Automation Control Networks: The Field Bus

Automation Control Networks: The Field Bus

Gerry CellucciApr 1, '21

As control systems evolved from centralized systems to distributed systems, the need arose to interconnect distributed input and output points over a communications network. Individual controllers were joined together via a 2 or
3 wire network, usually in a “daisy chain”.
These 2 or 3 wire networks, called Field Buses, are important as the trend to distribute control to the sensing/actuating devices continues. There are numerous Field Bus networks on the market, each with their own benefits and drawbacks, but the future of control networks looks to be based upon internet standards.

As building control systems transformed from a pneumatic industry to electronic industry -Electronic Control Systems were born.

Originally, the cost of computing power was high, electronic control systems tended to consist of a large central computer “home run” wired to sensors and actuators. It was common to see “fist” sized bundles of wires running hundreds
of feet from a control panel to the actuators and sensors used in the control loop. While providing effective control, these systems were difficult to maintain and almost impossible to modify or upgrade.

As the price of computer processing power dropped, it began to make sense to distribute the control functionality closer to the actual points being
controlled. Small islands of control started to form, centered around mechanical equipment. For example, one control device might control an Air Handling Unit (AHU) and three other devices might control three floors of a building that the
AHU serves. These control devices needed to share information and, therefore, networked together over a Building Automation Bus.

These days the Automation Bus can be referred to an Operational technology (OT) network.

These controllers, typically called Unitary controllers, offered electronic control of small room level mechanical equipment like Variable Air Volume (VAV) boxes, Fan Coils, and Heat Pumps. With control distributed to the room, a typical
building could have hundreds of unitary controllers. Managing the data in these controllers lead Building Automation system designers to create a two-tier hierarchical network architecture. A Building Automation Bus, linking together
Network controllers, formed the top tier of this network architecture. Unitary controllers were then linked to Network controllers over the second tier network, called a Field Bus.

The major characteristic of a field bus is its ability to be cost effective for connecting distributed Input and Output (I/O) points. This has led to moderate performance characteristics; bus speeds of less than 100 kbs, fewer than 100 devices per bus segment, and cable distances of less than a mile.

The focus of these buses has been on Open Standards. Where at one time it was acceptable (even preferable) that each building controls manufacturer had its own field bus, the market direction has changed to where it is preferable to have a single field bus supported by multiple manufacturers.

These requirements have led to a few dominant open field buses.


BACnet had an almost immediate impact on- and has come to dominate - the  controls industry. BACnet is a communication protocol for Building Automation and Control (BAC) networks that leverage the ASHRAE, ANSI, and ISO 16484-5 standard protocol.


The Local Operating Network (LON) was developed to allow open interoperable communications between control devices. It has applications within home and industrial control but has been most widely adopted in building control systems. The communications protocol is designed and owned by Echelon Corporation and is an American National Standards Institute (ANSI) standard. The protocol provides peer-to-peer communications between devices.


Modbus is a data communications protocol originally published by Modicon (now Schneider Electric) in 1979 for use with its programmable logic controllers (PLCs). Modbus has become a standard communication protocol and is now a commonly available means of connecting industrial devices.

Modbus is popular in industrial environments because it is openly published and free. It was developed for industrial applications, is relatively easy to deploy and maintain compared to other standards, and places few restrictions - other than the datagram (packet) size - on the format of the data to be transmitted.

Modbus communicating equipment is also common in the HVAC marketplace.

Johnson Controls N2 OPEN Bus (LEGACY)

Often mistaken as a proprietary field bus,  the Johnson N2 OPEN bus links controllers and point interfaces to a Network Panel (NP). The N2 Bus uses a master/slave protocol, in which the master device, the NP, initiates all communication with the N2 Bus devices. The field bus allows up to 100 devices connected using 2 or 3 wires over a distance of up to 4572 m.

With over 300 products from third-party manufacturers that are N2 compatible, the N2 protocol arguably had the greatest product depth of the current HVAC field buses. The fact that it was developed and is owned by Johnson Controls has limited its wide spread adoption by other HVAC controls companies.

Older N2 field bus controllers can be replaced by the newer Facility Explorer field controllers that can be flashed for N2 or BACnet communication - allowing legacy N2 controllers and BACnet controllers to co-exist o the same automation network.

The Serial MSTP Field Bus is being supplanted by the Information Technology (IT) Network (the corporate intranet). As the costs of networking continue to decrease, the Field Bus may also be assimilated into the IT and OT infrastructure. To be fully supportable within an IT infrastructure, the future of BAS networks will support the communications standards used on the internet (WEB SERVICES).