RS-485 Polarity

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RS-485 comprises two signals, one that 'tracks' the UART 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, hopefully without adding to the confusion.

Identifying 'A' and 'B' signals

Remember that UARTs output a logic '1' (high voltage) for idle state! This signal will go to 0v for the start bit.

The A signal is the wire that, when in the idle state, is at a slightly higher voltage than the 'B' signal. This difference in voltage measurements is the result of the biasing and termination resistors of the circuit. See Wikipedia

Identifying 'Mark/Idle' and 'Space'

Look at the data bits. The idle condition is always the 'mark' condition. The start bit is always a 'space'. Drawings on the internet are in conflict with each other and cannot be relied upon.

Nominal Terminology
Our Terminology Description Terminonlogy Data (UART) Representation When Idle
A Tracking UART value Non-Inverting A is high when UART emitting 1, A is low when UART emitting 0 High
B Non-tracking Inverting B is low when UART emitting 1, B is high when UART emitting 0 Low

Manufacturer's Labeling
Non-inverting Inverting Class Manufacturer examples
A B Nominal BITS, FieldServer
+ - Symbol BACnet Specification
RT+ RT- EnOcean SmartServer IoT
+ - Symbol FT-Click, BASRouter LX, Onicon F-4300
B+ A- MBS UBR-01 Mk II
485+ 485- Symbol FieldServer
U+ U- Wikipedia
B A Wikipedia. See, what a fiasco
A+ B- Waveshare USB to RS-485
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 input/output Logic level A vs B Legacy RS-485 Bus State RS-485 Bus Logic 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