AVR-Chat

Hi James
 
I have a suggestion for you not a solution to your particular problem.
 
I've used RS485 for a long time and I've found the (more expensive) auto
bias self terminating IC's from maxim to work perfectly where others
failed!
 
/Regards
Lasse M.
 
-----Original Message-----
From: James Russo [mailto:jr@halo3.net] 
Sent: 1. oktober 2004 16:03
To: AVR-Chat@yahoogroups.com
Subject: [AVR-Chat] RS485 w/ MAX481, terminators not needed?
 
Hello Everyone,

    I am working on getting two MAVRIC-II (Mega128) boards talking using

RS485 (MAX481 to uart1). I am using a short segment of  (<5feet) of 3 
twisted wires (A,B,Gnd). My problem is that with the terminators 
installed on either node, my data output from the RS481 is not high when

the there is no driver active on the bus. This causes the uart to 
constantly see a start bit, and then return a 0 after every byte 
received. However, if I remove the terminators on both everything seems 
to work as expected.

According to the spec sheet for the 481, the DO pin should be high when 
the "inputs are open"? I have a feeling it has to do with the very short

RS485 run with just two nodes on it and the involved resistances, but 
hopefully someone else has more experience and can give me a pointer.

Thanks,

-james




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