Centronics Data Computer Corporation developed the interface in the mid-1960s to be an 8-bit unidirectional parallel host-to-printer connection.The interface became widely used; however, no industry-standard specification was developed to define the interface.A setup utility specifically for the system must be used to select bidirectional operation.The Register map for an IBM PS/2 parallel port is shown in Table 3.
During Type 2 or 3 DMA reads, a pulse on the ACK line generates a DMA request and initiates the transfer to system memory.
An extended mode (Type 1) parallel port has only the first three registers.
These registers are identical to the SPP register set with an additional Direction bit in the parallel port control register.
The last three registers in Table 3 are only present in Type 2 and 3 parallel ports.
Type 2 and 3 DMA transfers follow the SPP timing as described earlier.