Two Angle encoder connected to 6602; one signal is drifting

[chino1001]

Hello,

I have two encoders connected to a 6602 counter card. The signal connected to channel two is always drifting, regardless of which of the two encoder is connected to it. The encoders that I have are made by Renishaw, the hardware consists of:

 - Two encoder rings mounted on a common rotary axis (18,000 counts on the ring, interpolated 500 times by Renishaw electronics to 9,000,000 effective counts)

 - Each encoder ring has two read head that are offset 180* apart from each other

 - Each of the two read heads connects to a Renishaw DSi dual read head encoder system which converts the two readhead reading, from a common ring, to a differential A quad B signal (A+, A-, B+, B-)

By them selves, each encoder system is stable, it is when I try to use two of them at once, that there is drift, and even miss match between the two under slow displacement to another stopped position.

I'm lost on what to do next. The drift is always in one direction. In a 15 hour period I have observed about 1 degree drift in channel two.

thanks in advance,

Vincent

[Kevin_Price]

Do you have any conditioning electronics to convert your differential signals into TTL?

My memory's a bit fuzzy, but I *think* I've seen such a single-direction drift before when a differential encoder was wired directly to the counter inputs without converting to TTL.  Further, I *think* the troubleshooting showed that the encoder's negative (-) differential outputs wanted to drift relative to one another.  So when both were hooked to the daq board's common digital ground, problems ensued.

-Kevin P