Standard parallel port (SPP or AT)

Standard parallel ports send 8 bits of data in parallel mode, but receive data only 4 bits at a time.

Bi-directional (PS/2 or BPP)

Bi-directional parallel ports send and receive data 8 bits at a time.

Enhanced parallel port (EPP)

Many EPP ports can handle PS/2 bi-directional communications.

Extended capability port (ECP)

ECP can handle PS/2 bi-directional communications, but must be put in this mode by software. Several BIOS setup programs offer the ability to perform this task. You can also run a software application to select the bi-directional mode of an ECP port by setting the most significant bits (bits 7, 6, and 5) of (LPT1 PORT ADDRESS + 0x402) to 001. Relying on a software solution could cause problems if you forget to run the application or if other software changes the port mode. Therefore, if your parallel port supports only ECP, you should install a separate parallel port ISA card.

ECP+EPP

A standard ECP port can be put in a mode to emulate a bi-directional PS/2 port, but an EPP port cannot. An ECP+EPP port can be put into ad additional mode to emulate EPP. ECP+EPP ports have the same configuration challenge as described for ECP ports.