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FODtrack interface 7528 version?

I'm experiencing instability with my FODtrack interface card. It acts like
it frequently does not read a bit on the input port correctly. This is an
intermittent problem - send a command in FODTrack calibrate mode once, and
the G5500 rotor goes the wrong direction (usually to approx 020 deg.). Send
a second calibrate command with the same values and the rotor steers

When attempting to track a satellite using FODtrack I get similar results -
usually aims correctly, then a wild change in azimuth, usually to the
northeast. Elevation responses are stable. I've rewired the parallel port
connections and resoldered the circuit board pads, but no change. It acts
like the D/A is missing one of the mid bits occasionally. The results are
the same whether I'm running in DOS or Win98. PC is an old Toshiba laptop,
100 MHz, Pentium I.

I'm wondering if a) the 7528 chip is bad, or b) I bought the wrong flavor
7528. I'm using a TLC7528CN (digikey part # 296-1871-5-ND, page 276 in
catalog Q013).

Any suggestions?

Thanks (again),

Ken, W7KKE

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