RS-485 Standard confuse

[boogie]

Hello ,
I wanna ask about RS-485 Standar. i am rather confused about electrical characteristic of RS-485 signal. The RS-485 spesification defines a +1.5 V to +6V signal represents binary 0, and a -1.5V to -6V is used for binary 0. for receiver, a +200mV to +6V signal is interpreted by the receiver as a binary 0, and a -200 mV to -6V signal as a binary 1. But when i use SN75176B RS-485 line driver from Texas Instrument, the Receiver output will High if the differential input +200mV and will Low if the differential input -200 mV. It makes me confuse because its opposite with standard tell.
I hope you can give me an suggestion to solve my problem

Thanx in advanced

 

[IanP]

As the RS-485 is a differential standard the minimum potential difference between outputs is 1.5V or (-)1.5V and the maximum +/-6V. In most cases it is not related to 0V or GND; it is as I said difference in potential between the outputs, for example, if this is the 75176 which is supplied of 5V A output can be 1V and B output can be 4V, then the difference is (-)4V, what is within the RS-485 standard, but as you can see there is no negative voltage in the circuit.
As there is always voltage drop in the transmission cables the receiver has to more sensitive then the minimum transmitting differential voltage. For the RS-485 it is 200mV. So if potential difference between A and B (or input+ and input-) is ≥200mV the receiver sees logic 1, if the B input has higher potential than A by at least 200mV then the receiver sees logic 0. Compare it with the attached truth table for the 75176..
Regards,
IanP