EM-A1 is a small auxiliary PCB. It is designed to change control mode from unipolar to symmetric plus/minus control. This card makes it possible to use various Electromen driver cards with bipolar control.Card is only to be used with Electromen products which include the header socket for this EM-A1 option card.Also a symmetric potentiometer control is possible with EM-A1, as EM-A1 can supply negative voltage ( -5V ) for potentiometer. The negative voltage is usually made available from direction input terminal of the base card (option card EM-A1 plugged). All though EM-A1 option card uses +-5V as full control scale, the different base cards can have possibilities for even wider control input ranges like +-10V or even +-30V. The base cards input scale adjustment trimmer (attenuation) can then be used for adjusting the control scale to be suitable for the application.