Hi Bruce,
You're right. I have tried to find informations at its website - it is horrible! Datasheet is placed on cd, but here you can find only wiring and dimensions. No electrical characteristics, no maximum values of parameters, really "secret device"...
Encoder has serial output. I suppose J.Guy is right, it can be problem with clock signal. Encoder outputs only one bit, probably first. It is never changing, because built-in controller doesn't receive the clock signal. But why?
There is small drawing placed on the fax sheet I have got from Siemens. I can see 13 clock pulses and below data line which outputs first msb, then lsb. After data bits I can see "L" state coresponding to last rising clock pulse voltage (after 12..15us). It is stop bit, I think. There aren't any informations about special commands like "reset", "send position" or other. I think it should work sending data to the 8052's ordinary SPI unit.

Regards,
Jacek