapter 6—Serial Ports and Modems
Table 6.5 Upgrading UARTs Continued
Device Type UART Location Upgrade Method
Multi-I/O card Equivalent to normal Replace card
with Super I/O UART inside a highly
integrated surfacemounted
chip
Motherboard- Socketed MB chip Remove and replace 16450
with based I/O 16550AF if socketed; disable
serial I/O and install new multi
I/O card with 16550AF or
better UART
Newer systems— See the section “UARTs” Disable serial I/O and install
UART equivalent
inside Super I/O
earlier in this chapter new multi I/O card as above
Serial Port Configuration
Each time a character is received by a serial port, it has to get the
attention of the computer by raising an interrupt request line (IRQ).
8-bit ISA bus systems have 8 of these lines, and systems with a
16-bit ISA bus have 16 lines. The 8259 interrupt controller chip
usually handles these requests for attention. In a standard configuration,
COM 1 uses IRQ 4, and COM 2 uses IRQ 3.
When a serial port is installed in a system, it must be configured to
use specific I/O addresses (called ports) and interrupts. The best plan
is to follow the existing standards for how these devices should be
set up. For configuring serial ports in either Windows or Linux, use
the addresses and interrupts indicated in Table 6.6.
Table 6.6 Standard Serial I/O Port Addresses and Interrupts
COM x I/O Ports IRQ Equivalent to Linux2 COM 1 3F8–3FFh IRQ 4 ttys0
COM 2 2F8–2FFh IRQ 3 ttys1
COM 3 3E8–3EFh IRQ 4 1 ttys2
COM 4 2E8–2EFh IRQ 31 ttys3
1. Although many serial ports can be set up to share IRQ 3 and 4 with COM 1 and COM 2, it is
not recommended. The best recommendation is setting COM 3 to IRQ 10 and COM 4 to IRQ
11 (if available). If ports above COM 3 are required, it is recommended that you purchase a
special multiport serial board.
2. Linux users must use distributions based on kernel 2.2 or better to enable IRQ sharing. With
older distributions, use the setserial command (found in the Linux startup) to assign different
IRQs to devices using ttys2 (COM3) and ttys3 (COM4); this also requires you to configure the
cards to use those IRQs. For more information about setserial and serial ports under Linux,
refer to the Linux Serial How-To at www.linuxdoc.org/HOWTO/Serial-HOWTO.html.