BACnet MS/TP Best Practices - Revision history

BACnetEd at 18:49, 8 February 2024

2024-02-08T18:49:11Z

← Older revision		Revision as of 11:49, 8 February 2024
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	=== Resources ===		=== Resources ===
	A very good MS/TP article from [https://cgproducts.johnsoncontrols.com/met_pdf/12011034.pdf JCI]		* A very good MS/TP article from [https://cgproducts.johnsoncontrols.com/met_pdf/12011034.pdf JCI]
	Interesting load analysis (with hockey stick under high loads) ([https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=860960 link])		* Interesting load analysis (with hockey stick under high loads) ([https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=860960 link])
	Application Guide from [https://sid.siemens.com/r/A6V11623436 Siemens]		* Application Guide from [https://sid.siemens.com/r/A6V11623436 Siemens]
			* Table of RS-485 cable part numbers [https://ctlsys.com/support/rs-485_cables_for_bacnet_and_modbus/ Continental Control Systems]

BACnetEd at 15:10, 12 October 2023

2023-10-12T15:10:29Z

← Older revision		Revision as of 08:10, 12 October 2023
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	==== A-/B+ or Polarity Issues ====		==== A-/B+ or Polarity Issues ====
	Signal Polarity, in particular is fraught with contradictions and errors. We have tried to summarize the situation in the [[RS-485 Polarity]] page.		Signal Polarity, in particular is fraught with contradictions and errors. We have tried to summarize the situation in the [[RS-485 Polarity]] page.


			=== Resources ===
			A very good MS/TP article from [https://cgproducts.johnsoncontrols.com/met_pdf/12011034.pdf JCI]
			Interesting load analysis (with hockey stick under high loads) ([https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=860960 link])
			Application Guide from [https://sid.siemens.com/r/A6V11623436 Siemens]

BACnetEd at 12:08, 17 September 2023

2023-09-17T12:08:04Z

← Older revision		Revision as of 05:08, 17 September 2023
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	It should never be grounded at more than one point, since this is a) unnecessary, b) introduced ground-loop noise into the circuit.		It should never be grounded at more than one point, since this is a) unnecessary, b) introduced ground-loop noise into the circuit.


			==== A-/B+ or Polarity Issues ====
			Signal Polarity, in particular is fraught with contradictions and errors. We have tried to summarize the situation in the [[RS-485 Polarity]] page.

BACnetEd at 19:13, 12 September 2023

2023-09-12T19:13:01Z

← Older revision		Revision as of 12:13, 12 September 2023
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	BACnet MS/TP gets a bad rap in the field. Some installers do a great job and have networks that run with zero errors for years. Other networks suffer continuous (and largely invisible) communication errors. This page will accumulate a list of best practices for designers, installers and users.		BACnet MS/TP gets a bad rap in the field. Some installers do a great job and have networks that run with zero errors for years. Other networks suffer continuous (and largely invisible) communication errors. This page will accumulate a list of best practices for designers, installers and users.

			=== Two-wire vs Three-wire ===

			Firstly consider the following scenario.

			If there is only one pair pf wires (A and B signals), and if a device is floating (i.e. not connected to a ground anywhere, think battery powered or transformer isolated), then due to the various designed in or parasitic resistances, the circuit common (or 0v) will naturally be a few volts below the A/B potential. The RS-485 transceiver will then be able to extract the data signal quite happily.

			==== Two-Wire ====
			With this arrangement, the circuitry 0v is normally grounded to the building ground by manufacturer's specifications and common practices. There is nothing wrong with this approach if all the grounds of all the devices are within +/- 7 volts of each other.

			The problem comes when there are large ground differences between nodes. This happens all the time when different building supply phases are used for different BACnet nodes, or if there are large loads scattered around the building, which cause large currents, thus voltage drops in the various supplies.


			==== Three-Wire (or Isolated RS-485) ====
			The third, or common wire carries the 'reference signal' from node to node. Per the scenario above it is not needed for operation, since the isolated circuit will arrange itself around the A/B voltages. However, the common return is important to remove the common-mode noise on the wire, thus removing the for emitted noise from the network.

			It should never be grounded at more than one point, since this is a) unnecessary, b) introduced ground-loop noise into the circuit.

BACnetEd: Created page with "BACnet MS/TP gets a bad rap in the field. Some installers do a great job and have networks that run with zero errors for years. Other networks suffer continuous (and largely invisible) communication errors. This page will accumulate a list of best practices for designers, installers and users."

2023-09-12T19:00:33Z

Created page with "BACnet MS/TP gets a bad rap in the field. Some installers do a great job and have networks that run with zero errors for years. Other networks suffer continuous (and largely invisible) communication errors. This page will accumulate a list of best practices for designers, installers and users."

New page

BACnet MS/TP gets a bad rap in the field. Some installers do a great job and have networks that run with zero errors for years. Other networks suffer continuous (and largely invisible) communication errors. This page will accumulate a list of best practices for designers, installers and users.