LonWorks Twisted Pair Wiring: Free Topology, Distances and Link Power

Free topology is the reason people put up with LonWorks wiring. Bus, star, loop, or any mix of them, on one pair, with one termination you can put anywhere. No daisy-chain discipline, no polarity to get right.

It isn’t free of rules. It has distance limits, a termination that has to be the right value, and a power budget that depends on where devices sit rather than just how many there are. Get those wrong and the network works on the bench and fails on site.

Everything here is ISO/IEC 14908-2. The vendor world calls this channel TP/FT-10.

The channel

Data rate	78,125 kbit/s
Encoding	Differential Manchester
Polarity	Insensitive, for data and DC both
Max bit error rate	1 in 100 000
Cable	Per clause 9.4 of ISO/IEC 15018:2004

Polarity insensitivity is worth pausing on, because it’s the single biggest practical difference from RS-485. You cannot wire a device backwards. That applies to the DC as well as the data on a link-powered segment.

Note what the standard does not contain: there’s no 1,25 Mbit/s channel in ISO/IEC 14908-2. That backbone channel is a vendor channel. If a specification says “per ISO/IEC 14908-2”, it means 78,125 kbit/s.

Step 1 — Pick the topology

Two choices, and they have different termination and distance rules.

Free topology. Bus, star, loop, or any combination. One termination, and it can sit anywhere on the segment.

Bus topology. A proper bus. Two terminations, one at each end.

Free topology is what you want on a retrofit where the cable route is already decided. Bus topology buys you distance — more than double.

Step 2 — Check the distances

	Free topology	Bus topology
Max node-to-node distance	250 m	—
Max total wire length	450 m	—
Max bus length	—	600 m
Max stub length	—	3 m

Two things people get wrong on free topology.

Node-to-node means every pair. The distance from each device to every other device, and to the link power source, must be inside 250 m. Not the longest run from one end — every combination.

Where loops exist, measure the long way round. If the topology has more than one path between two points, the calculation uses the longest path, not the shortest.

Total wire length is exactly what it sounds like: all the cable connected to the segment, added up. It’s easy to blow this on a star with many short legs while every node-to-node pair is comfortably inside 250 m.

Step 3 — Get the termination right

The termination is an RC network — a resistor with a 100 µF, 50 V minimum capacitor on each side.

Topology	Terminations	R1
Free	One, anywhere on segment	52,3 Ω ± 1 %, 1/8 W
Bus	Two, one at each end	105 Ω ± 1 %, 1/8 W each

Those tolerances are tight and they’re specified for a reason. A 50 Ω resistor is not a 52,3 Ω resistor.

On a link-powered segment, the link power source provides one termination itself. Free topology then needs nothing else. Bus topology still needs a discrete 105 Ω network at the other end.

Step 4 — Count the nodes

Powering	Max nodes per segment
All link-powered	128
All locally powered	64
Mixed	(link × 1) + (local × 2) ≤ 128

A locally powered node counts double. That catches people who assume local power is “free” because it isn’t drawing from the network — it isn’t a power limit, it’s a loading limit.

Step 5 — Do the link power budget

Link power puts DC and data on the same two conductors. The source runs 41,0 to 42,4 V DC, centred on earth so you measure +21 V and −21 V at Net+ and Net−.

Load is counted in Link Power Unit Loads. One LPUL is 285 mW. A device may draw several. The network allows a maximum of 128 LPULs, and total steady-state draw across a segment must not exceed 36,5 W.

Then the part that gets missed. There’s a second constraint that weights power by distance. For each branch:

P₁×d₁ + P₂×d₂ + P₃×d₃ + … ≤ C × α

C = 1,9 × 10³ Wm
α = 1 / (1 + 0,003 93 × (temp°C − 25))

Power in watts, distance in metres from the link power source. The α term derates for average wire temperature — above 25 °C, your budget shrinks.

So two segments with identical device counts and identical total wattage can behave completely differently depending on where the hungry devices sit. Put them near the source. A high-draw device at the far end of a long run is what breaks this constraint, and the failure looks like intermittent dropouts rather than anything obviously power-related.

Device behaviour on the DC side:

Operational range	26 V to 42,4 V
Application power shuts off below	24 V
Charge storage per LPUL	4,7 mAs max
Startup holdoff after network exceeds 26 V	220 ms min

That 220 ms holdoff is a designed-in stagger. Every device waits before drawing application power so they don’t all hit the supply at once on power-up.

Hot plugging is mandatory. A compliant device must survive being plugged into a live powered network with no hard failure, and start up correctly.

Step 6 — Repeaters, if you need them

Three rules, and the first one is absolute:

• At most one physical layer repeater in the path between any two nodes. Not one per segment — one in the path.
• Delay through a repeater must not exceed 36 µs.
• Repeaters must never be interconnected to form a loop.

Beyond that, extend with routers rather than repeaters. A repeater gives you another segment; a router gives you another channel and actual traffic filtering.

What the transceiver has to do

Useful if you’re specifying hardware or chasing a marginal device rather than wiring one.

Transmit amplitude. Peak into 52,3 Ω connected directly at the transceiver output: 0,425 V to 0,900 V, across all manufacturing and operating variation.

Turnaround. Delay from transmit enable to start of transmission on the network must not exceed 1,125 bit times.

Packet framing. A preamble of Differential Manchester “1”s (bit-sync) followed by a single Differential Manchester “0” (byte-sync) marking the first byte. The packet ends with a deliberate line code violation — the transmitter holds the output transitionless for at least three bit periods after the final clock transition, and receivers read that as end of packet.

Impedance. During transmission, minimum 2 000 Ω from 0,2 f to 2 f, where f = 78,125 kHz. Link-powered devices are allowed one impedance notch for a fixed-frequency switching regulator: less than 3 kΩ deep, less than 0,25 f wide, and above 1,6 f. Minimum receive impedance is specified across frequency and is roughly double for link-powered devices compared with locally powered ones.

Channel timing

Communication rate	78,125 kbit/s
Priority slots	4
Minimum bit sync bits	4
Average packet cycle	4 020 µs
Preamble length	301 µs
Beta2 width	168 µs
Beta1 transmit width	868 µs
Beta1 receive width	895 µs

The packet cycle figure is the one you need for setting transport timers. It feeds the Xmit_Timer ≥ 3 × packet cycle rule, which puts a sensible retransmission timer above roughly 12 ms on this channel.

Environment

The standard doesn’t mandate environmental conditions — they depend on the application, and local regulation governs. It does give a representative set:

EMI	IEC/CISPR 22
ESD immunity	IEC 61000-4-2, Level 2
Radiated immunity	IEC 61000-4-3, Level 2
Fast transient / burst	IEC 61000-4-4, Level 2
Surge immunity	IEC 61000-4-5, Level 2
Transceiver operating temperature	−40 °C to +85 °C

Level 2 is modest. On a plant floor or in a substation you should be specifying above it.

What goes wrong

Wrong termination value. 52,3 Ω on a bus, or 105 Ω on free topology, or a nearby standard value substituted. Symptom: reflections, marginal reception, works at short range and fails as the segment grows.

Two terminations on free topology. Free topology takes exactly one. A second one — often left over from a bus that got rewired — over-loads the segment.

Total wire length exceeded on a star. Every node-to-node pair passes and the segment still fails. Add up all the legs.

Far-end power draw. Total wattage is inside 36,5 W, node count is fine, and the segment still drops out. Run the distance-weighted sum.

Locally powered nodes counted as one. They count as two.

Two repeaters in one path. Legal to have several repeaters on a network; not legal for two to sit between the same pair of nodes.

FAQ