A daisy chain for field instrumentation is a wiring scheme that reduces the amount and cost of cable between sensors, displays, and controllers. Not all types of outputs are supported on a daisy chain. Specifically, it is for sensors that use RS-485 communication and Modbus RTU protocol.
What is a Daisy Chain?
A daisy chain is a single cable that connects to multiple sensors. No return cable is needed to form a loop. It is very common in electrical wiring for the same reason we chose it for our Tank Cloud remote monitoring platform – it’s efficient and economical. Other examples of a daisy chain include Christmas lights, the phone system, and household electrical fixtures.
What is Modbus?
To put it simply, Modbus is how the sensors, displays, and controllers talk (and listen) to one another. It’s a protocol that determines how data is packaged and sent. Each device on the network has it’s own address that can be queried one at a time by a master device. It makes the daisy chain possible.
In addition, it allows us to deploy a network of level sensors and pressure transmitters for remote monitoring in an effective an inexpensive package. The daisy chain is a key part of this as it makes installation very easy and much less expensive.
What type of wire is used in a Daisy Chain?
A cable suitable for a daisy chain has very few requirements. It should be a twisted pair. While the name may suggest two wires, the cable actually has five – 2 for communications (twisted together), 2 for power (positive and negative twisted together), and 1 drain wire that connects the shield to ground. This cable is much less expensive than many of the alternatives, and is widely available.
However, for best performance with our Modbus RTU sensors, the cable should be well shielded and have an impedance of 120 ohms.
So how is a Daisy Chain wired?
This is the easy part. A daisy chain is wired by matching wires (red to red, black to black, etc) as you junction with each level sensor or pressure transducer. It is recommended that each sensor connect to a small junction box with terminal blocks. Here the wires can be inserted easily into the appropriate place.
Proper grounding is important for protection from both electrical noise and electrical surge. Be sure to connect the drain wire along with the other four. In addition, a 120-ohm termination resistor should be wired between the communications wires at each extreme end of the daisy chain.
Using a shielded twisted pair cable that is properly wired and grounded will avoid many issues that can cause application failure.
Can I just wire everything back to a single point?
Yes, but this quickly gets out of hand. Connecting two level or pressure sensors to a single point – either a master sensor or a controller – should not be a problem. However, doing so with five or more sensors is problematic.
First of all, you forfeit the clear benefits of the daisy chain. Secondly, fitting all the wires together at once is a mess. Finally, there are more communication issues with this method, and it will shorten the overall distance of the network. So it can be done if you only have a few sensors – but we don’t recommend it.
How does this approach differ from others?
One of the biggest advantages for using the Tank Cloud remote monitoring platform instead of one of the several competitors is the ability to network several sensors to a single Internet connection (hint – using a daisy chain). We designed this system from the ground up for remote tank farms.
In contrast, other systems use expensive hardware, or are designed to collect data from a single remote tank – which costs more in data fees. Using the Modbus protocol and daisy chain wiring, the Tank Cloud remote monitoring system reduces cost in both implementation and in long-term data fees.
Let us know if you still have questions about how to wire a daisy chain, or why it even matters. It may seem like a detail, but a “solution” is a poor solution if it’s difficult and costly to implement.