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

22.2 Serial Ports

PC serial ports, also
known as communications ports, comm
ports, asynchronous ports, or
async ports, connect slow bit-oriented
peripherals such as modems, mice, printers, and plotters to the fast,
byte-oriented system bus. Serial ports may reside on motherboards or
expansion cards, or be embedded on devices such as internal modems.
PC serial ports haven't changed much over the years,
although they're faster now and have larger buffers.

Serial ports were formerly used to connect almost anything to a
PCmodems, mice, printers, plotters, etc. Nowadays, serial
ports are used mostly to connect modems and other peripherals (such
as the Palm cradle) that do not require high-speed communications.
Serial ports have been replaced for most purposes by USB ports, but
most current motherboards and PCs have one or two serial ports,
although one or both may exist only as header pins on the
motherboard, rather than as visible ports on the rear panel
connector. Despite its obsolescence, though, using a serial port is
sometimes the best (or only) way to get the job done. The following
sections describe what you need to know to use serial ports
effectively.

The so-called "legacy-reduced" motherboards and systems that began coming to market in late 1999 may or may not provide serial ports. "Legacy-free" systems and motherboards began shipping in volume in mid-2000, and do not provide serial ports (or many other formerly standard connections, such as parallel ports, PS/2 mouse and keyboard ports, a floppy diskette drive interface, etc.). These systems depend entirely on USB for external connectivity. If you need to connect a legacy serial device (e.g., a Palm cradle) to such a system, there are three options. First, you may be able to find a newer USB cradle or cable for your old device, especially if it's a PDA. You can buy a USB cable that works with models as old as the Palm III (it may work with earlier models, too), but you may need to upgrade to the latest Palm Desktop (a free download) for USB support. Second, you can install a PCI serial card, which typically supplies two high-speed serial ports and perhaps a parallel port. Third, you can buy a USB-to-serial converter, various models of which are available from Belkin and others.

22.2.1 Serial Port Standards

Because they transfer only one bit at a time, using serial
communications interfaces to establish a unidirectional connection
between two devices in theory requires only one data circuit. This
circuit comprises one data (or signal) wire and a second wire (called
the ground, return, or common) that completes the electrical circuit.
In practice, most PC serial communications devices use additional
wires to enable bidirectional communication, to provide control
circuits between the devices, and so on.

Serial port interface and signaling specifications are defined by the
Electronic Industries Association (EIA) standard Interface
Between Data Terminal Equipment and Data Communication Equipment
Employing Serial Binary Data Interchange, Revision D. This
standard is properly abbreviated EIA-232D, but
is usually called RS-232C, for the earlier and
nearly identical revision C. The ITU (formerly CCITT) defines
essentially identical standards as V.24
(interface) and V.28 (signaling).

22.2.1.1 DB25 pin definitions

EIA-232D defines pin assignments, but does not specify physical
connectors. In the PC environment at least, a well-established de
facto standard does exist for 25-pin serial connections. By
convention, DTE devices (e.g., PC serial ports) use a 25-pin male
D-sub plug connector, designated DB25M. DCE
devices (e.g., modems) use a 25-hole DB25F
female socket connector. There are exceptions. For example, some
serial printers are DTE devices but use a DB25F connector. Figure 22-1 shows a DB25 connector.

Figure 22-1. A DB9 connector

Table 22-1 lists DB25 pin assignments. The column
headings are self-explanatory, except the following:

Signal direction, relative to the DTE. For example, a DCE (modem)
asserts voltage onPin 5 (CTS) to notify the DTE (serial port) that it
is ready to accept data from the serial port. We list this signal
direction (from the DCE to the DTE) as In. Pins 1 and 7 are ground
pins, and have no signal direction. Pin 11 is unassigned, and so has
no signal direction.

The circuit numbers/names used by CCITT, EIA, and RS standards
documents, respectively. Circuits with a hyphen (-) are defined by
the standard in question but not assigned a name. Circuits with a
blank box are neither defined nor named.

EIA-232D maps circuits, all of which are named from the point of view
of the DTE device, to pin numbers. Placing a voltage signal on a
circuit is called

asserting or

raising that circuit. For example, placing
voltage on Pin 20 asserts (or raises) DTR. Although you probably
don't need to understand these signals in detail,
knowing something about their purposes will help you choose the right
cable and troubleshoot connection problems. The signals asserted on
the EIA-232D pins and pin pairs commonly used in PC serial
communications have the following purposes:

Pin 1, if present, grounds one end of the cable shield to the
equipment chassis to reduce interference. Pin 7 is the common ground
reference for all signals on other pins. Pin 7

must be connected at both ends for communication
to occur. It is a relatively common (although poor) practice for
cables to connect the grounds together, forming a single circuit.

These pins are used to communicate data. A DTE device transmits data
on Pin 2 and receives on Pin 3. A DCE device transmits on Pin 3 and
receives on Pin 2. Pin 7 is the common return and ground reference
for both of these circuits.

The DTE asserts RTS when it wants to send data to the DCE, and the
DCE asserts CTS when it is ready to receive data. The DTE may not
transmit data until the DCE asserts CTS. CTS may be linked to RTS,
allowing the DTE to use RTS to assert CD, if the DCE is so
configured.

The DCE device asserts DSR when it is powered on and not in test
mode. The DTE asserts DTR when it is powered on. In modem
applications, DTR may be used to emulate an off-hook condition. If
the modem is in auto-answer mode, RI may be linked to DTR, causing
DTR to be asserted and the modem to answer when an inbound ring is
sensed.

Although the proper name of this circuit is Received Line
Signal Detector (RLSD), nearly
everyone calls it Data Carrier Detect
(DCD) or simply Carrier
Detect (CD). The DCE asserts CD to
tell the DTE that a carrier is present at the DCE. In modem
applications, this means that the DCE is receiving a telephone line
signal that meets its criteria for presence of a carrier. Many DTE
devices will not transmit or receive data unless the DCE is asserting
CD. Accordingly, in nonmodem (direct connect) applications, Pin 8
(CD) is normally tied to Pin 20 (DTR). Because the DTE normally
asserts DTR as soon as it is turned on, this
"spoofs" the DTE into believing
that DCD has been asserted.

The DCE asserts RI to notify the DTE that the phone is ringing. When
configured in auto-answer mode, the modem (DCE) asserts RI in step
with the ring cadence. That is, when ring voltage is present, the DCE
asserts RI; between rings, the DCE drops RI. The DTE instructs the
DCE to answer the call by asserting DTR on Pin 20.

PCs commonly use nine of the preceding 10 pins and signals (excluding
Pin 1, GND). Pins 12 through 17, 19, 21, 23, and 24 are not used for
standard PC serial connections, but may be used in nonstandard ways
by proprietary serial devices. Most commercial DB25 serial cables are
labeled as "9-wire,"
"10-wire," or
"25-wire." The first connects only
the most commonly used pins. The second also connects GND. The third
connects all 25 pins.

22.2.1.2 DB9 pin definitions

When IBM introduced the PC/AT in 1984, it used a nonstandard DB9M
9-pin serial connector that included only the commonly used pins.
Because this connector is physically smaller, it allowed putting both
a serial port and a DB25F parallel port on the same expansion card
bracket. The market presence of IBM and the passage of years have
combined to make this DB9M serial connector a de facto standard. Like
the DB25, DB9 connectors are available in male and female versions,
designated DB9M and DB9F,
respectively. By convention, DTE devices use the DB9M. Few DCE
devices use DB9 connectors. However, those that do usually use a
DB9F. Figure 22-2 shows a DB9 connector.

Figure 22-2. A DB9 connector

Although its pinouts don't map to either DTE or DCE,
the DB9M serial connector is considered an
"honorary" DTE device, with
DTE-to-DCE interfacing accommodated by the cable. The DB9 connector
includes the nine important signals, but the pinouts differ from
DB25, as Table 22-2 shows.

22.2.2 Universal Asynchronous Receiver-Transmitter (UART)

The Universal Asynchronous Receiver-Transmitter
(UART) is the heart of a serial port. When
transmitting, the UART receives the outbound byte stream from the
system bus, converts the bytes into a bit stream, and places the bits
onto the serial interface. When receiving, the UART receives an
inbound bit stream from the serial interface, buffers the inbound
bits, assembles them into bytes, and places those bytes onto the
system bus. There are two types of UARTs.

A dumb UART generates an interrupt each time it
sends or receives a byte. At low data rates this is acceptable, but
at faster data rates these frequent interrupts put a heavy burden on
the processor. Dumb UARTs have long been obsolete, but are often
encountered in older systems such as 486s and Pentiums that have been
converted to use as firewalls, fax servers, and other communications
servers. Dumb UARTs can be used with Windows and Linux in
applications for which data rates do not exceed 38.4 kb/se.g.,
14.4 kb/s fax modems. The following are dumb UARTs:

The 8250 is an eight-bit UART used in original IBM PCs and
compatibles, and occasionally encountered in newer PCs with
secondhand serial cards installed. If you find an 8250, pull the card
and throw it away.

The 16450 is a 16-bit UART used in PC/ATs and compatibles. It is
common on 16-bit serial cards for 80286, 80386, and early 486
systems, and on the motherboards of such systems. It runs 19.2 Kb/s
reliably, and may be usable at 38.4 Kb/s. It has only a one-byte
buffer, making it unsuitable for multitasking operating systems such
as Windows 9X/2000/XP and Linux. The 16450 may exist as a discrete
chip on the system board or an expansion card, or as a part of a VLSI
chipset. If the 16450 is socketed, you can replace it directly with a
$5 16550AFN chip. If the 16450 cannot be replaced easily, disable it
and install a modern serial card.

The 16550 is a 16-bit UART used in late-model 486 systems, in Pentium
and later systems, and in expansion cards intended for those systems.
The 16550 has a 16-byte FIFO buffer, but early models have a bug that
disables the FIFO buffer. This bug was fixed in later models of the
16550. Fixed versions are designated 16550A, but some chip makers did
not change the labeling, making it difficult or impossible to
discriminate visually between buggy models and fixed models. Windows
(usually) reports an early 16550 as a 16450. Linux reports an early
16550 as a 16550, and a 16550A (regardless of labeling) as a 16550A.
Early 16550 UARTs are unacceptable for multitasking use under Windows
or Linux.

The 16650 is a faster 16550-class UART that doubles buffer size to 32
bytes. Unfortunately, like early 16550 models, early 16650 models
have a bug that disables FIFO buffering. Linux and Windows treat
these early 16650 UARTs as 16450s, using only one byte of the buffer.
Windows reports the 16650 as a 16550 or 16650, regardless of whether
the 16650 is an older, buggy model or a later, fixed model. However,
Windows generally refuses to enable the FIFO buffer on an older
16650. Linux, as usual, reports the UART type accurately. If Linux
reports the UART as 16650, it is an older model and should not be
used for multitasking or high data rate applications. If Linux
reports the UART as 16650V2, it is a fixed model and can be used.

A FIFO UART uses a first-in, first-out buffer to
reduce the number of interrupts that occur when transferring data.
The FIFO buffer may range from 16 bytes to 64 bytes or more. A FIFO
UART buffers incoming data until the buffer reaches the
trigger level (is nearly full). The UART
generates an interrupt only when the trigger level is reached, which
means a FIFO UART generates many fewer interrupts than a dumb UART
and is much less likely to drop incoming data bytes. The following
are FIFO UARTs:

16550-family UARTs use a 16-byte FIFO buffer to support multitasking
operating systems, which cannot always give immediate attention to
interrupts generated by the serial port. Early 16550 models had a bug
that prevented the FIFO buffer from being used. This bug was fixed in
later models, which are officially designated 16550A. However, some
chip makers produced fixed 16550s without changing the labeling,
making it difficult or impossible to tell visually whether the UART
is the buggy older model or the fixed newer model. Windows usually
reports an early 16550 as a 16450, and uses it as though it were a
16450. Linux reports the buggy early model as a 16550, and the fixed
newer model as a 16550 (regardless of how the chip is labeled). The
16550A functions properly under Windows and Linux, and supports 115
Kb/s throughput. The 16550A is the minimum acceptable UART for a
modern system.

The 16650 is essentially a faster 16550-class UART that doubles
buffer size to 32 bytes and adds support for various modern BIOS
features, including power management. Early 16650s suffer the same
bug as early 16550s, and cannot be used reliably for multitasking or
high data rates under Windows or Linux. It is usually impossible to
tell visually whether a particular 16650 is the older, buggy model or
the newer, fixed model. Windows generally disables the FIFO buffer on
a buggy 16650, but enables the buffer for a fixed 16650. Linux
reports an original 16650 as a 16650, and the fixed model as a
16650V2. The 16650 supports data rates up to 450 Kb/s, making it
better suited than a 16550 for use with ISDN adapters and similar
high-speed devices. Dual-16650 serial cards are available for less
than $50.

The 16750 is an improved 16550-class UART that further expands the
FIFO buffer to 64 bytes for transmit and 56 bytes for receive, and
increases the maximum data rate to 900 Kb/s. The 16750, as well as
similar enhanced versions such as the 16850 and 16950, are uncommon
on PCs, and are used primarily on specialized, very high-speed
devices such as T3 interface cards.

22.2.2.1 Determining UART type

Under DOS and Windows 9X, you can determine the UART type with
Microsoft Diagnostics (MSD.EXE), with a general-purpose diagnostic
utility such as SiSoft Sandra, or with a dedicated port diagnostics
program such as PortMaster. Either of the latter two utilities can be
downloaded from any of numerous Internet sites.

Figure 22-3 shows SiSoft Sandra displaying
information about COM1 on an AMD Duron system, built on a Microstar
motherboard that uses a VIA KT133 chipset, and running Windows 98.
Sandra reports that COM1 uses a National Semiconductor 16550AN UART,
which is not precisely true. The COM1 UART is actually just one of
the functions provided by the Super I/O portion of the Southbridge of
the VIA chipset. The UART is emulated rather than physically present,
but as far as Windows and connected serial devices are concerned,
this system might just as well have a real physical 16550AN UART.

Figure 22-3. Using SiSoft Sandra Ports Information to view serial port configuration on a Windows 98 system

Utilities that run under Windows NT/2000/XP are limited in the amount
and accuracy of the information they can provide because Windows
masks the underlying hardware from the utility. For example, Figure 22-4 shows the results of running SiSoft Sandra
Ports Information on a system running Windows NT 4. The Windows NT
Hardware Abstraction Layer (HAL) prevents user-mode applications
(such as Sandra) from accessing the underlying hardware, so Sandra
can report only what Windows sees fit to tell it, which may be little
or nothing.

Figure 22-4. Using SiSoft Sandra Ports Information to view serial port configuration on a Windows NT 4 system

If your system boots Windows NT/2000/XP, the only way to obtain
detailed information about the hardware is to boot DOS and run a
DOS-mode diagnostic utility. You can use either a comprehensive
commercial diagnostics program such as CheckIt for that purpose, or a
dedicated small utility such as PortMaster.

If the device has not yet been installed in the computer (or if
you're willing to pop the lid), you can sometimes
determine the UART type by examining the chips on the device to
locate a UART number. However, many devices use VLSI/ASIC components
rather than discrete UART chips, making it difficult or impossible to
identify the UART type visually. In these cases, if you
don't have the documentation, the only convenient
way to determine the UART type may be to install the board, boot DOS,
and run a diagnostics program.

Some "enhanced" VLSI/ASIC-based
16450 serial ports with FIFO buffering added may falsely report
themselves to the operating system and applications as 16550s,
although PortMaster usually detects them properly. These UARTs are
commonly found on early 486 motherboards and
"high-speed" serial cards designed
for 486 systems. These bumped-up 16450s may or may not function
properly with any particular combination of operating system and
application, and should be replaced with a serial port that uses a
16550AFN or higher UART.

22.2.2.2 Choosing UART type

Which UART you need to use for maximum throughput depends on three
factors:

DCE-to-DCE rate specifies the actual link speed
between the two communications devices. For example, V.90
"56K" modems actually talk to each
other at a maximum of 53 Kb/s (rather than 56 Kb/s, due to FCC
regulations limiting signal amplitude on a phone line). ISDN devices
communicate with each other at 64 Kb/s when using one B channel, or
128 Kb/s when using two, and so forth.

Data compressibility is the degree to which
source data can be compressed by the communications device before
being placed on the DCE-to-DCE link. Some data (e.g., text, web
pages, and databases) contains a great deal of embedded slack space,
and can be compressed as much as 4:1 by the communications hardware.
Other data (e.g., images, executables, etc.) is much less
compressible.

DTE-to-DCE rate specifies the actual link speed
between the local serial port and the local communications device.
Setting this speed faster than the DCE-to-DCE rate allows the serial
port to provide compressible data to the communications device
quickly enough that the DCE-to-DCE link never runs out of data to
transfer. The compression algorithms used by most communications
devices allow 4:1 compression in theory, although 1.5:1 or 2:1 is
more common in practice. This means that the DTE-to-DCE data rate
must be at least one and one-half or two times the DCE-to-DCE rate
(and, ideally, four times) to make sure the DCE is never kept waiting
for data.

After you determine the UART types used by your serial ports, use the
following guidelines to decide if these ports are adequate for their
intended purpose:

Serial ports that use these UARTs are long obsolete, and are
completely unsuitable for use in any modern system. 8250-class UARTs
were used on original IBM PC- and XT- class systems, and 8250-class
add-on cards were installed in some 486 and early Pentium-class
systems. Replace any serial port that reports an 8250-class UART. If
the UART is embedded in the motherboard, disable the motherboard
serial port(s) in BIOS Setup, and install an add-on serial card with
16550AFN or higher UARTs.

These ports are usually adequate for mice and similar low-speed
devices, even on slow and/or heavily loaded computers.

These ports are usually adequate for modems that use V.32bis (14.4
Kb/s), V.34 (28.8 Kb/s), or V.34+ (33.6 Kb/s). These ports are
marginal for V.90 (56 Kb/s) modems, and single-channel (56/64 Kb/s)
ISDN applications because they allow for only about 2:1 compression.
A 16550A is unacceptable for dual-channel ISDN because it limits the
DTE-to-DCE rate to 115.2 Kb/s, which is actually lower than the 128
Kb/s DCE-to-DCE rate. All other things being equal, 16550A ports are
less likely to be a bottleneck on faster and lightly loaded
computers. On midrange 486s and less, and on heavily loaded slow
Pentiums, the 16550A may be a bottleneck, and should be replaced with
a 16650.

These ports are necessary to guarantee full throughput potential for
any application that requires a DTE-to-DCE rate higher than the 115.2
Kb/s DTE limitation of the 16550A and slower UARTs. In other words,
you really need a 16650 or better for anything above a 28.8 Kb/s V.34
DCE-to-DCE link. In practice, you probably won't
notice much difference with anything up to a 56K modem, but you may
notice a big difference with ISDN.

22.2.3 Resources Required by Serial Ports

Each standard serial port requires an IRQ and an I/O port. The serial
port generates an interrupt on its assigned IRQ to get the
processor's attention when it has data to transfer.
The I/O port is a range of addresses, named for the first address in
the range, that defines a
"scratchpad" area that the serial
port uses to exchange data with the computer. For example, I/O Port
0x3F8 includes the address range 0x3F8 through 0x3FF.

Serial ports are named

COM[xxx] :, where

[xxx] may be a number from 1 through 256.
Although the serial port name technically includes the trailing
colon, the colon is usually dropped for convenience, and that is the
practice we follow. IBM originally defined only COM1 and COM2 for the
PC BIOS, although it later defined standard values for COM3 and COM4
as well. Table 22-3 lists the standard COM port
assignments for ISA/PCI systems. (Systems that use the EISA or MCA
bus use additional COM ports with different settings, but these
systems are obsolete and immaterial.) The values for COM1 and COM2
are invariant due to long usage. The values for COM3 and COM4 are
semistandard, but may differ on some systems.

Note that ports 1 and 3 and ports 2 and 4 share an IRQ. Although EISA
and MCA systems permit IRQs to be shared gracefully, ISA/PCI systems
do not. This means that, in effect, you can use only two serial ports
safely on a system that uses standard COM port assignments. You may
be able to get away with such sharingsay, by putting a serial
printer on COM3 and a mouse on COM1but you must make sure
never to use both devices that share one IRQ at the same time.

Windows 95 went a long way toward solving this problem by adding
support for up to 128 COM ports. Windows NT 4.0 goes further still,
supporting up to 256 COM ports and allowing you to define custom
values for IRQ and base address for each installed COM port. Because
Windows applications use COM ports by name and are not concerned with
underlying settings, it is relatively straight-forward to support
multiple COM ports in the Windows environment.

A final note: PCs assign a priority to each IRQ, and process
interrupts on the higher-priority IRQ first. The highest-priority IRQ
is 0, followed by 1, 2 through 9, 10 through 15, and then 3 through
8. Put high-priority devices such as modems on a high-priority serial
port (e.g., COM2/IRQ3) and low-priority devices such as mice on a
lower-priority serial port (e.g., COM1/IRQ4). This is less important
with recent UARTs than it used to be, but is still worth doing as a
matter of good practice.

22.2.4 Enhanced Serial Ports

The proliferation of high-speed external serial devices, notably
serial ISDN terminal adapters, made it obvious that something faster
than the standard 16550 UART was needed. The standard serial ports on
most PCs top out at 115 Kb/s or (rarely) 230 Kb/s.
That's not fast enough to support the full
throughput of a dual B-channel ISDN terminal adapter, which operates
at 128 Kb/s natively and up to four times that fast when data
compression is enabled.

To address that need, various manufacturers supply enhanced serial
ports, which are add-on cards that include one or two serial ports.
These use 16650, 16750, 16850, or 16C950 UARTs, which are essentially
16550 UARTs with additional buffering added and support for higher
data rates. Some cards also include an embedded processor, typically
an 80186 or 80286, which offloads interrupt handling from the main
processor.

These adapters are overkill for normal serial port applications, may
provide some benefit for 56K dial-up modems, and often provide a
noticeable performance increase for ISDN applications. If you buy
such a card, make sure that it includes any necessary drivers for
your operating system.

22.2.5 Multiport Serial Adapters

Each standard serial port occupies one IRQ, which limits how many
standard serial ports can be installed. Most computers need one or
two serial ports at most, so this is seldom a problem. But there are
applicationssuch as running a terminal server under
Linuxfor which it is desirable to have eight, 16, or more
serial ports available. That's clearly impossible if
each serial port requires one IRQ. The solution is a special board
called a multiport serial adapter.

These adapters are available in ISA and PCI form, and are installed
in the PC just like any other expansion card. They provide from four
to 32 serial ports using only one IRQ. The serial port connectors
reside in a separate external box that connects to the multiport
serial adapter via multiple 8P8C
("RJ-45") connectors or a single
proprietary octopus cable with many conductors.

Such adapters are available in two forms:
"dumb" multiport serial adapters
are little more than a collection of UARTs and connectors, use 8-bit
transfers, generate an interrupt for each byte transferred, and
depend on the host PC for processing power.
"Smart" multiport serial adapters
include a microprocessor (typically an 80286 or 386) that offloads
serial interrupt processing tasks from the host PC. Smart adapters
usually buffer 1 KB of data or more between transfers, greatly
reducing the interrupt burden on the host system. Nowadays, most
multiport serial adapters are of the smart variety.

Dumb terminals have become rare in most
applicationsLAN-connected PC clients are much more common
nowadays, even on Linux systemsand so the need for multiport
serial adapters is much less than formerly. If you do need one, for
example, for process control, automation tasks, or building a
terminal server for a modem poolmake sure that drivers are
available for your operating system.