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What are communication means available for networking industrial field devices?

Mumbai University > Electronics Engineering > Sem 7 > Embedded System Design

Marks: 10 Marks

Year: May 2016

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PROFIBUS (Process field bus):

  1. PROFIBUS (Process Field Bus) is a standard for fieldbus communication in automation technology.
  2. There are two variations of PROFIBUS in use today; the most commonly used PROFIBUS DP, and the lesser used, application specific, PROFIBUS PA:

    a. PROFIBUS DP (Decentralized Peripherals) is used to operate sensors and actuators via a centralized controller in production (factory) automation applications. DP acts as a backbone network for transmitting process signals to the controller.

    b. 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. The Physical Layer 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.

PROFIBUS Technology:

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Application layer: To utilize these functions, various service levels of the DP protocol were defined:

  1. DP-V0 for cyclic exchange of data and diagnosis
  2. DP-V1 for acyclic data exchange and alarm handling
  3. DP-V2 for isochronous mode and data exchange broadcast (slave-to-slave communication)

Security layer: The security layer FDL (Field bus Data Link) works with a hybrid access method that combines token passing with a master-slave method. In a PROFIBUS DP network, the controllers or process control systems are the masters and the sensors and actuators are the slaves.

Bit transmission layer: Three different methods are specified for the bit-transmission layer:

  1. With electrical transmission pursuant to EIA-485, twisted pair cables with impedances of 150 ohms are used in a bus topology. Bit rates from 9.6 kbit/s to 12 Mbit/s can be used. The cable length between two repeaters is limited from 100 to 1200 m, depending on the bit rate used. This transmission method is primarily used with PROFIBUS DP.
  2. With optical transmission via fiber optics, star-, bus- and ring-topologies are used. The distance between the repeaters can be up to 15 km. The ring topology can also be executed redundantly.
  3. With MBP (Manchester Bus Powered) transmission technology, data and field bus power are fed through the same cable. The power can be reduced in such a way that use in explosion-hazardous environments is possible. The bus topology can be up to 1900 m long and permits branching to field devices (max. 60 m branches). The bit rate here is a fixed 31.25 kbit/s. This technology was specially established for use in process automation for PROFIBUS PA.

Modbus

  1. Modbus is a serial communications protocol for use with its programmable logic controllers (PLCs).
  2. The main reasons for the use of Modbus in the industrial environment are:

    a. developed with industrial applications in mind

    b. openly published and royalty-free

    c. easy to deploy and maintain

    d. moves raw bits or words without placing many restrictions on vendors

  3. Modbus enables communication among many devices connected to the same network. Modbus is often used to connect a supervisory computer with a remote terminal unit (RTU) in supervisory control and data acquisition (SCADA) systems.

Protocol versions of Modbus:

  1. Modbus RTU is used in serial communication & makes use of a compact, binary representation of the data for protocol communication. The RTU format follows the commands/data with a cyclic redundancy check checksum as an error check mechanism to ensure the reliability of data.
  2. Modbus ASCII is used in serial communication & makes use of ASCII characters for protocol communication.
  3. Modbus TCP/IP or Modbus TCP is a Modbus variant used for communications over TCP/IP networks, connecting over port 502. It does not require a checksum calculation as lower layers already provide checksum protection.
  4. Modbus over TCP/IP or Modbus over TCP or Modbus RTU/IP — This is a Modbus variant that differs from Modbus TCP in that a checksum is included in the payload as with Modbus RTU.

Communication and devices:

  1. Each device intended to communicate using Modbus is given a unique address.
  2. On Ethernet, any device can send out a Modbus command, although usually only one master device does so. A Modbus command contains the Modbus address of the device it is intended for (1 to 247).
  3. Only the intended device will act on the command, even though other devices might receive it (an exception is specific broadcastable commands sent to node 0 which are acted on but not acknowledged).
  4. All Modbus commands contain checksum information, to allow the recipient to detect transmission errors.
  5. Different implementations of modbus use wireless communication, such as in the ISM band, and even Short Message Service (SMS) or General Packet Radio Service (GPRS). One of the more common designs of wireless networks makes use of mesh networking. Typical problems that designers have to overcome include high latency and timing issues.
  6. A Modbus frame is composed of an Application Data Unit (ADU) which encloses a Protocol Data Unit (PDU):

$$\text{ADU = Address + PDU + Error check} \\ \text{PDU = Function code + Data}$$

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