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Question about the wire for RS485 and calculating its impedance

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ktec1

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hi
i have studied about rs485
but i have a question about the wire i would use
is there any good suggestion about kind?(optimum point)
now estimate i use normal wire that are used to connect modem to telephone line
or the same telephone to telephone line too
how can i caculate the impedance of the wire in realation with the cable length
and (probbly frquency of data transmission?) for choosing a good resistor
at the end of wire to prevent distortion in pulses shapes
thanks
 

Re: help on rs485

Hi,

Since the RS485 bus has differential signals, you can use twisted pair cable to reduce the noise. Shielded twisted pair/foiled twisted pair cable can be used for better noise immunity.

Regards,
Joby
 

help on rs485

thanks joby
how can i get the impedance of the wire
thanks
 

help on rs485

dear joby16
thanks for your help
although it will take me time to study these and
apply them but those are what i want
in any unexpected problem i hope you
help again
 

Re: help on rs485

If you want off the shelf cable, probably the best choise would be ethernet cable. You can obtain them cheap, they are available in shielded version or unshielded one. 100Ω impedance.
 

help on rs485

sinisa you have said the impedance is fixed =100
but i have two questions
1) what is the defference betwen 100 meters or 1000 meters of the ethernet cable in the impedance
2) can i change the ethernet cable with phone cable
thanks for your help
 

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