Many laser printers last for years, so businesses often keep older laser printers around even after upgrading other hardware. Many older laser printers use a parallel port interface rather than the newer USB type, but many newer computers do not include the parallel ports needed to connect to the printers. Likewise, many older computers did not ship with the ports needed for modern USB printers. Nevertheless, you can use USB to printer port adapters to connect newer computers to older printers and vice versa.
You can use a USB-to-parallel adapter cable to connect most parallel printers to computers that have USB ports but not a Centronics-type 36-pin adapter port. USB-to-parallel adapter cables come in two primary varieties: active and passive. Active adapters are compatible with virtually all parallel printers, and contain circuitry in the cable that draws additional power from the USB port. The active circuitry also helps to increase port speed for the connection, and enables bi-directional data flow between the computer and printer — this is important for printing to many laser printers. Conversely, passive USB-to-parallel adapter cables have no active circuitry, and are compatible with far fewer parallel printers. Passive adapters simply pass data from the computer to the printer, and offer no error correction or bi-directional features. Because of this, passive adapter cables are generally only suitable for older dot matrix printers.
Limitations of USB-to-Parallel Adapters
Active USB-to-parallel adapter cables generally enable most printing features supported by Printer Command Language, such as PCL 4, PCL 5, PCL 5e, PCL 5c and PCL 6. HP designed the PCL protocol for their early inkjet printers, but later adopted the language for use with their successful line of laser printers. PCL is the de facto industry standard for printer protocols, and many printer manufacturers pattern their drivers and printer commands on the HP language. However, some advanced printing features, such as duplex printing or reverse sequence printing, might not function properly on some printers when using a USB-to-parallel adapter cable. Additionally, if the parallel printer is an all-in-one type with an integrated scanner and fax, these devices may work intermittently or not at all with an adapter cable. For all-in-one printers or high-end laser printers that support collating, sorting, duplexing or other advanced printers, installing a parallel port add-on card in the computer provides better results in most cases.
Troubleshooting USB-to-Parallel Adapters
If you have trouble printing to a printer connected to the computer with a passive adapter cable, the easiest fix is to simply the replace the adapter with a cable with active circuitry. You can usually differentiate an active adapter cable from a passive one by looking for a circular or oval housing in the cable near the USB connector. If the adapter has the housing integrated in the cable, it is probably an active adapter cable; if it does not and is a straight cable length from end to end, it’s passive. Printing problems can also arise if you connect the adapter cable to a front-panel USB connector, rather than one on the rear of the computer. Rear-panel USB ports provide a more stable current than the ones on the front of the computer case. If you still have problems after changing the adapter cable to an active type and plugging the USB connector in a port on the rear of the PC, ensure that you are using the correct Windows driver for the printer.
In some rare cases, businesses might find it necessary to connect a USB printer to an older computer with only a parallel printer interface. For these types of scenarios, parallel-to-USB adapters do exist, but they are relatively expensive and don’t work with all applications or support advanced printing features in many cases. Parallel-to-USB adapter work best with older computers that run DOS applications that don’t use the Windows Print Spooler to process print jobs. Parallel-to-USB adapters usually require an external power supply to function properly, and do not support high-speed USB connections. You can save money and increase compatibility with more types of USB printers by installing a simple USB add-on card if the computer has an open PCI slot.