Hack 14 Configure Parallel Ports 
Set up your parallel ports in ways that
accommodate your peripherals and your other expansion
needs.
Although LPT ports are
seldom used for much of anything (even printers, since most now use
USB) these days, the occasion may arise when you need to use the port
to hook up an old external disk drive to recover some important files
or an old scanner to capture a document or photo, or to connect two
PCs with a special transfer cable to migrate data from an old PC to a
new one. Knowing the basics of your LPT ports will make these tasks
easier.
Before you can use an LPT port, you need to have one, so you need to
find out if you do. There are specific ways to tell if you have one
or more LPT ports on your system:
Look at the back of your PC to see if there is a connector with 25
female pin holes in two rows of 13 and 12, respectively, surrounded
by a trapezoidal or D-shaped metal shell. This connector is known as
a DB-25 female. Unfortunately, other types of interfaces use the same connector
style—typically older SCSI ports and, rarely seen in the
general PC population, connections for special test equipment. The presence of these connectors does not tell you specifically that
there are LPT port electronics wired between the connectors and your
system board—these may be fillers for the addition of an LPT
port later on. The presence of these connectors also does not tell you if they are
connected to an add-in card plugged into an ISA or PCI slot or
directly to LPT port electronics on the system board. Only a physical
inspection of the inside of your PC can tell you for sure if the
connectors go anyplace and where they go. Look into the BIOS setup program for references to parallel ports in
the I/O port menu sections. If setup refers to LPT ports and the
system contains the 25-pin external connectors, chances are you do
have LPT ports. It is possible your BIOS could refer to LPT ports although you have
no physical connectors for them, indicating that you are missing some
cables to connect to the system board or the manufacturer never
intended the ports to be used. If you have the connectors but do not see any references to LPT ports
in your BIOS, then it's likely the LPT ports are
provided by an add-in card or the connectors are not parallel/LPT
ports at all but have some other use. In Windows, go to the Device Manager (My ComputerProperties Hardware)
to see if any LPT ports are present. This will not tell you how the
LPT ports are provided—by system board or add-in card—but
will tell you if a port exists and give information about its
configuration. Use a system information or diagnostic program like
SiSoft's Sandra- (http://www.sisoftware.net), Windows Device
Manager [Hack #12], or a similar
program to detect and reveal the port information. These programs
will not tell you how the LPT ports are provided, by system board or
add-in card, but will tell you if a port exists and information about
its configuration.
If you've determined that you have LPT port
connectors and the ports are configured in the BIOS,
you'll want to set their configuration to known
values so that other Plug and Play devices don't try
to use their resources later on. The proper address and IRQ settings
for LPT ports are listed in Table 2-2.
Table 2-2. LPT Port address and IRQ assignments|
Port number
|
Address
|
IRQ
|
|---|
|
LPT 1
|
378
|
7
| |
LPT 2
|
278
|
5
|
Figure 2-6 shows the BIOS screen for typical Plug
and Play LPT ports. Auto is not the setting you want if you are
concerned about establishing and maintaining a proper, known PC
configuration.
To avoid the unknown and potential confusion in the future you should
manually set the Parallel Port configuration to known values rather
than Auto mode. Figure 2-7 shows a manually
configured parallel port using default LPT1 values—address 378
and IRQ7—plus two other settings of note—Mode and DMA.
On any given system the parallel port may be capable of any of four
modes of operation: the original
Standard or Output-Only mode,
Bi-Directional, Enhanced Parallel
Port (EPP), or Enhanced Capability Port
(ECP). Of these, Standard, Bi-Directional, EPP, and ECP are typically
the only ones available as normal configurations for BIOS. The
definitions and significance of each of these modes are as follows:
- Standard or Output-Only
-
Just as indicated—the port is normally used to send data out to
a printer and get no data back.
- Bi-Directional
-
The port is capable of reading data back from a printer or other
device on demand from a program intended to do so. This (or its
variants, EPP and ECP) is the most common mode in use today.
- EPP
-
A faster variant of Bi-Directional mode with some other device status
capabilities—rarely, if ever, used. ECP fixes some issues with
EPP mode and adds much higher data rates and the ability to do DMA
data transfers.
- ECP
-
An upgrade to EPP. Made the LPT port the cheapest and fastest I/O
port available for such things as page scanners and external storage
devices before USB hit the market.
 |
With the availability of USB and IEEE-1394 (FireWire), ECP mode is
not used much if at all. Most parallel printers, even those with
programs that tell you ink or toner status, use Bi-Directional mode
to free up the IRQ and DMA lines that would be used by ECP mode.
|
|
The instructions for the device you want to connect to your
LPT port should indicate which mode the
LPT port is required to be in for the device to work properly. Some
devices do not work with EPP or ECP ports.
|
