According to our definition local communication concerns everything from extending the distance between two serially connected units, to the creation of an industrial network via fibre or RS-485. The common denominator for these is they have there own wiring. Over the years our range of equipment for local communications has grown and today we can offer solutions for virtually all applications irrespective of media.

Fibre

Fibre optic communications involve the transfer of light signals through thin glass fibre or plastic cable. Historically, copper cable has been used in industrial networks. Fibre optic cables were traditionally expensive to install and difficult to terminate. Instead of conducting electrical signals, which is the case with copper cable, fibre optic cables propagate light signals. A fibre optic cable has a light-propagating central core of glass. The core is surrounded by a thicker layer known as the cladding.

The cladding acts as a reflector around the core, which results in the light signals being transferred through the cable. Equipment for fibre optic transmission converts the electrical current to light signals, these are transmitted into the cable using LEDs or a laser and are received with a photodiode.

There are two types of fibre cable, single-mode and multi-mode, the most appropriate cable to use depends on conditions such as transmission range and data rate. Single-mode fibre has a very thin core, 9–10 µm. The transmitter is usually a laser with a wavelength of 1300 or 1550 nm, this permits high speed transfers over long distances. This is due to the light pulse not becoming distorted in the cable, as it is the only mode being transferred and it is not affected by other modes reflected in the cable.

Multimode fibre tends to have core diameters of 50 or 62.5 µm. More distortion of the light pulses mean that transmission distances are much lower than for single mode fibres. Components for multimode systems tend to be much cheaper than those for single mode thus compensating for the lower transmission ranges.

The greatest advantage of the fibre optic cable is that it is immune to electrical and magnetic interference. Consequently, it is highly suitable for harsh industrial environments, guarantees secure transmission and has a very high transmission capacity.


Fibre optic cable structure

Fibre optic cable consists of:

• Core
  This is the medium for the transfer of optical (light) signals and varies from 5 to 200 µm in diameter.
  
  
• Cladding
  The optical material that surrounds the core and which causes the light in the core to reflect. The cladding increases the diameter of glass fibre to between 125 and 235 µm.
  
  
• Connectors
  There are many connectors available on the market for fibre optic cable, however there are four main connectors that are used for professional installations, these are:

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Point-to-point

A point-to-point connection is a dedicated communication link between the transmitter and receiver.Using two fibre cables a connection is established in both directions, this then allows full duplex communication.There are several options depending on the interface of the connecting unit, RS-232 or RS-422/485. Interface conversion from RS-232 to RS-422/485 is also possible via the fibre cable.

Multiplexers

Multiplexing involves assigning each serial connection a time-slot where each channel is a point-to-point connection to the opposite side.The serial rate of respective channels can amount to 38.4 kbit/s and communication is then fully transparent. In multiplexing several channels are transferred on a common fibre connection and as several units can communicate on the same cable this is cost effective. The fibre is completely insensitive to external interference, which makes it ideal for applications in exposed situations and in environments with high levels of interference. Long transfer distances are possible, up to 3.5 km (2.17 mi), using fibre despite the transfer rate on the common line being high.

Multidrop/ring

Multidrop or ring connected networks require that there must be a master that polls all the slaves. Selection of the topology depends on the installation options and the selected level of security.
A ring network offers high reliability as failure of a fibre link does not affect communications. Should this happen the modem automatically re-establishes the connection. Digital alarm outputs are also integrated in the modem, which can be connected to an external relay for control of alarm indicators. Serial equipment is connected via RS-232 or using RS-422/485, the ring is connected together via fibre, which guarantees a secure conenction insensitive to interference.
In a bus network all units are connected in series, different variants can be chosen depending where on the chain the modem is installed. Consequently there is equipment for the start/end points as well as units with two fibre interfaces, which forward the fibre connection and also act as a repeater.
We have also developed special products that implement supplier specific protocols. As all units have a fibre optic line interface all connections are fully immune to all types of external interference, for example, lightning.

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Fieldbuses, M-Bus
and LONWORKS®

Fieldbuses

There is a tendency for the need to transfer data between different equipment to constantly increase. in a fully integrated, industrial communication solution, three hierarchical communication levels are usually involved. For example, on a field level communications take place between transducers, I/O-modules usually to a PLC unit. On the automation level several PLC units can be connected to a master control system, which in turn can be connected to a comprehensive administrative system.

Different types of communication protocols and electrical interfaces are used on a field level depending on the application. LonWorks® is a common example of a communication standard within building management, M-bus is used to read different types of meters (electricity/water) while PROFIBUS and Interbus are standard protocols within industrial process automation.

The most common electrical interface on a fieldbus level is RS-485. This is a bus oriented interface using 2-wire transfer which makes it well suited for industrial applications as the balanced transfer technology is relatively insensitive to external electromagnetic interference. This interface is used by protocols such as PROFIBUS and Interbus.

Historically these protocols were independent, however, there are now demands for the retrieval of comprehensive management information at an administrative level, to allow for instance economic forecasting. Consequently, we can see Ethernet, which is an international standard, used more and more on all levels as the “carrier” of data between different pieces of equipment as well as between the different communication levels.

Westermo fieldbus units are industrial PROFIBUS DP-modules. There are different types of slave modules, a master module and a field bus converter making it possible to transmit PROFIBUS DP I/O data via serial communication. This can be done via PSTN, GSM, ISDN, or radio modems and via an Ethernet network without using special drivers or a PLC card.
Our PROFIBUS DP master module works in the same way on the serial side as other adapters, but acts as a master on the PROFIBUS DP-side. Furthermore, in combination with our Ethernet adapters it is possible to connect PROFIBUS DP to Ethernet. The PROFIBUS DP converter is used to establish a transparent connection with different serial interfaces to a PROFIBUS DP network.

M-Bus

M-Bus (Meter-Bus) is a European Standard for remote interrogation of heating or electricity meters. M-bus can also be used for other types of consumer meters.
The M-Bus interface is designed for communication across 2-wires. Our adapter
provides numerous functions in a single unit, offering great flexibility when constructing a network. For example, you can use the adapter as a converter from RS-232 to M-Bus, or as a repeater to increase the communication distance.
The adapter can be used to expand the M-Bus network with a modem.

LonWorks®
Echelon Corporation has, through the introduction of LonWorks® technology, provided a complete platform to develop openly distributed control systems based on an intelligent network architecture. A LonWorks® system usually consists of a number of intelligent devices, known as nodes, where each node manages a specific task, for example, measuring a temperature or controlling a valve. The nodes exchange essential information with each other via the network. The nodes do not normally send commands to each other, but exchange data packets that contain information about e.g. the temperature, pressure, status, date and time. The nodes can then use the information in the data packets in different ways depending on the specific function of the node.

Within LonWorks® these data packets can be seen as global variables available on the network and in view of this have been called network variables. When a node updates a network variable, this is automatically sent out on the network so that other nodes become aware of the new value. Interoperability is a keyword in LonWorks® technology. One of the conditions for interoperability is that nodes from different manufacturers exchange and understand data without requiring any special adaptation of either the software or hardware. In order to conform to this it is not sufficient to just be on the same network, to have the same type of transceiver and be able to send network variables. Nodes also need to understand the contents of the network variables.

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Short-haul modems, line sharers,
switches and converters

Industrial interfaces

Local communication in industrial applications is very common, however, many parameters need to agree these include: Interface type, transmission rate, transfer distance or which transmission media should be used.

One of the most common interfaces for data communication is RS-232, which usually uses a 9/25 way D-sub connector. In conformity with the RS-232 standard, the cable between the connected devices must not exceed 15 metres (50 ft). In order to attain longer transmission distances you can use different modems, depending on the requirements and communication media, for example, fibre, copper or leased line. All these variants and combinations have resulted in a large selection of products adapted to a specific purpose.

Converter RS-232 to RS-422/485

RS-422

RS-422 is an ideal standard for industry as the interface is created to build data buses, typically multidrop, between central computers and a number of substations. The interface is balanced and relatively insensitive to interference. RS-422 was originally designed to handle 10 devices, but today can be expanded to 32 units. The recommended maximum distance is 1 200 m (4000 ft) at a transmission rate of 100 kbit/s. RS-422 utilises a 4-wire connection for communication between master and slaves, which permits full duplex communication.

RS-485

RS-485 is a further development of RS-422 and is now the more common of the two. RS-485 uses 2-wire in different master/slave systems, where each slave is addressable. In a 2-wire solution the data direction must be controlled by a handshaking signal (RTS/DCD) alternatively the device can change the direction with the help of the data flow. A prerequisite to connect devices on a common bus is that the equipment must handle “tri-state” signals, i.e. listening mode.

When a single device is in listener mode you must govern the line to a standard specified state, this is done by failsafe terminating the line.

Repeater for RS-422/485

There are three reasons to use a repeater.

1. According to the standard the maximum transfer distance in a segment is 1200 metres (4000 ft).
   When the network covers a greater distance, a repeater is installed , the signal is regenerated and a further 1200 metres (4000 ft) can be covered.
   
   
2. A maximum of 32 loads can be connected on a segment.
   
   
3. RS-422/485 must be installed as a bus network, otherwise communication interference will occur sooner or later. When there are branches on the network,
   it should be segmented using repeaters, and in doing so the bus structure in the installation is maintained.

Short-haul modems point-to-point

Range and short-haul modems

As mentioned earlier, the RS-232 standard does not recommend cabling longer than approximately 15 metres (50 ft). Short-haul modems are used to allow longer links to be made. These convert RS-232 to electrical or optical signals that are adapted for transmission over greater distances (up to several kilometres/miles) with fixed 4-wire cable or fibre. The recipient short-haul modem then converts these back to an RS-232 signal again, the connection is fully transparent, which means you can see the modems as an extension of the RS-232 interface.

10 mA balanced current loop (W1)

Westermo has developed its own transmission technology for short-haul modems that ensures communications over greater distances and in environments with a high level of interference.The technology is based on converting the signals to ±10 mA balanced current loop, where the current direction is shifted on the wire pair, depending on whether it is a high or low signal from RS-232. W1 also means that it is always galvanic isolation between connected units.‑This eliminates errors caused by transients or when problems occur due to differential ground potentials.
It is a tried and tested technology that has shown to be extremely reliable and insensitive to interference.‑The interface allows data to be transmitted over distances up to 18 km (11.81 mi).

Multidrop with line sharer
Multidrop is usually used in applications where a large number of devices need to communicate with a master computer or control system. This is usually a “polled” system where a master sends an enquiry to connected devices and where the addressed equipment, a slave, responds. As the connected equipment is frequently intelligent (detects its own address) fully transparent modems can be used.
There is a wide range of products available for multidrop:‑4-wire, 2-wire, fibre
and FSK-modems. Line sharers for local communication are available in two designs, for 4-wire or star coupled in a rack, 4-wire permits distances up to 18 km (11.81 mi).
It is also possible to design a multidrop network using addressable modems and unintelligent devices.‑These modems can act as a master or as slaves in a multidrop network, each slave can be set with a unique address.

Isolators

Investigations show that the majority of data communication interference is due to earth potential differences between interconnected equipment. This problem can occur when equipment is powered from different distribution panels with different ground potentials when referenced to earth. This can cause ground currents to go to the wrong earth point and generate communication interference. RS-232 is an unbalanced interface and thus is sensitive to all types of interference. Westermo isolators provide galvanic isolation between connected equipment and in doing so can save a large part of the timely and costly fault tracing that is necessary with communication interference.

20 mA current loop (TTY)

The 20mA current loop or TTY is not a standard.‑Despite this the technology has been used in industrial system for decades now and was a de facto standard long before the standards used today, like RS-232 and RS-422/485.
Westermo current loop converters can be set to either active or passive on both transmitter and receiver using DIP switches.‑It is possible to transmit up to distances of 6 km (3.71 mi) at data rates of up to 19.2 kbit/s.

Multiplexer

Multiplexers save both installation and cable costs. 4 asynchronous RS-232 ports can communicate at up to 115.2 kbit/s via a common 4-wire connection. The transfer distance is between 600 (2000 ft) and 1200 metres (4000 ft), depending on the transmission rate on the common channel. Several devices can also be connected in series to increase the number of ports up to 16. The maximum rate on the common channel is 204.8 kbit/s, which is shared by the connected devices. Communication parameters can be set individually for each channel and are made via a terminal.

Line switch

Line switch for 4-channels with RS-232 interface. The device has one input channel that can be connected to each one of the four other channels or all simultaneously. Selection of the output channel is made through external digital signals or through activation of the RTS signal from the connected equipment. There is also an expansion channel, which allows the interconnection of more switches. Transmission rates up to 115.2 kbit/s are possible on each channel.

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