PC Hardware in a Nutshell, 3rd Edition [Electronic resources] نسخه متنی

22.4 Installing and Configuring Serial Port Hardware

Before you buy a serial port,
verify that you don't already have what you need.
Serial ports are cheap, ubiquitous, and easily disabled, so you may
already have one or more of them installed that you
don't know about. Before you purchase a serial port
expansion card, take the following steps:

Use the operating system or a diagnostics program to determine the
number and configuration of installed serial ports.

For Windows NT 4.0, choose Start Programs
Administrative Tools (Common)
Windows NT Diagnostics, click the Resources tab, and double-click the
Serial item.

For Windows 2000/XP, right-click the My Computer icon and choose
Properties. Click the Hardware tab and then the Device Manager
button. Locate the Ports item in the Device Manager tree and
double-click it to expand the listing to show individual ports. All
installed COM ports (and LPT ports) are displayed. Double-click a
port to display a tabbed dialog that provides information about that
port, including details about General configuration, Port Settings,
Driver details, and the Resources that port is using.

For Windows 9X, right-click the My Computer icon, click the Device
Manager tab, expand the Ports (COM and LPT) item, highlight a port,
click Properties, and finally click the Resources tab. Note the COM
port number, IRQ, and I/O address assignments for each port present.

For Linux, open a terminal window. Display
/proc/devices and look for tty* devices in the
Character devices section (use the cat command to
display the file; if the file is so large that it scrolls off the
screen, you can use the more or
less command to display one screen at a time). The
first column lists the interrupt assigned to the device and the
second column the device name. Ordinarily, the primary serial port
uses IRQ 3 and the secondary serial port (if present) uses IRQ 4.
Display /proc/ioports to view the base address
and memory range assigned to each of the installed ports.

While you have this information displayed, record the details about
each port (particularly which resources it is using), which you will
need to avoid conflicts if you install another port. Also record
unused IRQs that are available for use by another serial port. In
addition to the minimal information supplied by Windows, diagnostic
software may also report additional details. (Linux provides more
details than you'll ever need to know about serial
ports.)

Examine the rear of the PC case to locate an unused serial port
connector. It may be a DB9M or a DB25M, and may reside on an
expansion card bracket or on the chassis itself. If you find such a
connector, it may or may not be connected to an unused serial port.
If it is connected, that port may not be enabled. Conversely, active
serial ports may not be represented by a back panel connector. For
example, an internal modem uses a serial port, but does so
internally, so no connector for that port appears on the back panel.
Also, a port may be
"orphaned"installed and
active, but not connected to a back panel connector. If that is the
case, you can purchase a standard motherboard-to-back panel cable and
connector at most computer stores for a couple of dollars.

Restart the computer, enter BIOS Setup, and locate the serial port
settings section, if present. Determine if one or more serial ports
are available on the motherboard but currently disabled. Such ports
will not be reported by the BIOS, and so will not be recognized by
Windows. The diagnostics program may or may not detect disabled
ports. If BIOS Setup displays a disabled port, you can use Setup to
enable and configure the port as described in the following section,
assuming that the port uses a UART adequate for your intended
purpose.

If you do need to buy a serial port card, get a dual-16650 card,
which costs only $25 or so. Dual-16550 cards are still available,
sometimes for less than $15, but having the 16650s is worth the small
additional cost. Even if the system is old enough to have ISA slots,
buy a PCI card rather than an ISA card. Serial ports are too slow to
really need PCI, but you should avoid installing ISA cards in a PCI
system whenever possible.

Once the port is physically installed and enabled, the next step is
to configure the port hardware by assigning an IRQ and base address
to the port. This is done differently according to the port being
installed, as follows:

Most motherboards include two serial ports, typically designated
Serial 1/2 or A/B. You map these physical serial ports to logical COM
ports by setting jumpers on older motherboards or by using BIOS Setup
on newer ones. Many motherboards restrict you to assigning Serial A
and B to one of the four standard COM port settings for IRQ and base
address. Some motherboards allow you to define custom IRQ and base
address settings for each serial port, which you can use to configure
unique settings that allow three or more serial ports to coexist. Any
motherboard allows you to disable the embedded serial ports, which
you may need to do if the embedded ports use less-capable UARTs and
you are installing a serial expansion card to replace them.

Simple serial expansion cards typically support two serial ports,
although only one may be physically installed. If this is the case,
empty sockets will exist to allow you to install an individual UART
chip to add the second port. Each installed UART requires two UART
support chips, designated 1488 and 1489. These chips may or may not
be preinstalled for the empty UART socket. The chips necessary to add
a second port are normally available from the same place you bought
the card. Such simple cards are usually set by jumper to one of the
four standard COM port settings. Newer cards, particularly those that
use 16650 UARTs, may be set by jumper or by a setup utility bundled
with the card. In the latter case, the utility may allow you to
assign the serial port to any available IRQ and base address.

If you are installing an add-on serial port card, don't install just any serial card you happen to find lying around. The best choice is a PCI card with 16650 UARTs, although 16550AFN UARTs will do in a pinch. Avoid ISA add-on cards in general, and in particular avoid 8-bit ISA serial port cards. These 8-bit cards have 8250-class UARTs and support only 8-bit IRQs, either of which renders them useless in a modern system.

If you need
only one or two serial ports, configure them with standard settings
for COM1 and/or COM2. If you need more than two, configure the
least-flexible serial ports as COM1 and COM2 and use the ports that
allow custom IRQ and base address settings for COM3 and above. Assign
the standard ports 0x3E8 to COM3 and 0x2E8 to COM4, but assign unused
IRQs to those ports. For example, you might configure COM3 for 0x3E8
and IRQ 10 (rather than the default IRQ 4). Doing so allows you to
use all COM ports simultaneously with no risk of IRQ conflicts.

22.4.1 Configuring COM Port Settings

Once you have installed and
configured the serial port hardware, the next step is to configure
the operating system to use it. Windows automatically recognizes
proper IRQ and base address values for serial ports reported by the
BIOS that use standard BIOS settingsCOM1 and COM2 in this
example. Depending on the BIOS, nonstandard COM port assignments are
often detected, but the hardware settings may not be reported
correctly. For example, when we installed COM3 on an older system
using 0x3E8 and IRQ 10, Windows 95 detected COM3 on 0x3E8, but
assumed that it used IRQ 4.

22.4.1.1 Configuring COM port settings in Windows 9X

To configure COM port settings in
Windows 9X, right-click the My Computer icon and choose Properties to
display the Device Manager. If necessary, mark the View devices by
type option button. Expand the Ports (COM and LPT) item, and
double-click the port you are configuring to display the port
Properties dialog, shown in Figure 22-5. The General
page reports port status (in this case the port is working properly)
and whether the port is available for use in the current profile.

Figure 22-5. The General page of Communications Port Properties displaying the operational status of the port

Click the Port Settings tab to display the Port Settings page, shown
in Figure 22-6, and use the drop-down lists to
configure port defaults for speed, framing, and flow control.

Figure 22-6. The Port Settings page, where you configure communication parameters for the port

The values you enter on the Port Settings page determine only the default settings for that port. Modem applications (such as Dial-Up Networking or fax software) change these settings programmatically to the values they require. The primary use of this page is to configure the correct settings for relatively simple devices such as a serial mouse or printer.

Windows automatically detects and enables
16550 FIFO buffering by default, but may use overly conservative
settings. To optimize port performance, click the Advanced button in
Port Properties to display the Advanced Port Settings dialog, shown
in Figure 22-7. Move both slider bars to the far
right for fastest performance, and leave them there unless you have
problems with the port.

Figure 22-7. Advanced Port Settings, where you configure UART
settings

If a 16550-family UART is present, Windows nearly always detects it and enables the Use FIFO buffers checkbox automatically. If Windows does not enable FIFO buffers on your system, that means that the serial port has either a very old UART, or a FIFO-capable UART, but Windows is confused about the type of UART installed. That sometimes happens if the UART is embedded in a chipset rather than an actual standalone chip. For example, we have encountered some embedded UARTs that report themselves correctly as 16450s, but have supplementary buffering which emulates a 16550. In a case like this, the port may operate properly with FIFO buffering enabled. But be cautious about enabling buffering if Windows has left it disabled. There may be good reason.

To modify IRQ and/or I/O port settings, display the
Resources page, shown in Figure 22-8. Clear the Use
automatic settings checkbox, and select one of the Basic
Configurations shown in Table 22-11. Highlight
Input/Output Range or IRQ and click Change Setting. Select an
available I/O port or IRQ, making sure that the Conflict information
pane indicates that no conflicts exist. Save the changes and restart
the system.

Figure 22-8. Windows 98SE
displaying automatic settings for COM1

Table 22-11 lists Windows 98 (not Windows 98SE) Basic
Configurations for COM ports. Basic Configurations 0, 2, 4, and 6
define standard uneditable values for COM1 through COM4,
respectively. Basic Configurations 1, 3, 5, and 7 assign standard
settings for I/O Port, but allow you to modify the IRQ. Choosing
Basic Configuration 8 allows you to assign any IRQ and I/O port to a
COM port.

Table 22-12 and Table 22-13 list Windows 98SE (not Windows
98) Basic Configurations for COM ports 1 and 2, respectively. In each
case, Basic Configuration 0000 uses the most common settings for that
port, and allows editing neither IRQ nor I/O Port. Basic
Configurations 0001 through 0004 allow you to choose a standard I/O
Port address range and assign any available IRQ to that COM port. The
Windows 98SE method differs from that of Windows 98 and Windows 2000
(described in the following section) in that it forces you to choose
a predefined I/O Port address range, and allows editing only the IRQ
assigned to that port.

22.4.1.2 Configuring COM port settings in Windows NT 4

To configure port
settings in Windows NT, run the Control Panel Ports applet. The Ports
dialog lists the COM ports that NT recognizes. Double-click the port
to be configured and use the drop-down lists in the Settings dialog,
shown in Figure 22-9, to configure default port
settings for speed, framing, and flow control.

Figure 22-9. The Windows NT 4 Settings dialog, where you configure communication parameters for the port

To view or change settings for I/O port and IRQ, click the Advanced
button in the Settings dialog, and use the drop-down lists to specify
parameters for the port (see Figure 22-10). You can
enable or disable FIFO buffering by marking or clearing the checkbox,
but Windows NT 4 makes no provision for tuning FIFO performance from
the GUI. Although it's probably not worth the time
to do, you can modify FIFO buffering settings using the Registry
Editor. FIFO settings are contained in the key

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Serial .
The relevant value entries are:

The RxFIFO value entry specifies the number of
bytes that must accumulate in the receive FIFO buffer before a COM
port interrupt is triggered. RxFIFO is a REG_DWORD value. The default
value for this entry is the value set for the Serial service, which
is eight bytes.

The TxFIFO value entry specifies the number of
bytes that must accumulate in the send FIFO buffer before a COM port
interrupt is triggered. TxFIFO is a REG_DWORD value. The default
value for this entry is the value set for the Serial service, which
is one byte.

Figure 22-10. The Windows NT 4 Advanced Settings dialog, where you configure base address and IRQ for a selected COM port

Note the following when you are configuring COM ports under NT:

NT may or may not recognize a newly installed port, depending on its
settings. If the new port is not listed, click Add from the Ports
dialog and use the drop-down lists to define port number, I/O port,
and IRQ for the new port, as described earlier.

You can enter a custom value for I/O port address instead of using
one of those listed.

NT does not prevent you from using settings that will cause port
conflicts, so verify that the IRQ and I/O port address are available
before configuring a port to use them.

22.4.1.3 Configuring COM port settings in Windows 2000/XP

To configure port settings in Windows
2000/XP, right-click the My Computer icon and choose Properties to
display the System Properties dialog. Display the Hardware page and
click the Device Manager button. If necessary, mark the View devices
by type option button. Expand the Ports (COM and LPT) item, and
double-click the port you are configuring to display the port
Properties dialog, shown in Figure 22-11. The General
page reports port status and whether or not the port is available for
use in the current profile. The Port Settings page, shown in Figure 22-11, allows you to change the default speed and
framing parameters for the port.

Figure 22-11. The Windows 2000/XP Port Settings, where you change default speed and framing parameters for the selected port

Windows 2000/XP automatically detects and enables 16550 FIFO
buffering by default, but sets both Receive Buffer and Transmit
Buffer to their fastest settings by default. On most systems, these
settings work properly. If you encounter frequent retries or
corrupted data on the serial port, try using these sliders on the
Advanced Settings dialog, shown in Figure 22-12, to
reduce the buffer settings in small increments.

Figure 22-12. The Receive Buffer and Transmit Buffer sliders, which you can adjust for optimum performance

To modify IRQ and/or I/O port settings, display the Resources page,
shown in Figure 22-13. Clear the Use automatic
settings checkbox, and select one of the Basic Configurations shown
in Table 22-14 (for COM1) or Table 22-15 (for COM2). Highlight Input/Output Range or
IRQ and click Change Setting. Select an available I/O port or IRQ,
making sure that the Conflict information pane indicates that no
conflicts exist. Save the changes and restart the system.

Figure 22-13. Windows XP Professional displaying automatic settings for COM1

Table 22-14 and Table 22-15
list Windows 2000/XP Basic Configurations for COM ports 1 and 2,
respectively. In each case, Basic Configurations 0000 through 0003
define standard settings for the COM ports, with 0000 corresponding
to the most common settings. For either port, choosing Basic
Configuration 0004 allows you to assign any available IRQ and I/O
port to that COM port. The Windows 2000 method is similar to that of
Windows 98SE, except that Windows 2000 allows editing both IRQ and
I/O Port address range in Basic Configuration 0004, whereas Windows
98SE forces you to select from among the listed I/O Port address
ranges, and allows editing only the IRQ assigned to the port.

On some i440BX-based systemsand perhaps systems with other
chipsets as well, although we have not encountered itWindows
2000/XP refuses to recognize serial ports and parallel ports. From
reports we receive from readers, this is apparently not an uncommon
problem. The symptom of this problem is that the Device Manager
displays a yellow exclamation point for the port(s) in question.
Double-clicking the port displays the Device status information shown
in Figure 22-14.

Figure 22-14. Windows displaying Device status as disabled when it does not recognize a port

According to Microsoft, the message "This device is
disabled because the firmware of the device did not give it the
required resources. (Code 29)" indicates either that
the port is disabled in the BIOS but is still being enumerated, or
that the system BIOS needs to be updated. Each time we have
encountered this problem, updating the system BIOS to the most recent
version indeed fixed the problem.