From DCS to OCS: A change in process control system

Compared to a traditional distributed control system (DCS), an industrial optical bus control system (OCS) has a lower investment cost of automation, shorter installation time and reduces control system maintenance costs.

By Stone Shi March 6, 2022
Stone Shi is executive editor-in-chief, Control Engineering China. Courtesy: Control Engineering China


Learning Objectives

  • Examine attributes of an industrial optical bus control system (OCS)and its technological advantages.
  • Identify OCS benefits.
  • Look at OCS cybersecurity protections.

Compared with a traditional distributed control system (DCS), the use of the industrial optical bus control system (OCS) can lower automation investment costs, shorten project time, and reduce control system maintenance expenses. That was the answer given by Jinsong Huang, vice president of Hangzhou Hollysys Automation Co. Ltd., while discussing Hollysys’ OCS industrial optical bus control system.

The DCS, since its birth in the 1970s, has many technical advantages and has become a standard in the fields of oil refining, petroleum, chemical industry, electric power, metallurgy and other large-scale process control industries. Because of this, the DCS has not changed much in the past 30 or 40 years.

Today, with the rapid development of technologies in information technology (IT), information and communication technology (ICT) and Internet of Things (IoT) have brought many societal changes. As optical communication technology and software have been introduced into DCS, the DCS has evolved into the OCS. In an OCS, the system architecture, the controller and input/output (I/O) connections have undergone qualitative changes, resulting in changes in project cost, space between cabinets and implementation cycles, outperforming a traditional DCS.

What is OCS? Why can it change a traditional DCS? What technologies does it use? What benefits can it bring to users? Which applications is it suitable for?

What is an OCS?

The biggest difference between OCS and DCS is optical fiber is used in OCS as the network transmission medium between the controller and I/O, instead of the copper-core twisted pair cable usually used in a DCS.

In a traditional control system, the signal transmission of the field equipment to the control system needs to go through a series of intermediate links such as field junction boxes, cable trays, terminal cabinets, safety barrier cabinets and I/O cabinets. The OCS changes the system architecture of the traditional DCS. OCS field devices are connected to the remote controller through the industrial optical bus data transmission unit (iDTU) located in the field, eliminating a series of intermediate links.

OCS industrial optical bus control system is composed of engineer station, operator station, redundant historical station, equipment management station, redundant control station, redundant industrial optical bus connection unit (RJU) and industrial optical bus intelligent data transmission unit (iDTU) and other equipment in which optical fiber is used for communication from iDTU to RJU and redundant control station.

The redundant control station uses triple redundant links, a pair of Gigabit Ethernet, a pair of RS-485 links, and a set of general purpose I/O connections (GPIOs). It ensures the redundancy and disturbance-free, functional safety and stable control of the control system, and the mean time between failures of redundant switching can be increased by 22.2% compared with controllers of other systems.

RJU is equivalent to the role of a switch. Each RJU can connect 16 or 32 iDTUs in a star shape, and adopts passive optical device technology. The physical iDTU is used to connect field devices and communicate with the control station through an industrial optical bus.

The iDTU adopts a modular design and has redundant virtual I/O (VIO) modules, redundant optical bus interface modules and redundant power supply modules. The VIO module supports six signal types: Analog input (AI), analog output (AO), digital input (DI), digital output (DO), process interface (PI) and Namur. iDTU also supports Modbus communication protocol, which can transmit field data to and third-party systems as a Modbus slave station.

What technological innovations does OCS have?

Installing OCS industrial optical bus control system is not as simple as replacing the original copper cables with optical fibers. OCS has patents for its technological innovation for signal input/output and transmission. Huang said OCS integrates industrial optical bus and software-defined I/O.

Industrial optical bus is a technological innovation application of Hollysys in OCS. Although the technology of using optical fiber to transmit the Ethernet protocol has been very mature, many technical difficulties still need to be overcome to transmit the industrial bus protocol on the optical fiber. Hollysys has developed the Onet redundant industrial optical bus based on optical communications experience in the control network Cnet on DCS. Different from the 7-layer architecture of Ethernet, Onet has only three layers: physical layer, data link layer and protocol layer. This concise layer improves the transmission efficiency. Onet complies with IEC 61158 international standard, JB/T 10308.3-2001 national standard and EN50170 European standard, supports star network topology, supports 128 node devices, communication rate is 24Mbps, transmission medium is single-mode fiber and transmission distance is 20 km (without relay).

Software-defined I/O is a technology that has emerged in the DCS field in recent years, and Hollysys is well-prepared in this field. Software-defined I/O is embodied in two aspects. One is to define the channel type of I/O through software. All channels of I/O are consistent in hardware, and it is not necessary to configure different I/O modules for different signals. It is only necessary to set the channel to the same signal type as the field instrument output through the software. The other is to switch the backup channel through software that is the channel redundancy mode of N+1. When the I/O channel fails, it is only necessary to perform software switching on the engineer station, and switch the faulty channel to the standby channel, without the need for maintenance personnel to arrive at the fault site.

What are the benefits of OCS?

The technical innovations of the OCS industrial optical bus control system have brought fundamental changes to the traditional process control system. The OCS also brings benefits to users.

First, OCS brings a substantial reduction in initial project cost. Use of optical fiber communication between the control station and the field data acquisition unit saves money compared to expensive instrument cables. The cost of intermediate links, such as cable trays, is also reduced. Industrial optical bus technology is more cost-effective than Ethernet-based fiber-optic transmission. With elimination of many intermediate links, later control system maintenance cost and the cost of spare parts decreases.

Second, the project implementation cycle of OCS is less. Thanks to the simplified OCS architecture, the construction volume is reduced. With software-defined I/O technology, the signal orchestration (marshalling) link between field devices and controllers in traditional engineering design is eliminated, shortening the project execution cycle. The standard engineering design simplifies the work processes, and the modularized standard chassis design shortens the supply and delivery cycle of manufacturers. On-site guidance and installation can be implemented upon arrival, easing efforts for users, system designers and manufacturers.

Third, the system’s reliability is higher. The application of industrial optical bus is superior in improving the anti-electromagnetic interference ability and lightning strike ability in the process of on-site signal transmission. One optical fiber can transmit at least 512 on-site signals, and the transmission distance can be as long as 20 km without relays, as mentioned. The OCS can realize redundant transmission of on-site signals, with higher transmission efficiency and stronger usability.

How does OCS ensure the security of the system?

System availability is very important since process control systems often are used in petrochemical, chemical and other key applications that require high continuous production. Huang said the OCS has a special design for reliability on the Onet industrial optical bus, RJU and iDTU.

In the Onet optical bus communication, redundant design is used between the slave controller and the iDTU, which ensures fast switching between the main and standby controllers. Also the entire optical bus link has real-time diagnosis functionality, which ensures communications reliability.

Information security of industrial control systems is facing more severe challenges. Advanced persistent threat (APT) attacks occur frequently, ransomware technologies are constantly being upgraded and historical loopholes are being exploited. Huang said the OCS has implanted the latest information security technology into the control system.

Trusted computing technology has been integrated into the OCS controllers. Different from the passive defense mechanisms of information security, such as firewalls and defense-in-depth of traditional industrial control systems, trusted computing technology realizes an active immune mechanism based on endogenous security. According to Jinsong Huang, the controller adopts a high-performance main processor and a co-processor based on the national secret algorithm, and achieves three information security defense strategies: static trusted startup, dynamic trusted measurement and trusted firmware update.

Stone Shi is executive editor-in-chief, Control Engineering China. Edited by Mark T. Hoske, content manager, Control Engineering, CFE Media,

KEYWORDS: Distributed control system (DCS), industrial optical bus control system (OCS) 


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Stone Shi
Author Bio: Executive editor-in-chief, Control Engineering China