Rx is the input. It should be connected to KStep's AnalogOut. 5V should be connected to KStep's AnalogVcc.Apologies, what do you mean the 5V and Rx are wrong, how should they be?
This isn't clear. That's why it is better to use a diagram. The DC current would be very high (28Amps?) and more than the relay's rating. You should probably switch the 120VAC to the supply instead.I have the DC power connected through the relay to and then the relay to the motor controller. The spindle uses a 7.5V DC motor supply.
Please update the diagram.