> there are 3 wires in the cable: one is the ground and the 2 other ones can > put high ( +5 V ) or low ( -5 V ). Or low ( 0 V ). Default state is high. It won't make sense to use -5 V with TTL logic, and, also it is not easy to generate that with four 1.5V batteries. Really, the calcs don't generate -5 V, they just pull the wires to the ground. Vojtech.