Though not a problem on computers supplied by Sherline for their CNC systems, some customers have had problems when using the Sherline 8760 driver box on computers they have supplied themselves. It has been determined that the output of some parallel ports is marginal and can cause stepper motors to run in only one direction or suddenly change directions in the middle of an operation. This small signal amplifier plugs between the computer and the parallel cable to the driver box. Power is supplied via a short USB cable that plugs into an unused USB port in the host computer. If your CNC system has developed this problem, a signal booster is usually a more economical solution than trying new computers until you find one that works.