RE: [CAD_CAM_EDM_DRO] Major encoder trouble, need advice.

You say: "USDigital recomends the 26C32 receiver while the HP-UHU has the
SN75115... big difference?"

Yes there is quite a difference, the 26C32 is an RS422 receiver, which has
200mV hysteresis, whereas the 75115 has something like 50mV to 150mV
hysteresis depending on operating temp. However it should work if the noise
is less than this.

Note: For RS422 drivers and receivers, the termination resistor should be
across the inputs, not in series. (There should also be one across the
output driving the line.)

The 75115 has an optional internal 130 ohm termination resistor (one end is
connected to the +ve input, the other end comes out on a pin, and can be
connected to the -ve input. You should be able to check with a meter to see
if it is connected. (If it is not connected, you need to add the external
termination resistor.)

In summary: Assuming it is supposed to be RS422, the twisted pair line must
be terminated at each end.

Use a meter to check for 120 ohm (or so) across the encoder output. (US
Digital should have one there)

Use a meter to check for 120 ohm (the internal resistor can be between 80
ohm and 180 ohm) across the HP-UHU input.

Bruce



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