LonWorks SNVT Reference & XIF Mapping Guide

Overview

LonWorks (Local Operating Network) is an open standard (ISO/IEC 14908) control networking platform created by Echelon Corporation, now maintained by Renesas Electronics. It is widely deployed in building automation — HVAC controllers, lighting systems, and fire safety panels — and is one of the most common legacy protocols encountered in BACnet and Modbus integration projects.

Two concepts are central to every LonWorks gateway integration:

• SNVTs (Standard Network Variable Types) — the standardized data types that define how LonWorks devices expose sensor readings, setpoints, and commands
• XIF files (eXternal Interface Files) — device descriptor files that list all network variables, their SNVT types, and their addressing — the “register map” equivalent for LonWorks

This guide covers both in depth, with practical mapping guidance for QuickServer gateway configurations.

LonWorks Protocol Fundamentals

Network Variables (NVs)

LonWorks devices communicate through network variables — typed data points published or consumed across the network. Every sensor reading, setpoint, and command is a network variable.

Network variables come in two directions:

Abbreviation	Full Name	Direction	BACnet Equivalent
NVI	Network Variable Input	Write to device	Analog Output, Binary Output
NVO	Network Variable Output	Read from device	Analog Input, Binary Input

[!NOTE] The direction is from the device’s perspective. An NVO (output) is data the device publishes — you read it. An NVI (input) is data the device accepts — you write to it.

Neuron IDs

Every LonWorks node has a unique Neuron ID — a 48-bit hardware identifier burned into the Neuron chip at manufacture. Neuron IDs are required for device binding and cannot be discovered remotely in most configurations. They must be collected from device labels, commissioning records, or the installing contractor.

[!WARNING] Neuron IDs are a hard prerequisite for any LonWorks integration. Without them, the gateway cannot bind to devices. Collect Neuron IDs before ordering hardware or starting configuration.

Physical Layer

LonWorks uses FT-10 (Free Topology, twisted pair) at 78 kbit/s as its primary physical layer. Fiber optic networks require an FTT-10 USB converter for gateway connectivity.

Media	Speed	Notes
FT-10 (twisted pair)	78 kbit/s	Most common in building automation
TP/XF-1250	1.25 Mbit/s	Higher speed, transformer-isolated
Power line	3.6–5.4 kbit/s	Used in lighting and metering
IP (CEA-852 tunneling)	Varies	LonWorks over IP backbone

Standard Network Variable Types (SNVTs)

SNVTs (pronounced “sniv-its”) are the standardized data types maintained by LonMark International. Each SNVT defines the encoding, resolution, range, and engineering units for a particular type of measurement or command.

Why SNVTs Matter for Integration

Unlike Modbus (where registers are raw integers and the integrator assigns meaning), LonWorks network variables carry built-in type information. The SNVT tells the gateway exactly what data type to expect and what units it represents.

This is both a strength and a constraint:

• Strength: No guesswork about whether a register is a temperature, pressure, or setpoint — the SNVT declares it
• Constraint: The SNVT defines the unit — and that unit cannot be changed at the protocol level

Common SNVT Types

SNVT	Description	Data Type	Range	Resolution	Unit
SNVT_temp	Temperature (signed)	16-bit signed	−274.0 to 6279.5 °C	0.1 °C	°C
SNVT_temp_p	Temperature (unsigned, positive)	16-bit unsigned	0 to 6553.5 °C	0.1 °C	°C
SNVT_press	Pressure	16-bit signed	−3276.8 to 3276.7 kPa	0.1 kPa	kPa only
SNVT_lev_percent	Level / percent	16-bit signed	−163.840% to 163.835%	0.005%	%
SNVT_flow	Flow rate	16-bit unsigned	0 to 6553.5 L/s	0.1 L/s	L/s
SNVT_count	Count	16-bit unsigned	0 to 65,535	1	(unitless)
SNVT_switch	Switch state	Struct (value + state)	0.0–100.0% + on/off	0.5%	—
SNVT_hvac_mode	HVAC operating mode	Enum (8-bit)	0–255	—	—
SNVT_occupancy	Occupancy status	Enum (8-bit)	0–255	—	—
SNVT_time_stamp	Date/time	Struct (7 bytes)	—	1 s	—
SNVT_str_asc	ASCII string	31-byte string	—	—	—
SNVT_elapsed_tm	Elapsed time	Struct (6 bytes)	—	1 ms	—

[!CAUTION] SNVT_press is kPa only. There is no PSI option at the protocol level. If the BMS requires PSI, the unit conversion must be done in the gateway configuration (1 kPa = 0.145038 PSI) or in the BMS itself. This is a frequent source of surprise at commissioning.

SNVT Scaling and Encoding

Most SNVTs use fixed-point integer encoding with an implicit scale factor. The raw integer on the wire must be converted to engineering units using the SNVT-defined resolution:

SNVT	Raw Wire Value	Scale Factor	Engineering Value
SNVT_temp_p	2350	× 0.1	235.0 °C
SNVT_lev_percent	20000	× 0.005	100.000%
SNVT_press	1013	× 0.1	101.3 kPa
SNVT_flow	500	× 0.1	50.0 L/s

When configuring a QuickServer, the gateway handles SNVT-to-engineering-unit conversion automatically if the SNVT type is correctly specified in the configuration. If the SNVT type is wrong, readings will be off by orders of magnitude.

[!TIP] If a temperature reading shows 3000 instead of 30.0, the most likely cause is a missing or incorrect scale factor in the gateway configuration. Verify the SNVT type assignment in the config matches the device’s XIF file.

Metric-to-Imperial Conversion

LonWorks SNVTs use metric units exclusively (°C, kPa, L/s). Building Management Systems in North America typically expect imperial units (°F, PSI, CFM).

Configure the unit conversion in the gateway point map:

SNVT Unit	BMS Unit	Conversion
°C	°F	°F = (°C × 9/5) + 32
kPa	PSI	PSI = kPa × 0.145038
L/s	CFM	CFM = L/s × 2.11888

The QuickServer supports slope/intercept math in the point map configuration to perform these conversions inline.

XIF Files (eXternal Interface Files)

What a XIF File Is

A XIF file is the LonWorks equivalent of a Modbus register map. It is a binary or XML file extracted from a LonWorks device that describes:

• All network variables (NVs) the device exposes
• The SNVT type of each network variable
• The NV index (address) for each variable
• Whether each NV is an input (NVI) or output (NVO)
• Any configuration properties (SCPTs/UCPTs)

Without a XIF file, you are configuring the gateway blind — guessing at NV indexes, SNVT types, and read/write capabilities.

[!WARNING] XIF files are a hard prerequisite for LonWorks gateway configuration. Do not start a configuration build without the XIF file for every device type on the network.

How to Obtain XIF Files

XIF files can be obtained through several methods:

Method	Tool Required	Notes
Export from commissioning tool	LNS (LonMaker), Tridium Niagara, or OEM tool	Most reliable — contains the exact NV list as configured
Extract from device firmware	Echelon NodeUtil or equivalent	Extracts the default XIF from the Neuron chip directly
Request from device manufacturer	None	Ask for the XIF file matching the firmware version installed
Extract via gateway discovery	QuickServer NodeUtil mode	QuickServer can discover and extract XIFs from connected devices

[!TIP] Always verify the XIF file matches the firmware version installed on the device. Manufacturers sometimes add or remove network variables between firmware revisions. A mismatched XIF will produce “phantom” points that cannot be read or missing points that should exist.

XIF File Contents

A typical XIF file for an HVAC controller contains entries like:

NV Index	Name	Direction	SNVT Type	Description
0	nvoSpaceTemp	NVO (read)	SNVT_temp_p	Space temperature sensor
1	nvoDischargeTemp	NVO (read)	SNVT_temp_p	Discharge air temperature
2	nviSetpoint	NVI (write)	SNVT_temp_p	Temperature setpoint
3	nvoOccupancy	NVO (read)	SNVT_occupancy	Occupancy status
4	nviHvacMode	NVI (write)	SNVT_hvac_mode	Operating mode command
5	nvoDamperPos	NVO (read)	SNVT_lev_percent	Damper position (0–100%)
6	nvoFanSpeed	NVO (read)	SNVT_lev_percent	Fan speed (0–100%)
7	nvoAlarm	NVO (read)	SNVT_alarm	Alarm status

UCPT and SCPT Properties

Beyond standard NVs, LonWorks devices may expose configuration properties:

• SCPT (Standard Configuration Property Type) — predefined by LonMark (e.g., SCPTminRnge, SCPTmaxRnge for sensor calibration ranges)
• UCPT (User Configuration Property Type) — manufacturer-defined proprietary properties

[!NOTE] UCPT/SCPT properties often contain critical calibration and configuration parameters. Some devices expose runtime data through UCPTs rather than standard NVs — a pattern seen frequently in Siemens LON TEC controllers. Verify with the device manufacturer whether any data points of interest are UCPTs rather than NVs, as the gateway configuration uses a different access method for properties vs. network variables.

Mapping LonWorks to BACnet Objects

The most common LonWorks integration pattern is LonWorks → BACnet/IP using a QuickServer gateway. The gateway reads LonWorks NVs and exposes them as BACnet objects.

NV-to-BACnet Object Mapping

LonWorks NV	Direction	SNVT Type	BACnet Object	Notes
NVO (analog sensor)	Read	SNVT_temp_p, SNVT_press, SNVT_flow	Analog Input (AI)	Read-only measurement
NVI (analog setpoint)	Write	SNVT_temp_p, SNVT_lev_percent	Analog Value (AV) or Analog Output (AO)	Writable setpoint
NVO (binary state)	Read	SNVT_switch (state byte)	Binary Input (BI)	On/off status
NVI (binary command)	Write	SNVT_switch	Binary Output (BO)	On/off command
NVO (enum)	Read	SNVT_hvac_mode, SNVT_occupancy	Multi-State Input (MSI)	Map enum values to state names
NVI (enum command)	Write	SNVT_hvac_mode	Multi-State Value (MSV)	Writable mode selector

For detailed guidance on BACnet object type selection, see BACnet Object Types & Properties Reference.

Write Functions: NVUI vs NVPO

When the gateway needs to write to a LonWorks device, the correct write function must be selected:

Function	Full Name	Use Case
NVUI	Network Variable Update (Input)	Standard write to an NVI — the default for most setpoints and commands
NVPO	Network Variable Poll (Output)	Force-read an NVO — used to explicitly poll a device output
WRBX	Write Byte Extended	Direct memory write — advanced, used when NVUI doesn’t apply
WRBC	Write Byte Compact	Compact direct memory write — advanced

[!TIP] Start with NVUI for all write operations. If writes succeed on some devices but not others (e.g., AHU controllers accept writes but FCU controllers don’t), the issue is often that the target device requires a different function or has write protection enabled. Consult the device manufacturer’s documentation for write capabilities.

LonWorks Application IDs

LonWorks devices are classified by application ID (also called functional profile). Common application IDs in building automation:

App ID	Description	Typical Points
8101	VAV Controller	Zone temp, setpoint, damper position, fan speed
8501	AHU Controller	Supply/return air temp, fan status, valve positions
8515	Chiller/Heat Pump	Entering/leaving water temp, compressor status
9086	Energy Meter	kWh, kW demand, voltage, current
9641	Boiler Controller	Supply water temp, firing rate, flame status

[!NOTE] Devices with the same application ID from different manufacturers may not have identical NV lists. Always verify against the actual XIF file — the application ID is a guideline, not a guarantee of compatibility.

Pre-Project Intake Checklist

Collect this information before starting any LonWorks integration. Missing items at this stage are the #1 cause of project delays.

Device Information

• Device manufacturer and model — needed to source the correct XIF file
• Firmware version — XIF files may differ between firmware revisions
• Neuron ID(s) for every device on the network — 48-bit hardware address; cannot be discovered remotely
• XIF file for each device type — required before configuration can begin
• Subnet/Node IDs — required if replacing an existing gateway or node
• Number of devices on the LonWorks trunk

Network Information

• Physical media — FT-10 (twisted pair), fiber optic, or IP tunneling?
• Existing network topology — is there a LonWorks router, repeater, or network manager?
• Other devices on the trunk — any existing gateways or controllers sharing the network?

Integration Requirements

• SNVT types needed — which measurements, setpoints, and commands are required?
• Unit preferences — metric (°C, kPa, L/s) or imperial (°F, PSI, CFM)?
• Desired BACnet point names — LonWorks NVs have technical names (e.g., nvoSpaceTemp) but no human-readable descriptions; the BMS integrator must specify the desired object names
• Read/write requirements — which points are read-only monitoring vs. writable setpoints/commands?
• Destination protocol — BACnet/IP, BACnet MS/TP, Modbus TCP, or Modbus RTU?
• Device Instance number for the gateway (if BACnet)

[!CAUTION] LonWorks network variables have no description metadata. Unlike BACnet objects (which have a Description property) or Modbus maps (which have documented register descriptions), LonWorks NV names like nvo3 or nvoSpaceTemp are all you get. The BMS integrator must provide the desired point names separately.

Troubleshooting Common Issues

Symptom	Likely Cause	Fix
Temperature reads as 3000 instead of 30.0	Missing scale factor — raw integer not divided by resolution	Verify SNVT type in config matches XIF; check slope/intercept settings
Pressure values in kPa but BMS expects PSI	SNVT_press is always kPa	Add slope = 0.145038 in gateway point map for kPa → PSI conversion
Write commands have no effect	Wrong write function (NVUI vs NVPO vs WRBX)	Start with NVUI; consult device documentation for write capabilities
Writes work on AHUs but not FCUs	Device-specific write protection or different NV function	Check if FCU requires different NV index or has write-protect enabled in device firmware
”Phantom” points — NVs in config but no data	XIF file doesn’t match installed firmware version	Re-extract XIF from device or request correct version from manufacturer
Gateway cannot discover devices	Missing or incorrect Neuron IDs	Verify Neuron IDs from device labels; check physical wiring
Intermittent communication on fiber optic	FTT-10 USB converter needed	Standard USB-to-serial adapters don’t work on fiber; use Echelon 75010R or equivalent
Config builds fail or produce errors	UCPT/SCPT properties not handled as standard NVs	UCPT/SCPT properties require different access method; check gateway documentation
Values show but are stuck / not updating	Device not bound to gateway node	Re-bind using Neuron ID; verify Subnet/Node ID assignments
Multi-year project reactivated — nothing works	Neuron IDs or network topology changed	Re-collect Neuron IDs; re-extract XIF files; verify physical network

Related Articles

• BACnet Object Types & Properties Reference
• Modbus to BACnet Protocol Conversion Guide
• BACnet Discovery & Network Architecture Reference
• Troubleshooting Guide for BACnet MS/TP Networks

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• QuickServer — LonWorks FT-10 to BACnet/IP or Modbus TCP gateway with built-in NodeUtil discovery
• QuickServer — Multi-protocol gateway supporting LonWorks, BACnet, Modbus, and 35+ other protocols
• CAS BACnet Explorer — Verify BACnet object exposure from the gateway after LonWorks point mapping
• Chipkin Support — XIF file analysis, SNVT mapping, and LonWorks gateway configuration services