how many legs does the detector have? Perhaps it actually has two IR receivers in it. You only need one Emitter, but I would think you need two receivers. If you can imagine a fixed light source behind a spoked wheel, as the wheel turns the shadow of the spoke moves passed the detectors (which are side-by-side in a single plastic mould) in the opposite direction of the wheel's travel. Theis would output grey-code, and when you get a change in direction, the phases of output reverses (thus telling the 8051 that the wheel has changed direction). Even if it changes half-way through a "spoke", the phase of the code has still changed, so you don't get any errors in reading the output. It is then up to your code as to whether or not you include the half-count and "round it up" to a full count.