In SX Microcontrollers, SX/B Compiler and SX-Key Tool, dkemppai wrote:

[quote="metron9"]
I have a question about RS485 voltage levels though, Can the same benifits of in phase noise be obtained using +5 and Ground levels on a chip to chip communication as an alternitive to just serial when longer lines connecting devices becomes a problem. I am still not sure in my mind if say +10 and Ground would be escentially the same operation as +5 and -5. I have a hunch though that with a -5V signal it can be regulated the same as the +5 side so you know you have a 10V potential difference. After clicking on the bottom link and reading the authors tutorial on DMX systems that he builds I realize this is the first time I have read anything about RS485. There should be 32 hours in a day so after work I have more time to learn about all this stuff and get some sleep.

[/quote]RS485 is a differential signaling. It means that there is a + and - signal. 
The minus line does exactly the oppostie of what the positive line does. 

When the +signal is 5V volts, the -signal is 0V. When the +signal is 0Volts, the

-signal is 5 volts. The reciever looks at the DIFFERENCE between the two

lines. The reason this is done, is that if you induce noise on the wire between

your RS485 devices, both the +signal and -signal get the same noisevoltage,

and the difference remains the same (This is key to noise immunity) 

Since the reciever looks at the difference only, the noise is ignored.

This is what gives RS485 immunity to noise that RS232 doesn't have.  

 

Note that you are talking about RS485. There is also an RS422 specification.

The RS485 and RS422 use the same signaling (Voltage levels). However, One

is specified as unidirectional, and the other is bidirectional. RS422 has a TX pair

and RX pair of wires (5 wires total). On RS485 line (3 wires total), data flows

in both directions on the same wire. In reality it is a little more complex 

than that...  ...but this should give you the rough idea. 

 

Check out: [url=http://www.national.com/an/AN/AN-216.pdf]http://www.national.com/an/AN/AN-216.pdf[/url]

National semiconductor also has some more info on the subject, but you will

have to search for it...

 

Hope this helps. 

-Dan

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