RS-485 Polarity - Revision history

BACnetEd at 20:37, 26 June 2026

2026-06-26T20:37:43Z

← Older revision		Revision as of 13:37, 26 June 2026
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	! UART input/output !! A vs B !! Legacy !! RS-485 Bus State !! RS-485 Bus Binary state		! UART input/output !! A vs B !! Legacy !! RS-485 Bus State !! RS-485 Bus Binary state
	\|-		\|-
	\| 1 \|\| A > B\|\| Mark/Idle \|\| On \|\| ~~Logic~~ 0		\| 1 \|\| A > B\|\| Mark/Idle \|\| On \|\| Binary 0
	\|-		\|-
	\| 0 \|\| A < B \|\| Space \|\| Off \|\| ~~Logic~~ 1		\| 0 \|\| A < B \|\| Space \|\| Off \|\| Binary 1
	\|}		\|}

BACnetEd at 20:32, 26 June 2026

2026-06-26T20:32:34Z

← Older revision		Revision as of 13:32, 26 June 2026
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	\|+ Nominal Terminology		\|+ Nominal Terminology
	\|-		\|-
	! ~~Our Terminology~~ !! Description !! ~~Terminonlogy~~ !! Data (UART) Representation !! When Idle		! Specification !! Description !! Terminology !! 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		\| A \|\| Tracking UART value \|\| Non-Inverting \|\| A is high when UART emitting 1, A is low when UART emitting 0 \|\| High

BACnetEd at 20:31, 26 June 2026

2026-06-26T20:31:40Z

← Older revision		Revision as of 13:31, 26 June 2026
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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.		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. 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.

	'''Basic Rule from the specification (and I have the actual specification)		'''Quotations directly from the Standerd. Basic Rule from the specification (and I have the actual specification)

	* A > B means "binary 0 on the bus" or "ON".		* A > B means "binary 0 on the bus" or "ON".
	* A < B means "binary 1 on the bus" or "OFF".		* A < B means "binary 1 on the bus" or "OFF".
			* "The logic function of the generator and the receiver is beyond the scope of this standard, and therefore not defined"

	And A is non-inverting (and thus B is inverting)		And A is non-inverting (and thus B is inverting)


	'''Identifying 'A' and 'B' signals'''		'''Identifying 'A' and 'B' signals'''

BACnetEd at 09:28, 22 June 2026

2026-06-22T09:28:33Z

← Older revision		Revision as of 02:28, 22 June 2026
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	\| + \|\| - \|\| Symbol \|\| FT-Click, BASRouter LX, Onicon F-4300		\| + \|\| - \|\| Symbol \|\| FT-Click, BASRouter LX, Onicon F-4300
	\|-		\|-
	\| B+ \|\| A- \|\| ~~This is obviously contrary to everyone else~~ \|\| MBS UBR-01 Mk II		\| B+ \|\| A- \|\| \|\| MBS UBR-01 Mk II \|\| This is obviously contrary to everyone else
	\|-		\|-
	\| 485+ \|\| 485- \|\| Symbol \|\| FieldServer		\| 485+ \|\| 485- \|\| Symbol \|\| FieldServer

BACnetEd at 09:28, 22 June 2026

2026-06-22T09:28:05Z

← Older revision		Revision as of 02:28, 22 June 2026
Line 46:		Line 46:
	\| + \|\| - \|\| Symbol \|\| FT-Click, BASRouter LX, Onicon F-4300		\| + \|\| - \|\| Symbol \|\| FT-Click, BASRouter LX, Onicon F-4300
	\|-		\|-
	\| B+ \|\| A- \|\| \|\| MBS UBR-01 Mk II		\| B+ \|\| A- \|\| This is obviously contrary to everyone else \|\| MBS UBR-01 Mk II
	\|-		\|-
	\| 485+ \|\| 485- \|\| Symbol \|\| FieldServer		\| 485+ \|\| 485- \|\| Symbol \|\| FieldServer

BACnetEd at 14:41, 31 July 2025

2025-07-31T14:41:58Z

← Older revision		Revision as of 07:41, 31 July 2025
Line 54:		Line 54:
	\| B \|\| A \|\| \|\| [https://en.wikipedia.org/wiki/RS-485#Signals Wikipedia. See, what a fiasco]		\| B \|\| A \|\| \|\| [https://en.wikipedia.org/wiki/RS-485#Signals Wikipedia. See, what a fiasco]
	\|-		\|-
	\| A+ \|\| B- \|\| \|\| [https://www.waveshare.com/product/iot-communication/wired-comm-converter/usb-to-rs485.htm Waveshare USB to RS-485]		\| A+ \|\| B- \|\| \|\| [https://www.waveshare.com/product/iot-communication/wired-comm-converter/usb-to-rs485.htm Waveshare USB to RS-485], AirBox
	\|-		\|-
	\| A \|\| B \|\| \|\| [https://e2e.ti.com/cfs-file/__key/telligent-evolution-components-attachments/13-143-00-00-00-26-49-60/RS485-_2D00_-Polarity-Conventions.pdf TI Polarity Conventions]		\| A \|\| B \|\| \|\| [https://e2e.ti.com/cfs-file/__key/telligent-evolution-components-attachments/13-143-00-00-00-26-49-60/RS485-_2D00_-Polarity-Conventions.pdf TI Polarity Conventions]
			\|-
			\| ? \|\| ? \|\| \|\| KMC router, A- and B+, true polarity TBC
	\|}		\|}

BACnetEd at 13:46, 1 July 2025

2025-07-01T13:46:51Z

← Older revision		Revision as of 06:46, 1 July 2025
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	\|+ Manufacturer's Labeling		\|+ Manufacturer's Labeling
	\|-		\|-
	! Non-inverting !! Inverting !! Class !! Manufacturer examples		! Non-inverting !! Inverting !! Class !! Manufacturer examples !!
	\|-		\|-
	\| A \|\| B \|\| Nominal \|\| BITS, FieldServer		\| A \|\| B \|\| Nominal \|\| BITS, FieldServer
	\|-		\|-
	\| + \|\| - \|\| Symbol \|\| BACnet Specification		\| + \|\| - \|\| Symbol \|\| BACnet Specification \|\| The non-inverting input of the
			EIA-485 transceiver is designated in this specification as "plus" or "+"
	\|-		\|-
	\| RT+ \|\| RT- \|\| \|\| EnOcean SmartServer IoT		\| RT+ \|\| RT- \|\| \|\| EnOcean SmartServer IoT

BACnetEd at 23:03, 29 June 2025

2025-06-29T23:03:05Z

← Older revision		Revision as of 16:03, 29 June 2025
Line 1:		Line 1:
	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.		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.

	'''Basic Rule from the specification		'''Basic Rule from the specification (and I have the actual specification)

	* A > B means the ~~line is logic '1' (MARK)~~.		* A > B means "binary 0 on the bus" or "ON".
	* A < B means the ~~line~~ is ~~logic '0'~~ (~~SPACE~~).		* A < B means "binary 1 on the bus" or "OFF".

			And A is non-inverting (and thus B is inverting)

	'''Identifying 'A' and 'B' signals'''		'''Identifying 'A' and 'B' signals'''
Line 64:		Line 66:
	\|+ RS-485 Bus State Terminology		\|+ RS-485 Bus State Terminology
	\|-		\|-
	! UART input/output !! ~~Logic level !!~~ A vs B !! Legacy !! RS-485 Bus State !! RS-485 ~~A volt !! RS-485 B volt~~		! UART input/output !! A vs B !! Legacy !! RS-485 Bus State !! RS-485 Bus Binary state
	\|-		\|-
	\| 1 \|\| ~~high\|\|~~ A > B\|\| Mark/Idle \|\| On \|\| ~~high \|\| low~~		\| 1 \|\| A > B\|\| Mark/Idle \|\| On \|\| Logic 0
	\|-		\|-
	\| 0 \|\| ~~low \|\|~~ A < B \|\| Space \|\| Off \|\| ~~low \|\| high~~		\| 0 \|\| A < B \|\| Space \|\| Off \|\| Logic 1
	\|}		\|}

BACnetEd at 20:16, 29 June 2025

2025-06-29T20:16:37Z

← Older revision		Revision as of 13:16, 29 June 2025
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	\|}		\|}

			The BACnet Specification (135 - 9.2.2) says: The non-inverting input of the
			EIA-485 transceiver is designated in this specification as "plus" or "+" and the inverting input as "minus" or "-". It is recommended, but not required, that the black or red insulated wire of the twisted pair be designated as "plus" and the white, clear, or green insulated wire be designated as "minus."

	TI has a very definitive [https://e2e.ti.com/cfs-file/__key/telligent-evolution-components-attachments/13-143-00-00-00-26-49-60/RS485-_2D00_-Polarity-Conventions.pdf 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 [https://en.wikipedia.org/wiki/RS-485 Wikipedia article] on bus states is WRONG ! :		TI has a very definitive [https://e2e.ti.com/cfs-file/__key/telligent-evolution-components-attachments/13-143-00-00-00-26-49-60/RS485-_2D00_-Polarity-Conventions.pdf 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 [https://en.wikipedia.org/wiki/RS-485 Wikipedia article] on bus states is WRONG ! :

BACnetEd at 20:02, 29 June 2025

2025-06-29T20:02:51Z

← Older revision		Revision as of 13:02, 29 June 2025
Line 1:		Line 1:
	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.		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.

			'''Basic Rule from the specification

			* A > B means the line is logic '1' (MARK).
			* A < B means the line is logic '0' (SPACE).

	'''Identifying 'A' and 'B' signals'''		'''Identifying 'A' and 'B' signals'''