In the original design of Indexer LPT, the reason we allocated the pins the way we did was to make each set of signals associated with an axis of motion symetrical, so each axis could be used as every other axis.
Also, we decided to use only internally pulled up signals for the limit switch inputs for stability, and both high and low inputs for each axis - for simplicity and to account for conditions such as when a machine is powered up when an axis may be resting against one of the switches - which could be problematic for shared limit inputs.
So - given the number of inputs available, we decided to limit the number of axis of motion per port to two. However, if you wanted to devote a card to raw inputs (and not motion), Indexer LPT can scan up to 48 switches!
This allocation of resources has served us well since the product was introduced in 1989, and we haven't changed it since.
Parallel ports are inexpensive and rugged - so we felt that adding a port or two would not burden the machine designer, nor appreciably increase the cost - it even helps keep the wiring neat and clean. With the new PCi port addressing, Indexer LPT supports up to four ports under Windows 98/98/Me and XP, allowing for seven simultaneous axes of control, since the resources for one "axis" is given up to implement the feed rate override feature.  |