What LonWorks FT-10 Is
FT-10 is the most common physical layer on installed LonWorks networks. It is a free-topology, transformer-isolated, twisted-pair channel that runs at 78 kbit/s and supports star, bus, loop, or mixed wiring at the segment level. It is the channel almost every brownfield LonWorks HVAC trunk runs on.
The “FT” stands for free topology. The “10” refers to the 1.25 megabaud chip rate divided down to a 78 kbit/s effective signaling rate. The channel is defined by the FTT-10A transceiver family from Echelon (now Renesas) and is standardized as part of the ISO/IEC 14908 LonTalk family.
Why The Physical Layer Still Matters
A LonWorks retrofit job often starts with the assumption that the network is healthy because devices respond. In practice, marginal FT-10 wiring is one of the most common late-stage problems on gateway projects. A trunk that grew over fifteen years of moves and adds usually drifts away from the original commissioning topology, and the symptoms only show up under load from the new gateway.
For deeper field troubleshooting steps, use the LonWorks Troubleshooting Guide.
Channel Specifications
| Parameter | Value |
|---|---|
| Signaling rate | 78 kbit/s |
| Encoding | Differential Manchester |
| Media | Twisted pair, transformer isolated |
| Topology | Free topology or doubly-terminated bus |
| Maximum nodes per segment | 64 with FTT-10A; 128 with LPT-11 link-power transceivers |
| Polarity | Non-polarity sensitive |
Topology Options
FT-10 is unusual among field buses because it tolerates two very different topology choices on the same channel type. Which one is in use changes the termination rules and the maximum cable length.
Free Topology
Free topology allows star, bus, loop, or mixed wiring on the same segment.
| Parameter | Limit |
|---|---|
| Maximum total wire per segment | 500 m |
| Maximum node-to-node distance | 400 m |
| Termination | One terminator anywhere on the segment |
Free topology is the most common choice in building installations because it tolerates the wiring patterns that real buildings actually have.
Doubly-Terminated Bus
A pure bus layout supports longer total cable runs at the cost of strict topology rules.
| Parameter | Limit |
|---|---|
| Maximum bus length | 2,700 m |
| Stub length | No stubs allowed |
| Termination | Two terminators, one at each end of the bus |
A trunk that started as a bus and later acquired stubs to reach new devices is technically out of spec. It often keeps working until a new gateway starts polling at higher rates.
Transceiver Families
| Transceiver | Notes |
|---|---|
| FTT-10A | Standard free-topology transceiver; non-polarity sensitive; transformer isolated |
| LPT-10 | Link-power transceiver; carries 42 V DC on the same pair to power the node |
| LPT-11 | Higher-density link-power variant; supports up to 128 nodes per segment |
Link-power variants share the wiring with FTT-10A devices but require a dedicated power supply on the segment. Mixing the two without the correct power coupler is one of the more painful site-wiring mistakes to diagnose because the segment can appear healthy when only a subset of nodes is active.
Termination
Termination on FT-10 is not optional. Free-topology segments need one terminator; doubly-terminated bus segments need two. The wrong number of terminators causes reflections that may look like intermittent communication rather than a wiring fault.
| Topology | Terminators Required |
|---|---|
| Free topology | 1 |
| Doubly-terminated bus | 2 |
Echelon and several third parties ship FT-10 terminators as small modules with screw terminals. Site-built equivalents using resistor-capacitor networks exist in older installations and should be replaced with documented terminators whenever access permits.
Connecting A Gateway Or Tool
A LonWorks gateway, laptop, or network management workstation connects to the FT-10 segment through a network interface. The interface drives the FTT-10A transceiver and presents a Neuron-compatible API to the host software.
| Interface | Notes |
|---|---|
| U10 / U20 USB Network Interface | Common modern USB-attached FT-10 interface |
| PCLTA-21 / PCC-10 | Legacy PCI or PC Card interfaces; still found on commissioning laptops |
| iLON SmartServer | Embedded IP gateway with an FT-10 port |
| QuickServer LonWorks gateway | Field gateway used to expose FT-10 data to BACnet, Modbus, or MQTT |
A gateway is not a passive listener on FT-10. It draws power from the segment in link-power deployments and contributes traffic in any deployment. Adding one to a marginal trunk can expose pre-existing wiring problems that were never seen under the previous traffic profile.
Common Failure Modes
| Symptom | Likely Cause | Action |
|---|---|---|
| Intermittent loss of a subset of nodes | Stub added to a doubly-terminated bus | Re-survey the topology; convert to free topology or remove the stub |
| Communication degrades after new gateway is installed | Marginal wiring exposed by higher traffic | Inspect terminators and segment length before blaming the gateway |
| Some devices respond, others do not | Transceiver family mix | Confirm FTT-10A versus LPT-10 mix and the power coupler |
| Reflections or random NAKs | Wrong number of terminators | Restore one terminator on free topology or two on bus topology |
| Long-distance node drops | Segment exceeds 500 m free-topology limit | Add a router or repeater; do not extend free-topology distance |