Industrial Networks (field bus networks)


Industrial BUS networks (or fieldbus) are used to connect field devices such as distributed I/O (input/output) stations or drives and other HMI (Human Machine Interface), which are distributed spatially, to automation systems.

According to WikiPedia, a field bus is the name of a family of industrial computer network protocols used for real-time distributed control, now standardized as IEC 61158. 



Side note: 

IEC 61158 specification

There were many competing technologies for fieldbus and the original hope for one single unified communications mechanism has not been realised. This should not be unexpected since fieldbus technology needs to be implemented differently in different applications; automotive fieldbus is functionally different from process plant control. The final edition of IEC standard IEC 61158 allows 8 technologies. These are the some hierarchic layer of the automation protocols.
IEC 61158 consists of the following parts, under the general title Digital data communications for measurement and control – Fieldbus for use in industrial control systems:
  • Part 1: Overview and guidance for the IEC 61158 series
  • Part 2: Physical Layer specification and service definition
  • Part 3: Data Link Service definition
  • Part 4: Data Link Protocol specification
  • Part 5: Application Layer Service definition
  • Part 6: Application Layer Protocol specification


Discussed here (very shortly, since each protocol can be discussed AT LENGHT) are the field bus protocols I have personally worked with in manufacturing plants during my career over the past 6+ years.

Interbus-S
Intebus was developed by Phoenix Contact between 1983 and 1987 with  the idea to create a standard for industrial field bus systems.  It is fully standardised under EN 50254 and IEC 61158.

The INTERBUS-S works as a distributed, regenerative shift register, where the various periphery components are connected to a connector board in a ring topology.

CAN bus (Controller Area Network)
This protocol was designed for communication between microcontrollers and devices in automotive applications, but is also used for industrial applications. CAN bus has many physical layer standards, one of which is ISO11898-2:2003.  One big advantage of CANbus is that the number of nodes is not limited by the protocol.

It works on a two wire (CAN-High and CAN-Low) design, and has high immunity to electromagnetic interference.  The bus logic uses a "Wired-AND" mechanism. This means there are "dominant bits" (equivalent to the logic level "Zero") and they overwrite the "recessive" bits (equivalent to the logic level "One" ).  This is shown in the digram below:

 


Profi Bus (Process Field Bus)
It is another standard for field bus communication and is classified as a Type 3 protocol as stipulated by the IEC 61158.

There are two PROFIBUS variations in use today, namely: PRODIBUS DP (Decentrilised Perephirals) and PROFBUS PA (Process Automation).

Here is a clear and basic description each taken from Wikipedia on Profibus:
  • PROFIBUS DP (Decentralized Peripherals) is used to operate sensors and actuators via a centralized controller in production (factory) automation applications. The many standard diagnostic options, in particular, are focused on here.
  • PROFIBUS PA (Process Automation) is used to monitor measuring equipment via a process control system in process automation applications. This variant is designed for use in explosion/hazardous areas (Ex-zone 0 and 1). The Physical Layer (i.e. the cable) conforms to IEC 61158-2, which allows power to be delivered over the bus to field instruments, while limiting current flows so that explosive conditions are not created, even if a malfunction occurs. The number of devices attached to a PA segment is limited by this feature. PA has a data transmission rate of 31.25 kbit/s. However, PA uses the same protocol as DP, and can be linked to a DP network using a coupler device. The much faster DP acts as a backbone network for transmitting process signals to the controller. This means that DP and PA can work tightly together, especially in hybrid applications where process and factory automation networks operate side by side.




Sercos (serial real-time communication system)
SERCOS was developed as a for applications where hard real-time, high performance communication is needed between industrial motion controls and digital servo drives. It is classified as standard IEC 61491 and EN 61491.

ARCnet (Attached Resource Computer NETwork)
ARCnet is a LAN network protocol similar to Ethernet and Token Ring and uses RG-62/U coax cable of 93Ω impedance.  ARCnet had the advantage, over Ethernet, that it's runs could be ...

DeviceNET
As an added extra, one i haven't worked with. DeviceNet uses CAN as it's backbone technology.  Typical applications include information exchange, safety devices, and large I/O control networks.

To conclude... I obviously bit off more than I could chew when I started this post and did not realise how much information there is to share about each field bus protocol.  I think this calls for a series of posts revolving around field bus protocols and some more technical details about each one...
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