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The important RS-485 transmitter output waveform is the differential signal that the receiver processes. VOD is defined as the difference between the transmitter’s non-inverting and inverting outputs, which are the Y and Z pins respectively. A common point of confusion arises from the fact that the differential waveform swings both negative and positive, causing one to think that the RS-485 transceiver must be outputting negative voltages. In reality, the negative VOD is just an artifact of its mathematical definition, and VOD is negative whenever the Z output is high and the Y output is low. The RS-485 signal’s unit interval is the same as the bit width and is the reciprocal of the data rate, and the data rate is 2x the switching frequency. Y and Z balance is very important to minimize EMI. If the Y and Z output transition times and propagation delays are well matched, then emissions from those outputs will cancel each other due to the use of twisted pair cabling.

PTM Published on: 2016-01-04