How Do You Size a LON TP/FT-10 Network?

Use the 100 m limit for a PIXEL LON installation unless the manufacturer approves a longer segment for the exact interface, cable, topology, node count, and termination arrangement. Longer TP/FT-10 distances appear in interoperability and cable guidance, but those network-level limits do not automatically override a product-specific rating.

Distance-limit comparison

The different values are not interchangeable. They describe different cables, conductor dimensions, layouts, termination schemes, and product boundaries. A bus does not automatically make the PIXEL rating greater than 100 m.

Recommended design basis

Before anything else, confirm whether the project requires a supported product configuration or an experimentally validated network. For normal commissioning, treat 100 m as the maximum PIXEL segment length. This is the only distance explicitly assigned to that port in the product description.

Use the longer TP/FT-10 figures only when the PIXEL manufacturer confirms that its interface may be engineered to the applicable interoperability or cable specification. The approval must identify the cable, topology, termination, total length, maximum node separation, branch length, and node count. A successful field test alone does not change the equipment rating.

If the route exceeds the approved segment length, divide it into compliant segments using network infrastructure suitable for the channel. Select and locate repeaters from their own installation data; do not infer their capacity from a previously operating site.

Pre-commissioning checks

1. Identify the physical channel. Confirm that every device uses the same TP/FT-10 twisted-pair channel. Do not substitute distance rules from another serial interface merely because it also uses twisted pair.
2. Draw the actual topology. Mark the trunk, every branch, each node, both cable ends, and the proposed terminator positions. Label the layout as a bus only when devices connect along a continuous trunk rather than through unrestricted star branches.
3. Measure cable paths. Record total installed cable length, not straight-line building distance. For free topology, also record the longest node-to-node path. Do not move on until the drawing and measured route agree.
4. Record the cable construction. Read the manufacturer and cable designation from the jacket or purchasing record. A generic Category 5 description is not proof that the cable matches BELDEN 8471 or the 1.3 mm cable condition.
5. Count nodes and branches. If applying the cited 2200 m bus guidance, keep the node count at or below 128 and each branch at or below 3 m. These values do not extend the PIXEL limit without product approval.
6. Check the terminal assignment. Obtain the LON network-card terminal designation from the PIXEL user manual before wiring. Verify polarity or polarity-insensitive behavior from that manual rather than assuming it.

Bus installation procedure

1. Set the design limit. Enter 100 m as the allowable segment length on the network drawing unless a written, interface-specific approval authorizes another value. Confirm the longest measured path is within that limit.
2. Route the trunk. Install one continuous twisted-pair path with short device drops. When the approved design invokes document 078-0014-01E, confirm that no branch exceeds 3 m.
3. Install termination. For the cited bus arrangement, place a terminator at each physical end of the bus. For the cited free-topology arrangement, use one terminator approximately at the center. Do not combine the two schemes.
4. Inspect cable separation and workmanship. Check for crushed cable, untwisted termination tails, loose terminals, shields or conductors bonded unintentionally, and long parallel runs beside power conductors. Operation beside power cable has been observed in individual installations, but it is not a design allowance.
5. Commission incrementally. Start with the shortest practical trunk and a small group of nodes. Confirm communication, then add cable sections and nodes in the planned order. Test the farthest node after every extension.
6. Stop at the first degradation. If retries, missed updates, or intermittent nodes appear, return to the last stable configuration. Correct topology, termination, cable, or noise coupling before adding more length.

Distance and topology mechanism

The 78 kbit/s data rate does not define cable reach by itself. Receiver margin changes with cable attenuation, conductor resistance, characteristic impedance, termination, branch reflections, node loading, and coupled electrical noise. That is why one TP/FT-10 table can state 2200 m for a controlled bus while another states 500 m for free topology.

A correctly terminated bus presents a more predictable transmission path. Long branches create impedance discontinuities, and the resulting reflections can reduce the signal margin at distant nodes. Free topology accepts less predictable paths, so its allowable total length and node spacing are normally more restrictive.

Field operation beyond a rating proves only that one installation had enough margin under the conditions tested. Reported examples include a 1200 m bus with less than 2% packet loss and an ordinary UTP-5 free-topology network of about 120 m with approximately 24 or 25 devices. Neither result defines a repeatable acceptance limit for the PIXEL port.

Verification and fault isolation

1. Verify communication with all planned nodes online.
2. Exercise normal network traffic and observe retries, lost updates, offline indications, and recovery behavior at the nearest and farthest nodes.
3. Repeat the test while nearby power equipment starts, stops, and changes operating state.
4. Power-cycle the network equipment and confirm that every node returns without manual intervention.
5. Record topology, cable, measured lengths, terminator locations, node count, and test results as the commissioning baseline.

Frequently asked questions