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 11
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.
for more on the PC parallel port
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continue onto 'The Software'
...by Mel Bartels