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I am trying to control a HMC-C018 serial control attenuator with a general GPI/O-Box (open or closed contacts).

For some reason it seems like the last bit clocked in on the serial input port is distributed to all the flip flops inside. The result is that I get either min og max attenuation. Nothing in between.

I wonder i there can be something wrong with my biasing of the attenuator, since I haven't found a documentation on how to do the biasing of this cirquit properly? Maybe ripple on the shift clock signal?

I would very much appreciate a diagram for proper biasing or any other advice!