I just got my solid state relay today, I have not hooked it up yet though.
The solid state relay can be turned on or off by the voltage provided by your parallel port. Some boards like the xylotex have a separate plug that just connects directly to the parallel port. When your toolpath software is configured correctly, it can send a signal through the parallel port and into your relay, turning it on or off. You hook the other side up to your spindle and away you go.
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