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Rs485 problems


I have an appliction running on the Axis 82 board which reads data from 
data loggers on a 4 wire Rs485 bus. In the lab with short cables the 
application works fine using the built in 485 connector (ttyS3).

In the field site we have 2 loggers at either end of a 500 - 600 metre 
cable with the Axis 82 about 70 metres from one end. Both ends of the 
cable are terminated with 120 ohms. In this situation the application 
can only (sometimes) see the nearer logger when using ttyS3.

If we attach an Amplicon RS232 - RS45 converter on to one of the other 
com ports on the Axis 82, the application can see both loggers and works 

Do you know of any reason why this should be so?
What is the 'range' of the 485 transceivers that you use?
Is there any termnination for the 485 bus (ttyS3) on the Axis 82?
Is there a kenrel config setting to solve my proble? :)

Thanks for your time


Bob Hibberdine