RS-485 Polarity

RS-485 comprises two signals, one that 'tracks' the logic levels (high voltage == 1, low voltage == 0). The other signal does the opposite. The meaning of these two signals has been polluted by chip and box vendors, which leads to a lot of confusion. A lot of further confusion arises when one considers the UART logic levels vs signal levels and labels. This article attempts to resolve the status without adding to the confusion

Our terminology:

Nominal Terminology

Our Terminology	Description	Logic levels	Data Representation
T+	Tracking	High voltage (typically +5 or +3.3)	1
T-	Non-tracking	Low voltage (close to 0v)	0

Labeling found in the field

Manufacturer's Labeling

Comment	Tracking	Non-tracking	Class	Manufacturer examples
Our terminology	T+	T-	Nominal	BITS
	+	-	Symbol	FT-Click, BASRouterLX
	B+	A-		MBS UBR-01 MkII
	U+	U-		Wikipedia
TI convention	A	B		TI Polarity Conventions

TI has a very definitive document, but one that has to be read carefully in terms of terminology. Read this document with the following table in mind. Note, according to TI (and our opinion), the Wikipedia article on bus states is WRONG ! :

RS-485 Bus State Terminology

UART bit	Logic level	A-B (T+ - T-) result	Legacy	RS-485 state	RS-485 A volt	RS-485 B volt
1	high	A > B	Mark/Idle	On (0)	high	low
0	low	A < B	Space	Off (1)	low	high