RS485 Hub: Star vs. Daisy Chain – When to Use Which

RS485 is supposed to work with one kind of wiring: a daisy chain. You have one device, another. They are all connected in a single line. This line has to be closed at both ends. That is what the books say.

Things do not always work out that way in real life.

When retrofitting video surveillance, building automation (HVAC), or legacy factory floors, technicians frequently encounter existing cables pulled in a star configuration.

That is where RS485 hubs come in. They help you work around the rules. Only when you really need to. RS485 hubs are useful when you have to deal with RS485. It is not working the way it is supposed to.

The Two Wiring Topologies

Daisy Chain (T-Connection)

This is the standard RS485 wiring method.

How to Daisy Chain RS485: One main cable runs from the master to the farthest device. Every other device connects to this main line with a short stub (as short as possible).

Pros:

• Best signal integrity
• Supports longer distances
• Reliable at high baud rates
• Only one termination needed at each end

Cons:

• Can be awkward to wire in existing installations
• Adding a new device may require running cable from the nearest point
• Stubs must be kept very short

Star Connection

Often sought as a daisy chain alternative, this is the wiring method used for most other cables—power, video, Ethernet.

In a star topology, each end-device is wired directly back to a master controller at the center, similar to standard Ethernet or power distribution layouts. This is very intuitive for physical installation, but it creates multiple parallel stubs on the RS485 bus.

Pros:

• Simple to understand and install
• Easy to add new devices
• Matches wiring of other systems
• Faults isolated to one branch

Cons:

• Uses more cable
• Not standard for RS485
• Can cause signal reflections if not handled properly (unless using an active RS485 Hub).
• No simple way to terminate

What the RS485 Standard Says

The RS485 standard, which is also known as TIA/EIA-485 says that devices should be connected in a line one after the other. This is called a daisy chain topology. The RS485 standard is pretty simple it just needs one cable and really short connections, to each device.

Why does this matter? Long stubs and star branches create signal reflection per transmission line principles as described in the Texas Instruments RS-485 Design Guide. At higher baud rates or longer distances, these reflected waves collide with the original data stream, corrupting the digital pulses and causing intermittent timeout errors.

👇 Unsure if your current distance and speed will cause data errors? Use our quick calculator to check your physical limits instantly:

The standard was written for factory floors where you can route cables any way you want. It doesn’t account for the reality of retrofitting into buildings that are already wired in a star pattern. In our field deployments, we’ve seen video surveillance systems successfully use star-wired RS485 for up to 400 meters with about 10 devices—well beyond what the textbook allows.

When to Use Daisy Chain

Daisy chain is the right choice when:

• You’re designing a new system from scratch
• You need maximum distance or speed
• You have control over cable routing
• You’re experienced with RS485 termination

This is the “textbook” scenario. It will give you the best performance and fewest headaches.

When Star Makes Sense

Star wiring makes sense when:

• You’re retrofitting into an existing building
• Other systems (video, power) are already star-wired
• Simplicity of installation matters more than absolute performance
• Device counts are low and distances are moderate

While strict RS485 guidelines prohibit unterminated star branches, real-world field experience dictates otherwise. In legacy video surveillance (PTZ camera) retrofits, it is common to see low-speed RS485 control signals successfully running over star topologies for up to 400 meters with around 10 devices—relying entirely on the transceivers’ tolerance to mild signal reflections.

The Problem with Mixing Topologies Without a Hub

What if you have a large system with devices spread out? What if you need a modbus star topology in some areas but a daisy chain makes sense in others?

This is where the standard breaks down. You can’t just connect a star network to a daisy chain network and expect it to work. Signal reflections will cause intermittent failures. Curious about the exact electrical difference between building a star network from a low-cost terminal block vs an active, opto-isolated hub? Check out our technical comparison: Industrial RS485 Hub vs. Passive Splitter.

Passive Splitters vs. Active Hubs: Breaking the Daisy-Chain Rule

If you are extending networks or splitting a bus, engineers have to choose between a cheap passive RS485 splitter (essentially a terminal block tying wires together) or an active Industrial RS485 Hub.

Creating a star topology by using a passive splitter is creating long, unterminated ‘stubs’. The electrical energy doesn’t just vanish when high-speed digital pulses race down a copper wire and hit the end of an unterminated stub. Due to the impedance-mismatch the energy will be reflected back down the wire. This is called signal reflection or standing wave.

Transmission line theory says that when these reflected waves interfere with the incoming data stream , they distort the square waves . A clean 1 or 0 is converted to a distorted voltage spike. The receiving PLC cannot read the bit causing a CRC error and a Modbus timeout.

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🛠️ Know Your Physical Limits (Offline PDF)

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How an RS485 Hub Solves the Topology Puzzle

Thousands of dollars in labor and downtime can be spent rewiring an entire facility to the daisy-chain standard. A good active RS485 hub will act as a topology translator at a fraction of the cost. You can keep your existing star wiring, and perfectly isolate ground loops and regenerate the signal.

What the hub does:

• Signal regeneration: It doesn’t just pass the signal through; it receives it and transmits a fresh copy.
• Isolation: Each port is electrically isolated (on isolated models), preventing ground loops.
• Topology conversion: You can connect a daisy chain to one port and stars to others.

The hub becomes the central point that makes different topologies work together.

Other Reasons to Use a Hub

Exceeding the 32-Device Limit

RS485 drivers are typically rated for 32 unit loads. If you need more devices, a hub lets you add another segment.

Extending Distance

The hubs are really helpful because they make the signal strong again. This means the distance limit starts over. With each hub you can actually go 1200 meters more.

Isolation

So you have devices in buildings or they are on different ground levels. This is where an isolated hub comes in handy. The isolated hub protects your devices. It stops the ground loops from forming. It also blocks the voltage spikes that can damage your equipment. The isolated hub is really good, at keeping your devices safe.

Rewiring an entire facility to meet the daisy-chain standard can cost thousands in labor. For a fraction of that cost, an active RS485 hub acts as a topology translator—allowing you to keep your existing star wiring while perfectly isolating ground loops and regenerating the signal.

Surviving VFD Noise: Why Opto-Isolation is Critical

Industrial environments are brutally noisy. Engineers constantly battle Electromagnetic Interference (EMI), with Variable Frequency Drives (VFDs) being the primary culprits. VFDs control motor speed using Pulse Width Modulation (PWM), creating high-frequency voltage transients (high dv/dt).

These high-frequency transients radiate outward and couple into nearby RS485 communication lines, inducing “noise spikes”. Because passive splitters have no filtering or isolation, this noise travels freely across the entire network, overwhelming the PLC receivers. An active industrial hub features built-in opto-isolation. EMI transients and VFD harmonics cannot cross the microscopic physical gap within the optocoupler, creating an impenetrable firewall against electrical noise.

Choosing the Right Hub

Hubs come in different sizes and features. Here’s what to consider.

Port count: How many branches do you need? A 4-port hub can connect one master device (such as an Industrial 4G Router) to three star branches, or act as a central point for multiple segments.

Isolation: If cables run between buildings or outdoors, get an isolated model. The 2500V isolation protects against ground loops and surges.

Caching: Some 2-port hubs have a mode. This mode helps when two masters read data from one slave. It also helps when one master reads data from two slaves. This feature is useful in systems where monitoring needs to be on like, in redundant monitoring systems.

Baud rate range: Make sure the hub supports your communication speed. Most industrial hubs handle 1200 to 460800 bps.

Not sure which configuration fits your panel? Browse our [DIN-rail Isolated RS485 Hubs] to compare port layouts and specs.

Wiring Tips from the Field

Based on hundreds of factory deployments, here are the non-negotiable rules for hub wiring:

Keep stubs strictly minimized: Even in a hub-based configuration, drop lines (stubs) connecting the device to the main branch should be kept as short as physically possible (ideally less than 10% of the total cable length) to minimize localized signal reflection.

Use shielded twisted pair. This is really important when you are outside or in a factory. The wires are twisted together which helps stop signals from getting in the way. The shield that is, around the wires also helps block these signals.

Ground the shield at one end only. Connect the shield to ground at the hub side, leave it floating at the device side. This prevents ground loops.

Terminate properly. In a star network it is not possible to terminate every branch. The hub is responsible, for handling the quality of the signal on its own. If you are using a combination of network topologies you should follow the instructions provided in the hubs manual to properly terminate the connections.

A and B labeling varies. Manufacturer Pinout Variations: There is no universal standard for labeling RS485 data lines. If a device remains unresponsive, swapping the D+ (A) and D- (B) terminals is a safe, standard diagnostic step that will not cause electrical damage to the transceivers.

Quick Reference: When to Use What

A Final Thought

RS485 is a standard that works well when you use it correctly.. In real life things are not always perfect like they are in books. A hub helps you make RS485 work for your building of making your building work for RS485.

You should pick a hub that has the number of ports and can keep things separate like it is supposed to. If you wire it up carefully it will last for a time. When you are looking at options look for things that are made for industrial use like being able to work in very hot or cold temperatures being able to mount on a DIN rail and being able to keep things separate properly. Companies, like Valtoris have a lot of options that have these features.

*Note: This guide references the TIA/EIA‑485 standard and practical experiences from the field. Always consult your device manuals for specific wiring requirements.*

Frequently Asked Questions

Can I use standard CAT5/CAT6 Ethernet cable for RS485 wiring?

Yes, this is extremely common in retrofits, but you must wire it correctly. You must use a single twisted pair for Data+ and Data- (e.g., solid Blue for D+, striped Blue for D-). Never split the data signals across two different colored pairs, or you will lose the noise-canceling benefits of the twist. Also, note that CAT5 uses 24 AWG wire, which has higher resistance than thicker 22 AWG industrial cables, slightly reducing your maximum distance.

Do I need to connect the third Ground wire (GND), or is 2-wire enough?

While RS485 is often called a “2-wire” network, a third reference ground wire is highly recommended to equalize the common-mode voltage between devices. If the voltage potential between two buildings drifts too far apart, it can destroy the transceivers. If you cannot run a ground wire, you must use an Opto-Isolated RS485 Hub to electronically sever the ground loop.

What are Fail-Safe Bias Resistors, and do I need them?

When no device is transmitting, the RS485 bus goes “idle.” In noisy environments, this floating state can cause receivers to read false data (ghost characters). Fail-safe bias resistors force the D+ and D- lines into a known state during idle times. High-quality Active RS485 Hubs usually have built-in bias resistors, relieving you from having to calculate and add them manually.

Can I use a cheap passive splitter (T-junction) instead of an active RS485 Hub?

No. Passive splitters merely twist wires together, creating long, unterminated stubs that violate the TIA/EIA-485 standard. These stubs act as antennas for signal reflection. An Active RS485 Hub physically receives the signal and transmits a fresh, regenerated copy on every port, which is the only way to safely mix Star and Daisy-Chain topologies.

Do I need to manually set the baud rate with DIP switches?

No. Valtoris 4-port and 8-port RS485 Hubs feature self-adaptive baud rates ranging from 300 to 460800bps. This true plug-and-play design completely eliminates the hassle of manually adjusting tiny DIP switches in a dark control cabinet.

Does it matter which port I connect the master PLC to?

Yes, it is critical. Valtoris hubs are directional to properly isolate and route data. You must connect your Master device (e.g., PLC or PC) to the designated “RS485 main port” (labeled 485A and 485B). Your field devices must be wired to the extended “slave ports” (labeled OUT1, OUT2, etc.). Wiring them backwards will break the communication logic.

How do I handle termination resistors in a Hub-based Star network?

In a Star network routed through an Active Hub, the hub manages the signal integrity for each port internally. You generally should not add 120Ω resistors to the individual branches unless a specific branch is exceptionally long (e.g., acting as its own daisy-chain sub-segment). Always follow the specific manual of your hub.

Can RS485 be daisy chained?

Yes. In fact, daisy chaining is the strict, standard-compliant (TIA/EIA-485) method for wiring RS485 devices. A single continuous cable routes through all devices, terminating only at the two physical extreme ends. This prevents signal reflection and ensures reliable data transfer.

If I use an active hub to create a star topology, where do I put the 120-ohm termination resistors?

Because an active hub isolates each port, every branch effectively becomes its own independent RS485 network. You should place a 120-ohm termination resistor at the far physical end of each branch (at the last sensor or drive). Do not place resistors at the hub’s output ports, as the hub’s transceivers internally manage the origin point.

Can an RS485 hub connect legacy devices running at 9600 bps to a new PLC running at 115200 bps?

A standard RS485 repeater hub requires all devices on all branches to share the exact same baud rate and parity. However, if you are dealing with mismatched serial parameters, you need a Caching Hub. It acts as a data buffer, allowing you to set independent baud rates for the Master port and Slave ports.

If I don’t put termination resistor on RS485 what happens?

If you don’t put a termination resistor on an RS485 network, high speed electrical signals hitting the end of the cable will reflect back. This results in signal reflection (or ringing) which distorts the original data stream and causes CRC data corruption, ghost characters and intermittent Modbus communication timeouts especially on long distance links or high baud rates.

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