Parallel Port Configuration

The configuration of the LPT port consists of

• Selecting the port's operating mode

• Selecting the IRQ, I/O port address, and DMA channel (for certain modes)

For an overview of how IRQ, I/O port addresses, and DMA channels are used, see "Hardware Resources," p. 75 .

tip

As you prepare for the A+ Certification Exam, you should note that the parallel designation for the LPT port comes from its use of eight data lines (pins 29) and that the port has provisions for printer status messages (pins 1012).

LPT Port Operating Modes

The LPT port can be configured for a variety of operating modes. The options available for a particular port depend on the capabilities of the system. Most systems you're likely to work with should offer all of these modes, although a few digital dinosaurs still kicking around in some offices might not have the IEEE-1284 modes.

Standard Mode

The standard mode of the LPT port is the configuration first used on PCs, and it is the only mode available on many 386-based and earlier systems. On some systems, this is also known as compatible mode . Although configuration for this mode typically includes both the IRQ and I/O port address, only the I/O port address is actually used for printing. If the parallel port is used in standard/compatible mode, IRQ 7 can be used for another device. The standard mode is the slowest mode (150 kilobytes per second [KBps] output/50KBps input), but it is the most suitable mode for older printers. In this mode, eight lines are used for output, but only four lines are used for input. The port can send or receive, but only in one direction at a time.

This mode will work with any parallel cable.

PS/2Bidirectional

The next mode available on most systems is the PS/2 or bidirectional mode . This mode was pioneered by the old IBM PS/2 computers and is the simplest mode available on some computer models.

Bidirectional mode is more suitable for use with devices other than printers because eight lines are used for both input and output, and it uses only I/O port addresses. This mode is no faster than compatible mode for printing but accepts incoming data at a faster rate than compatible mode; the port sends and transmits data at 150KBps.

This mode requires a bidirectional printer cable or IEEE-1284 printer cable.

IEEE-1284 High-Speed Bidirectional Modes

Three modes that are fully bidirectional (able to send and receive data 8 bits at a time) and are also much faster than the original PS/2-style bidirectional port include

• EPP (Enhanced Parallel Port ) Uses both an IRQ and an I/O port address. This is the mode supported by most high-speed printers and drives attached to the parallel port.

• ECP (Enhanced Capabilities Port ) Designed for daisy-chaining different devices (such as printers and scanners ) to a single port. It uses an IRQ, an I/O port address, and a DMA channel, making it the most resource hungry of all the different parallel port modes.

• EPP/ECP Many recent systems support a combined EPP/ECP mode, making it possible to run devices preferring either mode on a single port.

These modes, which transmit data at up to 2 megabytes per second (MBps) and receive data at 500KBps, have all been incorporated into the IEEE-1284 parallel port standard. Most Pentium-based and newer systems have ports that comply with at least one of these standards. All of these require an IEEE-1284compliant parallel cable.

These modes are suitable for use with

• High-speed laser and inkjet printers

• External tape-backup drives, optical drives, and Zip drives

• Scanners

• Data-transfer programs such as Direct Cable Connection, Direct Parallel Connection, LapLink, Interlink, and others

Basically, the list includes all of the most recent printers and peripherals that plug into the parallel port.

caution

If you use a switchbox or device cable in place of a data transfer cable, or vice versa, your device or your data transfer process won't work. Fortunately, data transfer cables are usually thinner than switchbox or device cables because only a few of the wire pairs are required for data transfer. However, you might want to label the different types of cables to avoid mixing them up.

Types of Parallel Cables

There are three major types of parallel cables:

• Printer Uses the DB-25M connector on one end, and the Centronics connector on the other.

• Switchbox/Device Most use the DB-25M connector on both ends.

• Data transfer Uses the DB-25M connector at both ends and crosses the transmit and receive wires at one end (meaning that two computers can send and receive data, much like a computer network, though much slower).

Printer and switchbox/device cables can support the IEEE-1284 or earlier bidirectional standards. Here's how they differ internally:

tip

When purchasing or selecting parallel cables, IEEE-1284compatible cables can be used with any parallel port mode and provide superior signal quality. New cables are clearly marked as IEEE-1284 compliant on the package. Existing cables often have IEEE-1284 marked on the rubberized outer shield of the cable.

IEEE-1284 cables feature several types of shielding in both the cable and at the printer end of the cable. This shielding is designed to minimize interference from outside sources. Normal cables have minimal shielding.

IEEE-1284 cables use a twisted wire-pair construction internally, running 18 wire pairs to the printer. The wire pairs help minimize crosstalk (interference between different wires in the cable). Standard (compatible) cables don't use as many wire pairs, and bidirectional cables use less shielding. As a result, IEEE-1284 cables are both a good deal thicker and more expensive than ordinary or bidirectional printer cables.

Standard and Optional Parallel Port Settings

Parallel ports can be configured as LPT1, LPT2 , and LPT3. When a single parallel port is found in the system, regardless of its configuration, it is always designated as LPT1. The configurations for LPT2 and LPT3, shown in Table 8.2, apply when you have a computer with more than one parallel port.

Table 8.2. Typical Parallel Port Hardware Configuration Settings

LPT Port #	IRQ	I/O Port Address Range
LPT1	7	3BC-38Fh or 378-37Fh
LPT2	5	378-37Fh or 278-27Fh
LPT3	5	278-27Fh

If one of the ports is an ECP or EPP/ECP port, DMA 3 is normally used on most systems along with the IRQ and I/O port address ranges listed here. Some computers default to DMA 1 for an ECP or EPP/ECP parallel port, but DMA 1 will conflict with most sound cards running in Sound Blaster emulation mode.

To learn more about DMA conflicts with sounds cards, see "Default Sound Card Hardware Configuration," p. 288 .

Some 16-bit ISA or PCI-based multi-I/O cards can place the parallel port at any available IRQ up to 15. PCI parallel port or multi-I/O (parallel and serial ports and possibly others on the same card) cards can share IRQs with other PCI cards. However, ISA parallel ports, including those built into the motherboard, cannot share IRQs when used in EPP, ECP, or EPP/ECP mode. (These modes use an IRQ.)

How to Configure or Disable Parallel Ports

Depending on the location of the parallel port, there are several ways to configure the port settings. These include

• BIOS setup program for built-in ports

• Jumper blocks or DIP switch configuration for I/O cards in expansion slots

• Plug and Play ( PnP ) mode for use with Windows 9x/Me and Windows 2000/XP

Follow these steps to adjust the configuration of a parallel port built into the system's motherboard:

1. Start the BIOS setup program.

2. Change to the I/O device or peripheral configuration screen (see Figure 8.3).

   Figure 8.3. A typical BIOS I/O device configuration screen with the parallel port configured for EPP/ECP mode with default settings.

   

3. Select the mode, IRQ, I/O port address, and DMA channel if required.

4. Save changes and exit; the system reboots.

Follow these steps to adjust the configuration of an ISA parallel port card using jumper blocks or DIP switches:

1. Turn off the system's power.

2. Remove the parallel port or multi-I/O card.

3. Adjust the jumper blocks or DIP switches to select the mode, IRQ, I/O port address, and DMA channel if required. Check the card's documentation for details.

   

   To learn more about jumper blocks and DIP switches, see "Adjusting Speed and Multiplier Settings," p. 141 .

   
   

4. Reinsert the card into the expansion slot.

5. Restart the system.

PCI-based parallel ports are configured by the PnP (Plug and Play) BIOS; some ISA cards can also be configured by the PnP BIOS. In some cases, you can adjust the settings used by a PnP parallel port or a motherboard-based parallel port with the Windows Control Panel. See Chapter 19 for details.

Adding Additional Parallel Ports

Although you can daisy-chain a printer and another parallel-port device to a single parallel port, you can't connect two printers to the same port unless you use a switchbox. If you want to have two parallel printers that can be used at the same time, or if you want to provide different parallel port devices with their own ports, you need to add a parallel port. What are your options?

tip

A USB-to-parallel-port adapter has a USB Type A connection at one end and a Centronics connection at the other end. This adapter enables you to connect a parallel printer to your USB port so you can use the parallel port for other devices. However, this type of adapter isn't designed to support other types of parallel port devices: If you want to connect a parallel port drive or scanner, you must use a real parallel port.

You can add additional parallel ports to a system with any of the following:

• ISA-based parallel or multi-I/O card

• PCI-based parallel or multi-I/O card

• USB-to-parallel-port adapter

Install a PCI card in a PCI slot and an ISA card in an ISA slot. ISA cards that are not PnP should be manually configured to use available IRQ, I/O port address, and DMA channel resources. Use the Windows Device Manager to determine available resource settings (see Chapter 19 for details).