Ron,

Thanks for the insight.

I used a B&B electronics RS 232 to RS 485 converter to connect the 4490 to the PC.

According to Aerocomm there is no need for a terminating resistor, but I have tried a 100 ohm resistor and it made no difference.

At this time I believe that the S75176 converters and the 4490 are just not compatable. I will look into the Max converter that you mentioned.

Thanks
Greg