1) your micro does not capture very fast outputs from the QD

The quadrature decoder output is 32 bits wide and is read as 4 bytes one after the other. The decoder presents this output on its data bus and latch it on data bus. The micro reads it directly through 8 port pins. The byte to be read [ ie. one among 4 ] is selected by SEL1 and SEL2 signals generated by micro. When OE is pulled low the selected byte is latched on encoder data bus. Micro reads the bytes one by one by first making proper byte selection using SEL1 and SEL2 signals and then making OE low, thus there is no question of very fast or very slow output from quadrature decode.
your answer does not answer my question, so I will rephrase
1) if the QD counts at 100kHz+ (possible at oscillations) will your micro offload the QD fast enough?

2) you are using a QD with single ended output or are treating a differential output as single ended.

fed to AM26LS32 to make it single ended.
Interesting concept, I never thought of using a 485 receiver to receive anything but output from a 485 transmitter.

3) you have some relays or such on the board that generate enough noise to upset the readings.

And if at all there would have been noise problem it wouldnt show effect only while stopping the motor.
There actually is a unique "activity" when stopping the motot: breaking the power to it.


In my case if i run motor continously then decoder is giving very accurate reading of shaft position. But every time i stop it there is an error produced which is cummulative and goes on increasing every time i stop the motor.
Are you, in effect, saying that the error always is too high a count?

4) you have a defective component.
I too thought the same initially and changed motor encoder and control board but still problem exists.
referring to nordic mythology: did you forget the mistletoe?

5) just a thought: If one or both of the differential lines give reverse output (a and b reversed) what would happen? My immediate thought is nothing, but maybe if you think about it there is a problem.

Erik

nordic mythology, mistletoe:
The story is that all trees and plants were asked by the gods not to be the arrow that killed Balder and they forgot to ask the mistletoe.

So very often you check "everything" and days later find the stupid little thing you forgot such as e.g. a cable.

THUS remember the mistletoe