Problem using Arduino UNO to drive card access controller data input lines

Trying to interface an Arduino UNO to a DSC 4820 card access controller. With the UNO disconnected, the controller and UNO D0 and D1 lines measure 5 VDC relative to their GND. After I connect the UNO output pins representing D0 and D1 to the controller, I try to write a 0 to the controller using the UNO command digitalWrite(VX_D0, LOW) followed by digitalWrite(VX_D0, HIGH). At the controller, in response to the low write I measure 2.5 VDC from D0 to GND and 7.5 VDC from D1 to GND. in response to the high write I measure 5.0 VDC for both DO and D1 to GND. When trying to write a 1 to the controller the same actions follow for the digitalWrite(VX_D1, LOW) followed by digitalWrite(VX_D1, HIGH). At the controller, in response to the low write I measure 2.5 VDC from D1 to GND and 7.5 VDC from D0 to GND. in response to the high write I measure 5.0 VDC for both DO and D1 to GND. I have used digital pins 7,8 as well as analog pins 5,6 using the analogWrite(VX_D1,0) and analogWrite(VX_D1,255) sequence in attempt to lower and raise the D0 and D1 voltage levels from 0 VDC to 5 VDC, but the results were the same as with the digital writes. If I issue the write commands with the UNO disconnected, At the UNO, in response to the low write I measure 0.0 VDC from D0 to GND and 0.0 VDC from D1 to GND. in response to the high write I measure 5.0 VDC for both DO and D1 to GND. Obviously, I want the voltages to behave as measured with the UNO disconnected. Suggestions on how to correct the incorrect response when the UNO is connected would be greatly appreciated.