Parallel Port Interface

The parallel port is an ideal interface for controlling telescopes, particularly with laptops in the field. The parallel port uses 8 bits of output, typically at port address 0x378. On the 25 pin connector, the 8 bits of output are on pins 2 through 9, from least significant to the most significant bit. These 8 bits of output are perfect for controlling the two stepper motors needed to drive a telescope in altazimuth mode. The parallel port cannot provide or sink large currents directly, hence, 74LS04 hex inverters are used to interface between the parallel port and the driver transistors.

The parallel port has 5 bits of input at port address 0x379. These input bits are on pins 15, 13, 12, 10, and 11 of the 25 pin connector, with pins 11 and 15 being inverted. Pin 15 activates bit 8, pin 13 activates bit 16, pin 12 activates bit 32, pin 10 activates bit 64 and pin 11 activates bit 128. Depending on which lines are raised high, the values can range from 8 through 248. In addition, the parallel port has 4 bits that can be either in or out at port address 0x37A. These 4 bits show up on pins 1, 14, 16 and 17 of the 25 pin connector. Bits 1,14 and 17 are inverted. These values when read range from 0 to 15.