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

23.1 Mapping Parallel Ports to LPTs

The PC BIOS allocates three prioritized
I/O port addresses to parallel printers. Port 0x3BC is the highest
priority, followed by 0x378 and then 0x278. At boot time, the BIOS
checks each of these addresses to detect parallel hardware. The
highest-priority parallel port detected (which may be on any of the
three I/O port addresses) is assigned as LPT1:. If a second parallel
port is detected, it is assigned LPT2:. If a third port is detected,
it is assigned LPT3:. Some BIOSes also make provision for LPT4:, but
this is nonstandard and not widely supported.

This automatic detection of port hardware and assignment of LPT
numbers means that installing another parallel port may change the
LPT designation of existing ports. For example, the embedded parallel
port on most motherboards is assigned port 0x378 (the second-priority
address) by default. As long as it is the only port present, it will
be mapped to LPT1: by the BIOS. If you add another parallel port
configured for port address 0x3BC, that new port will be mapped to
LPT1: and the existing port will be changed to LPT2:.

You set the I/O port address for most motherboard parallel ports in
BIOS Setup, although older motherboards may require changing a jumper
instead. You set addresses for parallel ports on most expansion cards
by changing a jumper. Avoid changes in LPT mappings when installing
parallel ports by verifying the port addresses for existing ports and
setting the new port for a lower-priority address, if possible.
Always make sure that the new port does not use the same address as
an existing port, or results will be unpredictable.

23.1.1 Parallel Port Types

Parallel port
hardware may be of five types, described next in the order of their
appearance in PCs. A computer may contain any of these port types,
and may include ports of more than one type. Earlier ports are
limited in functionality and performance. Later ports provide
increased functionality and performance, and may often be configured
to emulate earlier port types when necessary to support older
peripherals.

The unidirectional 4-bit parallel port, also
called a Standard Parallel Port
(SPP), is based on the de facto Centronics
standard, and was the type of parallel port supplied with the
original IBM PC and its clones. These ports are misnamed, as they are
not unidirectional and are not limited to 4-bit transfers. An SPP
does 8-bit output and can accept 4-bit (nibble) input.

In theory, these ports are limited to using a 2-meter (about 6 foot)
cable, but this distance can be extended to 3 to 5 meters (10 to 16
feet) by using a high-grade parallel cable. Unidirectional 4-bit
parallel ports are commonly found in older desktop and laptop
systems, and are still supplied on some low-end I/O cards. These
ports provide native throughput of 40 to 60 KB/s, although certain
design tricks can push this to the 150 KB/s range.

When IBM introduced the PS/2 line in 1987, all but the two
lowest-cost models (the Models 25 and 30) included a
bidirectional 8-bit parallel port. Initially,
these were non-DMA ports, called Type
1 ports by IBM. The parallel ports included with later
PS/2 systems could also be configured as Type 3
ports, which use DMA. These ports support both 8-bit input
and output, and provide about 75 KB/s to 300 KB/s throughput,
depending on characteristics of the port itself, how it is
configured, the speed of the external device, and the quality of the
port driver.

Recent notebook and desktop systems often provide a bidirectional
8-bit mode for their parallel ports, as do some add-on port cards.
Bidirectional 8-bit parallel ports provide better throughput than do
4-bit ports for connecting external devices such as tape drives and
parallel port network adapters, if the device can take advantage of
the 8-bit functionality. Note that some vendors also call 4-bit ports
"bidirectional," so the terminology
used to describe the port does not guarantee its level of
functionality.

The throughput limitations of even the Type 3 bidirectional 8-bit
parallel ports soon became obvious as page printer technology
improved. Manufacturers of scanners, storage devices, and other
external peripherals were also starting to use the parallel port as
an inexpensive alternative to expensive SCSI or proprietary
interfaces. A superior parallel port technology was clearly needed.
Xircom, Intel, and Zenith Data Systems got together and came up with
the Enhanced Parallel Port, or
EPP.

EPP offered performance and other advantages while maintaining
backward SPP compatibility, so it quickly came into widespread use.
There soon coalesced an informal confederation of manufacturers whose
purpose was to promote and enhance the EPP standard. This group
ultimately solidified as the EPP Committee, and successfully lobbied
the IEEE-1284 committee to include EPP as an advanced mode in the
IEEE-1284 specification described later in this section.

EPP supports 8-bit bidirectional communications at ISA bus speeds,
providing throughput similar to that of 8-bit ISA bus cards. EPP
provides theoretical maximum throughput of about 2 MB/s, and typical
real-world throughput of more than 1 MB/s. Many 386 and 486 systems
and most reasonably recent I/O expansion cards include EPP-capable
parallel ports.

EPP was a reasonably satisfactory solution
and was first to market, but Microsoft and Hewlett-Packard had been
working on their own improved parallel port technology, which they
named the Extended Capabilities Port, or
ECP. Like EPP, ECP supports 8-bit bidirectional
communications at ISA bus speeds. Unlike EPP, ECP uses DMA, provides
a FIFO buffer of at least 16 bytes, and includes hardware data
compression. These features allow ECP to provide better throughput
than EPPtheoretically more than 2 MB/s, but typically about 2
MB/s actual. Some 486 systems, most Pentium and higher systems, and
recent I/O expansion cards include ECP-capable parallel ports.

The increasing diversity of parallel port hardware and the resulting
potential for incompatibilities made it desirable to develop an
umbrella standard that would combine and codify these earlier ad hoc
standards into a single formal standard. The resulting document,
1284-1994 IEEE Standard Signaling Method for a
Bidirectional Parallel Peripheral Interface for Personal
Computers, does so by defining five parallel transmission
modes. IEEE-1284-compliant parallel port hardware, available on
recent computers, motherboards, and expansion cards, can use one or
more of the following modes to emulate earlier parallel port
hardware, thereby ensuring both compatibility and optimum performance
with almost any parallel peripheral:

Compatibility
Mode
, also called
Centronics Mode or Standard
Mode, is a forward unidirectional mode that corresponds to
the original SPP definition, and is included in the IEEE-1284
definition for backward compatibility with the installed base of
SPP-only peripherals. Transferring a byte in Compatibility Mode
requires four I/O instructions and additional overhead instructions,
which limits throughput to about 150 KB/s. Pure IEEE-1284
Compatibility Mode is seldom seen in practice. Compatibility Mode as
implemented by most integrated 1284-compliant controllers includes a
FIFO buffer, which is used in conjunction with the Compatibility Mode
protocol. This hybrid mode, which is not a part of the official
IEEE-1284 standard, may be called Buffered Mode,
Fast Centronics Mode, FIFO
Mode, or Parallel Port FIFO Mode. It
improves Compatibility Mode throughput to 500 KB/s or more by
substituting hardware strobing for the software strobing used in true
IEEE-1294 Compatibility Mode. The elimination of software handshaking
nearly eliminates latency, and can increase throughput to 500 KB/s or
more.

Nibble Mode
is the slower of the
two reverse channel modes defined by IEEE-1284. Nibble Mode may be
combined with Compatibility Mode or a proprietary forward channel
mode to yield full bidirectional capability. The advantage to Nibble
Mode is that it can be used with any parallel cable and any parallel
port hardware, including the original unidirectional 4-bit ports. The
disadvantage to Nibble Mode is that it is the slowest way to send
data from a peripheral to the PC. Like Compatibility Mode, Nibble
Mode data transfer is managed by a software driver, which restricts
throughput to about 50 KB/s. For printers, which use the reverse
channel to transfer only small amounts of status information, this is
not a significant limitation. For parallel interface disk and tape
drives, network adapters, and similar devices that need full
bidirectional bandwidth, Nibble Mode reverse channel throughput is
wholly inadequate and should be used only as a last resort.

Byte Mode
is the
faster of the two reverse channel modes defined by IEEE-1284. Byte
Mode corresponds to the reverse channel mode of the 8-bit
bidirectional parallel interface originally supplied with IBM PS/2
computers. In contrast to Nibble Mode, which transfers four bits at a
time and requires two data transfer cycles to transfer one byte, Byte
Mode transfers a full byte in one data cycle, using the eight data
lines to do so. Byte Mode reverse channel throughput is comparable to
forward channel throughput in unbuffered Compatibility
Modeabout 150 KB/s. Using Compatibility Mode and Byte Mode
together provides a half-duplex bidirectional connection that is
comparable to the original IBM PS/2 bidirectional parallel interface.

EPP Mode
corresponds to the ad hoc
Xircom/Intel/ZDS EPP definition. Intel first implemented EPP on the
82360 I/O chip that was part of the 386SL chipset. This pre-1284 EPP
implementation is called EPP 1.7. The IEEE-1284-1994 EPP Mode
definition formalizes EPP 1.7, but with some minor changes in signal
definitions. As a result, not all EPP peripherals work reliably with
all EPP ports. Any 1284-compliant EPP peripheral may be used with
either an EPP 1.7 port or a 1284-compliant EPP port. An EPP 1.7
peripheral may be used with an EPP 1.7 port, but may or may not
function properly with a 1284-compliant EPP port. EPP Mode can
achieve data throughput comparable to an ISA bus cardon the
close order of 0.5 to 2 MB/s.

ECP Mode
corresponds to the ad hoc
Microsoft/HP ECP specification.

The original IEEE-1994 standard is supplemented by IEEE-P1284.3, Standard for Interface and Protocol Extensions to IEEE-1284-1994 Compliant Peripherals and Host Adapters . For our purposes, the only notable new feature of IEEE P1284.3 is that it defines additional protocols that allow daisy-chaining parallel-connected devices. Windows versions prior to Windows 2000 do not support IEEE-P1284.3 functions. Windows 2000/XP has partial IEEE-P1284.3 support, including the ability to select and operate more than one IEEE-1284.3 daisy-chain device and an end-of-chain device, and basic support for the Datalink Layer. Windows 2000/XP does not support IEEE-1284.3 multiplexors or interrupts for IEEE-1284.3 daisy-chain devices.

23.1.2 Configuring Parallel Port Hardware

How you configure a parallel
port may significantly impact performance and overall capabilities.
Even on new systems that include capable parallel port hardware, the
parallel port mode is often set to SPP by default. Many people
unintentionally cripple the performance of their parallel ports
simply because they don't know that better choices
than the default are available.

The first step to configure a parallel port for optimum performance
is to determine the capabilities of the port hardware. Examining the
documentation may help, but documentation is often cursory,
misleading, or missing entirely. Without detailed documentation, the
easiest way to determine the capabilities of the parallel port
hardware is to download and run

Parallel.exe ,
which is available from many Internet file repository sites.

On older motherboards and expansion cards, you may have to set the
mode by using a jumper. On newer systems, you can usually set the
mode using the BIOS Setup program. The parallel port modes available
are determined first by the capabilities of the port hardware itself.
Even if the hardware supports all modes, however, the BIOS may not,
so you may be limited in the choices you can make. In general, use
the following guidelines when selecting a parallel port mode:

SPP Mode, which the BIOS may also call
Standard Mode,
Basic Mode,
4-bit Mode

, or
Unidirectional Mode,
is the least-common-denominator choice, and corresponds to
the original Centronics-compatible IBM parallel port. Use this mode
only after determining that none of the more-capable modes works with
your cable or peripheral.

Bidirectional Mode, which the BIOS may also call
8-bit Mode or PS/2 Mode,
corresponds to the parallel modes introduced with the IBM PS/2
parallel ports. If you choose this mode, you may also be able to
choose Type 1 sub-mode (also called
Non-DMA sub-mode) or Type 3
sub-mode (also called DMA sub-mode).
Choose Type 3 mode for better performance, as long as you
don't mind consuming a DMA channel. Use this mode if
only it and SPP work properly for your hardware. Also use this mode
if you are using Windows NT, which does not support EPP and ECP
modes.

EPP Mode, which the BIOS may instead call
Enhanced Mode, is sometimes the best choice even
if later modes are available. EPP includes some control features that
are not provided by ECP, sometimes making EPP better suited for
nonprinter peripherals such as parallel port storage devices and
scanners. Also, you may need to choose EPP mode explicitly to support
some early EPP-compliant devices that do not function properly with
the updated EPP mode supported by IEEE-1284-1994 compliant ports. EPP
uses IRQ channels for flow control. It does not use DMA or provide
data compression, making it somewhat slower than the ECP mode
described next.

ECP Mode, which the BIOS may call
Extended Mode, is usually the best choice for
transferring data to high-speed printers, although it does require a
DMA channel. Because it does not support some of the control features
provided by EPP mode, ECP mode may not be the best choice to connect
nonprinter peripherals.

IEEE-1284 Mode, which the BIOS may simply label

Auto , is the most flexible choice. If the BIOS
provides this option, choose it to allow the port to adjust
automatically to the optimum mode for the device to which it is
connected.

Parallel port support differs widely between Windows distributions,
as described in the following sections.

23.1.2.1 Configuring parallel ports under Windows NT

Windows NT through V4.0 does not support EPP
or ECP bidirectional communications ports. If a parallel port is
configured as EPP or ECP, Windows NT 4.0 and prior will not detect
the port.

23.1.2.2 Configuring parallel ports under Windows 2000/XP

Windows 2000/XP fully supports most
parallel hardware, including standard parallel port devices,
IEEE-1284-compatible and -compliant devices, and IEEE-1284.3
daisy-chain devices. Windows 2000/XP fully supports most parallel
modes, including Centronics mode, IEEE-1284 modes, ECP mode, and EPP
mode. Windows 2000/XP partially supports IEEE-1284.3 modes.

To configure a parallel port under Windows 2000/XP, right-click My
Computer and choose Properties to display the System Properties
dialog. Click the Hardware tab and then the Device Manager button.
Locate and expand the Ports (COM & LPT) item within the Device
Manager tree, and double-click the printer port you want to configure
to display a dialog similar to that shown in Figure 23-1.

Figure 23-1. Using Printer Port Properties to configure the port under Windows 2000/XP

The Filter Resource Method determines how Windows 2000/XP manages the
port, as follows:

Marking this option causes the Windows parallel port driver to
release any interrupt assigned to it if Plug-and-Play enumeration
determines that the installed parallel port hardware does not require
an interrupt to function properly. If the port hardware does require
an interrupt for proper functioning, the Windows parallel port driver
retains control of that interrupt. This setting works properly and
automatically on most systems that use ACPI, and we can only suppose
that Microsoft did not choose this as the default setting because the
potential exists for conflicts on older hardware.

This is the default setting for Windows 2000 and Windows XP. Marking
this option causes the Windows parallel port driver to release any
interrupt assigned to it for use by another device, even if
Plug-and-Play enumeration determines that an installed parallel port
requires an interrupt to function properly. This setting works
properly on most modern systems that use a default configuration.
However, if you reconfigure the parallel port in BIOS Setup to
function as an EPP port, this setting may cause that parallel port to
malfunction or not be recognized.

This option disables the Windows parallel port driver
interrupt-filtering function, and allows the parallel port driver to
accept and use any interrupt assigned to it. Enable this option only
if (a) because of hardware, BIOS, or driver issues, the system does
not operate properly unless an interrupt is available to the parallel
port hardware, or (b) you have installed a high-speed parallel
interface and driver that require an interrupt to function properly.
Note that enabling this option may cause an interrupt conflict with
legacy audio cards or network adapters. Enable this option as a last
resort. Some parallel ports configured as EPP may require that this
option be enabled.

Some older parallel port devices are not detected properly during
Plug-and-Play enumeration. If Windows fails to detect such a device
on your system, mark this checkbox and restart the system. If all
devices are detected properly, leave the checkbox unmarked.

23.1.2.3 Configuring parallel ports under Windows 9X

Windows 9X must be configured manually to
use ECP, but supports ECP devices in any of the following five
configurations:

Standard I/O ranges for LPT ports only

Standard I/O ranges for LPT ports and any IRQ

Standard I/O ranges for LPT ports, IRQ, and any DMA

Any I/O ranges for LPT ports only

Any I/O ranges for LPT ports and IRQ

To enable ECP support in Windows 9X, first use the system
documentation, BIOS Setup, or a diagnostic utility to verify the IRQ
and DMA settings assigned to the port you want to configure. In the
Device Manager, expand the Ports (COM and LPT) item and display the
Properties dialog for the port to be configured. On the Resources
page, the Resource Settings pane shows the I/O port that was detected
automatically and assigned to the device. Clear the Use automatic
settings checkbox and use the Setting based on drop-down list to
choose the appropriate Basic Configuration, according to whether the
ECP port uses standard or custom settings. Change the Input/Output
Range, IRQ, and/or DMA settings as needed to correspond to the port
hardware configuration, save the changes, and restart Windows.

23.1.2.4 Configuring parallel ports under Linux

Linux
has made great strides in adding support for various parallel modes.
Linux 2.0 supported only Compatibility Mode and Nibble Mode.
Beginning with 2.4, the Linux parallel port subsystem supports all
standard IEEE-P1284.3 modes. For complete details on the Linux 2.4
parallel port subsystem, see http://people.redhat.com/twaugh/parport/html/parportguidel.

23.1.3 Parallel Connectors and Cables

IEEE 1284-1994 defines both the
electrical and physical interface for cables and connectors. Cable
quality is critical for IEEE-1284, because various IEEE 1284 modes
support much higher transmission speeds than SPP.

23.1.3.1 Parallel connectors

Traditional parallel cables use a DB25M connector for the PC end and
a male, 36-pin, 0.085-inch centerline Champ connector with bale locks
(commonly called a Centronics C36M) for the
printer. The IEEE-1284-1994 specification allows these two
traditional connectors to be used as before. It designates the DB25M
the IEEE-1284-1994 Type A Connector and the C36M
the IEEE-1284-1994 Type B Connector. IEEE-1284
also defines a new type of parallel connector, called the
1284-1994 Type C Connector, which uses a 36-pin,
0.050-inch centerline mini-connector with clip latches, and is
usually called a mini-Centronics connector.
Printer cables are now available that use these connectors in many
combinations.

23.1.3.2 PC-to-peripheral parallel cables

It
used to be that a printer cable was a printer cable. Not anymore.
Printer cables now come in a variety of types, which use different
connectors and pinouts. The good news is that you can still use any
printer cable to connect a PC to a printeras long as the
connectors physically fitand that connection will work in some
fashion. The bad news is that using an old printer cable may cripple
the performance and functionality of the link.

When you buy a new parallel cablewhich you should if you are
now using an older cable to connect a recent port to a recent
peripheralmake sure it's labeled
"IEEE-1284-1994 Compliant." Table 23-1 through Table 23-4
show the pin connections for the standard IEEE-1284 cables you are
likely to need. To ensure optimum parallel performance, use an
IEEE-1284 cable with connectors appropriate for your PC parallel port
and the peripheral to be connected.

Table 23-1 shows the pinouts for a standard SPP
25-wire Centronics C36M-to-DB25M parallel printer cable, including
signal polarities and directions. The missing C36M pins are not
connected. The original IBM Parallel Cable and some inexpensive
currently available cables use only 18 wires, using a single wire to
tie DB25M pins 18 through 25 to C36M pins 19 through 30 and 33. These
18-wire cables may not work in all applications, notably with OS/2.

Table 23-2 shows the pinouts for an IEEE 1284
A-to-B adapter cable, used to connect a DB25M, Type A EPP, ECP, or
IEEE-1284-compliant PC parallel port to a peripheral with a
Centronics, C36M Type B connector. Note that DB25M pins 1 through 17
carry the same signals as the preceding cable, and that DB25M pins 18
through 25 are similarly used for ground returns, although with
slightly different definitions. Because it uses the same connectors
as the SPP parallel cable described in the preceding table, the only
way to differentiate this cable visually is to look for the
"IEEE-1284-1994 Compliant" label.

Table 23-3 shows the pinouts for an IEEE-1284
A-to-C adapter cable, used to connect a DB25M, Type A EPP, ECP, or
IEEE-1284-compliant PC parallel port to a peripheral with a
mini-Centronics, Type C connector.

Table 23-4 shows the pinouts for an IEEE-1284
C-to-B adapter cable, used to connect a mini-Centronics, Type C PC
parallel port to a peripheral with a Centronics, Type B connector.
This is an unusual cable for now, but will become more common if and
when PC parallel ports with IEEE-1284 Type C connectors become more
common. Because parallel ports are being deemphasized in new
motherboards and PCs, that day may well never arrive.

23.1.3.3 PC-to-PC parallel cables

Windows NT
does not support direct parallel connections, but Windows 9X Direct
Cable Connection can be used to establish a parallel-to-parallel link
between two PCs. You can use three types of DB25M-to-DB25M cables for
a DCC parallel connection, designated by Microsoft as follows:

The
Standard cable, shown in Table 23-5, is also called a Basic 4-bit
cable, LapLink cable, or
InterLink cable. This the slowest parallel DCC
cable, but can be used to link computers with any types of parallel
port, including dissimilar ports on the two computers. Expect
throughput of 40 to 70 KB/s when using one of these
cablespainfully slow, but still about 10 times the speed of
DCC over a serial connection.

The Extended
Capabilities Port cable, shown in Table 23-6, is also called an ECP
cable. This cable can be used to link computers that both
have ECP parallel ports (including IEEE-1284 ports in ECP Mode)
installed and enabled. It provides much faster throughput than the
standard cable500 KB/s or more, depending on the ports.

The Universal Cable Module
cable, also called a UCM cable, can
be used to link two computers that have different types of parallel
ports. It's not really just a cable because it
includes active electronic components that automatically optimize
throughput between PCs with differing port types. This cable can be
very useful when both PCs do not have ECP-capable parallel ports and
you want to get the highest performance available for the combination
of hardware being usedfor example, duplicating a standard PC
configuration to multiple PCs when those PCs do not have network
cards, or backing up a notebook computer to a desktop system.

The only source we've found for this cable is
Parallel Technologies (http://www.lpt.com

) . Its
Universal Fast Cable costs $70, and includes monitoring software.
When used to connect two ECP or two EPP ports, this cable can provide
throughput of about 500 KB/s, within striking range of a 10 Mb/s
Ethernet link. Note, however, that there is no real reason to buy
this cable if all your parallel ports are ECP-capableyou can
simply use the ECP cable described above. The benefit of this cable
is that it automatically detects the port types in use and optimizes
throughput for them.