Abstract: The signals of the ion source of 400 kV ion implanter are communicated between the ground potential and the high one through the fiber optic cables at frequency mode to seclude the high voltage. The fiber optic cable transmission system was updated because of its instability for the long time usage. In stead of the RC4152 chip, the AD650 chip was used to make the V-F (voltage-to-frequency) and the F-V conversion because of its simple outside circuit. The fiber optic cable of 1 mm diameter was used with T1521 and R2521 chips as its transmitter and receiver. The voltage followers were used in both input side and output side to enhance their impedance matching. The updated system has good linearity between V_IN and V_OUT, and it works well and stably.