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RE: Rs485 problems



Hi,

Except from termination the RS-485 network may also require biasing.
If you meassure the signal between the terminals with an oscilloscope
you may see that the default level of the signals are very close to
their minimum value which is bad. In this case the signals need
biasing through pull-up and pull-down resistors.

If I remember correctly it works fine with a couple of kohm. Tell
your hardware engineer to test with biasing to get the right
differential "look".

/Mikael

-----Original Message-----
From: owner-dev-etrax@xxxxxxx.com">mailto:owner-dev-etrax@xxxxxxx.com] On Behalf Of Robert Hibberdine
Sent: Friday, August 01, 2003 4:44 PM
To: dev-etrax
Subject: Re: Rs485 problems




Jarkko Tuomi wrote:
> Robert Hibberdine wrote:
> 
> 
>>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.
> 
> 
> Try connecting the external RS485 converter to the bus to see if the 
> problem is with transmission or reception
Can't do this for another week!

> Did you have the termination  resistors connected when you tested
 > with a short cable?
Yes. Actually, it worked both with and without.


> Have you checked the grounding, i.e. that there's no large common mode
 > voltage on the bus?
I'm just a software engineer! The hardware engineer says the grouding is 
fine. :)

> What kind of cabling and transmission rate you are using?
4 core screened cable (Belden 8273 if that means anything to you) and 
9600 baud.

Can't help thinking that if it works using the Amplcon Rs232 - Rs485 converter it ought to work using the Axis 82's ttyS3.


> Good luck,
Thanks!

> 
> Jarkko
> 
> 
> 
>