parapin makes it easy to write C code under Linux that controls individual pins on a PC parallel port. This kind of control is very useful for electronics projects that use the PC's parallel port as a generic digital I/O interface. Parapin goes to great lengths to insulate the programmer from the somewhat complex parallel port programming interface provided by the PC hardware, making it easy to use the parallel port for digital I/O. By the same token, this abstraction also makes Parapin less useful in applications that need to actually use the parallel port as a parallel port (e.g., for talking to a printer).
Parapin has two ``personalities'': it can either be used as a user-space C library, or linked as part of a Linux kernel module. The user and kernel personalities were both written with efficiency in mind, so that Parapin can be used in time-sensitive applications. Using Parapin should be very nearly as fast as writing directly to the parallel port registers manually.
Parapin provides a simple interface that lets programs use pins of the PC parallel port as digital inputs or outputs. Using this interface, it is easy to assert high or low TTL logic values on output pins or poll the state of input pins. Some pins are bidirectional--that is, they can be switched between input and output modes on the fly.
Parapin was written by Jeremy Elson (jelson@circlemud.org) while at the University of Southern California's Information Sciences Institute. This work was supported by DARPA under grant No. DABT63-99-1-0011 as part of the SCADDS project, and was also made possible in part due to support from Cisco Systems. It is freely available under the GNU Public License (GPL). Up-to-date information about Parapin, including the latest version of the software, can be found at the Parapin Home Page.
Warning:
Attaching custom electronics to your PC using the parallel port as a digital I/O interface can damage both the PC and the electronics if you make a mistake. If you're using high voltage electronics, a mistake can also cause serious personal injury. Be careful.
If possible, use a parallel port that is on an ISA card, instead of one integrated onto the motherboard, to minimize the expense of replacing a parallel port controller that you destroy.
USE THIS SOFTWARE AT YOUR OWN RISK.