I'd say that an absolute encoder gives you a binary (or Gray code) output indicating the absolute position at that point in time. An incremental encoder just indicates how far it moved and the direction, but does not indicate what the current position is. This is typically done with quadrature encoding (two streams of pulses. Which pulse has the edge first indicates direction, while you count the pulses to indicate how far you've moved).

Years and years ago I designed a Z80 system to steer a "Big Ugly Dish" for TV stations. This was written in Turbo Pascal and burned to ROM. Our software would let the user (through a dumb terminal) schedule the recording of different programs off different satellites. After recording a program, the system would steer the dish and tune the receiver to pick up the next program. The dish had a pretty big motor that would move the dish. There was a "flag" on the back of the motor with an opto interrupter. We'd count pulses out of the opto interrupter to see how far the dish moved. We assumed the direction was the direction we had TOLD the dish to move. We also had to account for overrun of the dish (it didn't stop immediately when you turned the motor off). So, we had to turn off the motor a bit before the proper position, considering which direction we were coming from. Originally, we thought we'd run the dish all the way to one end where it hit the limit switch to sync our counts with the dish position. Turned
out, though, that this was not very accurate. So, instead, after moving the dish, we'd store the current count in battery backed RAM (a TimeKeeper RAM that also had our real time clock/calendar). After a power interruption, we'd assume the dish had not moved while the power was out.

     So, summarizing, THAT was an incremental encoder...

Harold



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