Take advantage of Fieldbus

Fieldbus is the name of a family of industrial computer networks used for real-time distributed control.

The communication between control station, operating terminals and field devices simplifies the start-up and parameterization of all components. The communication functions allow diagnostic data, which are provided by up-to-date field devices, to be evaluated.

The essential objectives in fieldbus technology are to reduce installation costs, save time and costs due to simplified planning as well as improve the operating reliability of the system due to additional performance features especially by complex systems with a large number of in-/output signals.

Fieldbus systems are usually implemented in new plants or existing plants that must be extended.

Fieldbus is simply a means of communicating with input devices (sensors, switches, etc.) and output devices (valves, drives, indication lamps etc.) without the need to connect each individual device back to the controller (PLC, Industrial PC etc.). It allows two-way communications between field devices and control systems thanks to a simple All or Nothing type data or complex analog type data (setpoints, alarms, etc.).

Therefore, overall, Fieldbus can reduce costs.

Fieldbus Networks’ Topologies

Topology involves the manner in which the fieldbus devices are connected to the data highway. There are several possible topologies that are employed as per the needs of the plant geography. Topology options include : Ring, Branch, Star and Daisy Chain.

Ring Topology

Each node in a ring topology connects to exactly two other nodes.

Branch Topology

It is a hybrid topology that includes star networks that are interconnected via daisy chain topologies.

Star Topology

Also known as a spoke and hub configuration, in this setup devices are connected not to one another but rather to a central master/controller or hub. So messages can’t be passed from one device to another directly but must go through the central master/controller.

Daisy Chain Topology

The devices are series connected in a particular segment.Device connections to the segment should be such that dis-connection of a single segment would not lead to total isolation of that segment.

Advantages and Disadvantages of Fieldbus

Benefits of Fieldbus : Take advantage of Fieldbus

• Reduced Cabling Requirements
• Ease of Installation
• Maintenance and troubleshooting much easier
• Increased Reliability : Self-diagnosis of the system – for example, in the event of sensor / actuator failures
• Allows an information exchange that would be difficult otherways
• Offers flexibility in the design and installation of an automation system.
• Ease of future expansions or modifications

Fieldbus Disadvantages

• More sensitive to wiring and architecture faults
• Complexity: As a fieldbus represents a complete system, qualified personnel are required for its use.

What is a Fieldbus ?

Fieldbus has several different “layers.”

Level 1 : Physical layer

Physical layer is the Fieldbus layer connected with instrument devices in the field means how the data are transferred from one point to another point on the network.
Fieldbus allows options for three types of communication media at this layer, namely, Wire, Fiber-optic and Radio :

1-Wires :

Twisted pair copper cable is the most popular and cheapest medium in use.
Main serial communications are RS232, RS485 and CAN.

2- Fiber optics :

The fiber optics layer uses light to transmit data. Fiber optics operation begins with an electrical signal which is converted into light, transmitted along the fiber optic cable and then converted back from light to an electrical pulse at its destination.
Light can go both ways along the cable, travels very quickly, and can transmit the signal very long distances over high quality fiber. Fiber optics are also ideal for areas with much ambient electrical noise.

Advantages :
• extends network over large distances
• solves grounding problems
• effective in high noise applications
• offers more versatile topologies

3- Wireless :

Wireless solutions available on the market are radio and infra-red technologies.
Wireless technology can save on physical wiring costs. Laying wires from one device to another costs money, both in the actual wire used and in the labor to lay it properly.
Wireless is also the only option for some applications where it can be difficult, if not impossible, to run the wires, such as near rotating machines or through barriers such as roads.

To sump up, here are the Advantages :
• extends range
• solves grounding problems
• saves on wiring costs
• excels in applications where it is difficult or impossible to run wires

Every kind of Fieldbus has its physical layers and inside this there are components.

Level 2 – Data Link Layer

The Data Link layer is the protocol layer that transfers data between adjacent network nodes in a wide area network or between nodes on the same segment. The Data Link Layer provides the functional and procedural means to transfer data between network entities and provides the means for error detection and correcting events that occur in the physical layer.

The way the pulses are organized in a period of time make “frames”. Inside the frame there is the data. Frames structure and packets of frames correspond to the “language” of the fieldbus : the Protocol (Protocol : Means dialogue’s rules between the same comunicating entities’ layers).
For example, for Modbus you have 8 pulses (1 byte = 1 pulse)
As there could be some disturbances on the system and all these words could be disturbed. The total sentence must be true and then you have a control at the end. If there is something wrong in the CRC it means there is something wrong in the message : the salves will ask the Master « please tell your message again ».

Celduc’s ECOM module is with Modbus.

How to choose a FieldBus ?

There are hundreds of Fieldbuses available on the market.

Here are the criteria to choose the Fieldbus :

1.  The equipment : some manufacturers promote their fieldbus – In most cases, this is the equipment (the PLC for example) used which give the Fieldbus which will be chosen. In fact if you want another Fieldbus , you need to buy a card and the additional cost might be high.
2.  The physical configuration (distance,…) : machine to machine. The type of bus depends on the length of the wires. The longer are the wires more errors you could have : system protocol which can operate properly
3.  The quantity and speed of data : This can require high speed or local data storage. For real in time process you will need to have real-in-time communication with a specific modbus (the date to manager will be bigger)
4.  The service : webserver, … the number of local service you want on the device
5.  The cost per node :
   – Some proprietary fieldbuses requires certification and fees. The advantage is the software development is more secure and certified compatible with many devices. Modbus is the cheapest on the market.
   – It exist open fieldbus (CANOPEN, IO-LINK,…) mainly managed by associations (no fees), they are very flexible, but the development cost is higher and more risky especially if a certification is necessary.

Examples of Industrial Fieldbuses

–RS485 based connection with protocol MODBUS or CANOPEN (faster) are the cheapest and well adapted to sensor or relays
–IO-LINK (3 wires serial communication) is a free (no fees) new candidate very used for sensors and actuator
–Ethernet based fieldbuses makes the cost of each node too high and oversized in data to be used for simple relays.

Celduc’s products

Today, it is clear that communication and safety are our two biggest concerns ; these issues will challenge us further as we head into the future…

Solid state relays with Modbus Protocole :

SUx series with power and temperature controller ECOM (RS485/Modbus RTU serial Link).

More information needed ? Read our last blog article introducing celduc ECOM Module.