A parallel port is a 25-pin connector found on the back of older computers, designed to send data eight bits at a time across eight separate wires. For roughly two decades, it was the standard way to connect a printer to a PC. The port was a fixture on nearly every personal computer from the early 1980s through the early 2000s, when USB replaced it.
How a Parallel Port Sends Data
The key idea behind a parallel port is right in the name: it transmits multiple bits of data in parallel, meaning side by side at the same time. Eight data wires each carry one bit simultaneously, so a full byte travels from computer to printer in a single pulse. This is the opposite of a serial port, which sends bits one after another down a single wire.
Beyond the eight data lines, the connector includes four control lines and five status lines. The control lines let the computer send instructions to the attached device, like telling a printer to start reading data or to advance paper. The status lines work in the other direction, letting the printer report back whether it’s busy, out of paper, or ready for the next chunk of data. The remaining eight pins are ground wires that complete the electrical circuit.
The Physical Connector
On the computer side, the parallel port uses a DB-25 connector, a rectangular socket with 25 pins arranged in two rows. It looks nearly identical to a serial port connector (which also has 25 pins in the same layout), which caused plenty of confusion. On the printer side, the cable typically ended in a wider 36-pin Centronics connector, a bulky plug with wire clips on each side. You’ll sometimes hear the whole interface called a “Centronics interface” for this reason.
PCs could support up to three parallel ports, labeled LPT1, LPT2, and LPT3. In practice, most computers shipped with just one. The recommended maximum cable length was 15 feet. Longer cables existed (up to 50 feet), but signal quality degraded enough to cause unreliable connections and data errors at those distances.
Speed and Operating Modes
The original parallel port, sometimes called Standard Parallel Port (SPP) or Compatibility Mode, was designed strictly for one-way communication: the computer sent data out, and the printer could only send back basic status signals. It was fast enough for text documents but slow by modern standards.
As demands grew, the IEEE 1284 standard defined five operating modes to squeeze more performance and flexibility out of the same 25-pin connector:
- Compatibility Mode (the original one-way design)
- Nibble Mode (allowed limited data to flow back to the computer, four bits at a time)
- Byte Mode (full eight-bit data flow in both directions)
- EPP (Enhanced Parallel Port) (faster two-way transfers, useful for devices like external drives)
- ECP (Extended Capabilities Port) (the fastest mode, with built-in data compression)
At their best, EPP and ECP modes could reach roughly 3 megabytes per second. That was respectable for the 1990s, but for comparison, USB 2.0 tops out at 480 megabits per second (about 60 megabytes per second), roughly 20 times faster.
What People Connected to Parallel Ports
Printers were the primary use case, and that association was so strong that the port was commonly called the “printer port.” But as EPP and ECP modes enabled two-way communication, manufacturers started using the parallel port for all kinds of peripherals. External Zip drives, scanners, early external CD-ROM drives, and even some early digital cameras connected through the parallel port. It also found a niche as a simple hardware interface for hobbyists and industrial equipment, since the eight data lines and control signals could be directly read and written by software, making it easy to control custom electronics.
Why USB Replaced It
The parallel port had several practical drawbacks that USB solved. First, it wasn’t hot-swappable. You had to shut down the computer before plugging in or unplugging a device, or risk damaging the port or the device. USB lets you plug and unplug freely while the system is running.
Second, parallel ports lacked true plug-and-play support. Connecting a new device often meant manually installing drivers, setting IRQ values, and configuring the port mode in the BIOS. USB handles device recognition and driver loading automatically in most cases.
Third, the parallel port was physically large. The DB-25 connector took up significant real estate on the back of a computer, which became a problem as laptops shrank and desktops moved toward slimmer designs. A USB port is a fraction of the size and supports far more device types through a single standard connector.
Finally, speed wasn’t competitive. Even the fastest parallel mode topped out around 3 megabytes per second, while USB 2.0 offered roughly 20 times that bandwidth. As peripherals demanded faster data transfer, the parallel port simply couldn’t keep up.
Parallel Ports Today
Most modern motherboards have dropped the parallel port entirely. Some still include a serial port header on the circuit board, but the parallel header has largely disappeared from consumer hardware. Small form-factor motherboards (Mini-ITX) occasionally retain a parallel header because of demand from point-of-sale systems and industrial applications that still rely on legacy equipment.
If you need a parallel port on a modern computer, PCIe add-in cards and USB-to-parallel adapters are widely available. The adapters work well for basic printing but can be unreliable for more specialized uses, like controlling industrial hardware or communicating with older scanners, because they don’t replicate the exact timing and signal behavior of a native port. A PCIe card that provides a true parallel port is the more dependable option for anything beyond simple printing.