Keep the Petals on: Optimizing Daisy Chain Wired Communication

May 2, 2018 Altium Designer

Daisy chain wiring

 

As a child, I was fascinated by trains. I remember staring down an empty railway for hours, simply waiting for a train to pass by. From my youthful perspective at the time, it was magical how the train never fell off the track despite essentially being a moving chain consisting of many parts.

 

Although I eventually outgrew my obsession with trains, I became enamored by the daisy chain style of wiring for communication in electronics. As my career as an embedded system designer took off during a period where wireless technologies for industrial electronics had yet to mature, wired communications were the only choice. Learning how to optimize daisy chain wired communication, however, was another matter.

What is Daisy Chain Connection?

A daisy chain connection has little resemblance to a daisy flower, at least not visually. In electronics, a daisy chain connection involves connecting multiple devices sequentially in an uninterrupted cable connection. Depending on the requirements, the last device on the loop may or may not connect to the first.

 

The opposite of daisy-chain connection in electronics is the star connection, wherein every slave device is connected to the master with an individual cable. You’ll commonly notice daisy chain wiring in serial communications like RS485, where individual devices are connected sequentially.

 

For communications standards like RS485, daisy chain wiring is the only suitable wiring method when you have a large number of devices to deal with. Star connections would simply not work, as multiple stubs of connections result in reflections that affect signal integrity.

Advantages of Daisy Chain Wiring

When starting a design that involves daisy-chaining at the application level, you need to weigh the pros and cons of the wiring method and mitigate any potential issues beforehand. Otherwise, you may risk losing connections with half of your devices and lacking any recovery methods.

 

Daisy chain connections involve the smallest length of cable used. Compared to star connections, this equals faster installation time and cost. On paper, a daisy chain connection may look like a ring, but in practice, it is hardly ring-like. You may have multiple devices spread out at irregular distances and placed over a closed area, which results in the costly installation of star connections.

 

Daisy chaining means planning the shortest wiring path for all devices involved. As for a star connection, each device needs to have an individual cable back to the master controller. The cost of cable and installation alone can normally discourage clients.

 

For example, imagine a master controller in a server room. If it’s connected in a star topology to twenty monitoring devices in the manufacturing area, it would need twenty times more cable than if it were using a daisy chain connection.

 

Wires daisy chained together

Daisy chain wiring helps in keeping your accounts healthy.

 

Downsides of Daisy Chain Wiring

You’ve probably realized that there’s always a catch in the convenience offered by any solution. For example, the latest smartphone is packed with interesting features but would probably break at the slightest drop. Similarly, while daisy chain wiring saves on cost and installation time, a single cable fault can potentially cause total system disruption. For example, a cable breaking between the master and the first device would result in communication loss across all devices.

 

In such a case, a repair technician would likely attempt to trace the fault on the cable between a functional and an unresponsive device. However, troubleshooting could get more complicated if the communication integrated circuit (IC) on the device itself is damaged because a single damaged IC can cause an entire communication bus to malfunction.

 

Locating a faulty device on a daisy chain wiring is far more challenging than using a star connection. These major system breakdowns are the common problems involved in daisy chain wiring.

Minimizing Issues With Daisy Chain Connections

One may argue that the best way to eliminate the dependency of the entire bus on a single sub-connection or device is to use a star connection. However, that can reduce versatility and is simply not feasible with communication standards like RS485, which leaves two possible options.

 

The first option is to entirely discard wired communication and opt for wireless mesh network topology like Zigbee. Not only does this method eliminate any dependency on a single device, but the Zigbee standards also allow for a self-healing algorithm where a new path is automatically established if one of the devices fails.

 

WIFI mesh network

Redundancy can save your daisy chain connection from total failure.

 

However, if a daisy chain connection has to be used and the tolerance for system downtime is limited, a redundant secondary communication port can be created at the master controller. The daisy chain connection will have the last device on the loop connected to the master controller’s redundant port.

 

When the system can function without any issues, the redundant port only listens passively to the packets transmitted on the bus. But when a fault occurs on any part of the connection, the redundant port becomes immediately aware, as no transmission is detected. This puts the redundant port in ‘active’ mode and polls devices from the other end of the connection.

 

Of course, this solution only performs beautifully when the issue is related to broken daisy chain connections. It does not work as well when the bus is dysfunctional due to a damaged IC. The best way to minimize the possibility of a damaged IC is placing relevant surge protection components into your design. With multi-faceted PCB design software like Altium Designer, adding these key components is a simple process that can save you unnecessary problems in the future.

Need more tips for implementing redundancy in daisy-chained hardware? Contact an expert at Altium.

About the Author

Altium Designer

PCB Design Tools for Electronics Design and DFM. Information for EDA Leaders.

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