Fiber Optic Communications for the Premises
EXPLOITING THE ADVANTAGES OF FIBER OPTIC CABLE IN THE INDUSTRIAL ENVIRONMENT
5.1 Data Communications in the Industrial Environment
Our attention is now drawn to the problem of data communications in the industrial environment. This is the problem of data communications in the manufacturing facility. It is the problem of data communications on the factory floor or in the process control plant. Data communications in these premises can significantly benefit by using fiber optic cable as the Transmission Medium.
Let us begin by describing the industrial environment from a data communications perspective.
What type of data communications is going on here? Typically, the situation is illustrated in Figure 5-1. There is a Master Computer located somewhere in the manufacturing facility. In the past this was usually a mini-computer. Presently, it is either a workstation or PC. The Master Computer is communicating with any of a number of data devices. For example, it may be controlling automated tools and sensors. It may also be exercising control by querying and receiving data from different monitors. These data devices are located throughout the facility. The illustration provided by Figure 5-1 shows a machine tool, but in actuality the number of different automated tool types, sensors and monitors may be very large. By way of example, it may extend to well over 100 in a semiconductor fabrication facility.
The control procedure exercised by the Master Computer usually consists of sending a message out and receiving a message back. It may be sending automated tool or sensor an instruction. It may then receive back either an acknowledgement of instruction receipt or a status update of some sort. In like manner, the Master Computer may send queries to a monitor and receive back status updates.
Figure 5-1: Data Communications in the industrial environment
As is readily evident, the whole control procedure is executed using data communications with appropriate signaling devices (modems) and other needed equipment located at both the Master Computer and the data device locations. Required data transmission rates need not be significantly large. On the other hand, in the industrial environment reliability requirements are quite stringent. This is so regardless of whether reliability is measured by either BER or link up-time or some other parameter. The consequences of an unreliable data communications link may be a mere annoyance when it comes to office communications. However, consequences may be catastrophic in a manufacturing operation. Literally, an unreliable link could close down a whole plant.
Generally, the type of situation described above leads the data communications in the industrial environment to follow an inherently hierarchical architecture. This type of architecture is shown in Figure 5-2. The Master Computer is located near a communications closet. The modems and/or other communications equipment (e.g.,
surge suppressors, isolators,
interface converters) needed by the Master Computer to effect links to the data devices are usually rack-mounted in a card cage placed in the communications closet. Cabling then extends out from the card cage to the individual data devices. At the data device end the matching communications equipment may either be stand-alone or
DIN Rail mounted. With the latter, the communications equipment snap onto a rail mounted on a wall or mounted on some convenient cabinet near the data device. DIN Rail mounting will be discussed in greater detail toward the end of this chapter.
Figure 5-2: Data communications architecture usually found in the industrial environment
It is important to note that this is the general case not the absolute case. If the Master Computer has just 1 or a few ports there may be no need for a card cage. All data communications equipment may then be of the stand-alone type.
There are several topologies associated with this type of hierarchical architecture. The topology could be a star with a cable extending out from the card cage hub to each data device. Each ray of the star is simultaneously operating as data communications link. The topology could be a multi-dropped daisy chain, using the RS-485 interface standard. This is particularly suited to a polling, query-response, data communications scheme - the type of communications being carried out by the Master Computer. The topology could even be a broadcast bus, the type used by an Ethernet LAN.
5.2 The Problem of Interference
In considering data communications in the industrial environment a key concern is the problem of interference. This is an underlying concern regardless of whether or not the architecture is hierarchical or not and regardless of what topology is employed.
From an interference point of view the manufacturing facility represents a stressed environment. The presence of high current equipment such as the automated tools results in the propagation of electromagnetic pulses that interfere with the data communications links. Proper grounding is always difficult in the industrial environment. Ground loops and resulting ground currents can cause transmitted data to be demodulated in error.
In the past, UTP copper cable was the transmission medium of choice for the industrial environment. Why? Principally, because of there was a lot of experience in dealing with it. There are a number of different ways of handling the problem of intense interference when UTP copper cable is employed in this environment. Sponges can be inserted into a data communications link to protect against surges. Isolators can be inserted into a data communications link to protect against ground loops. Single ended serial communications can be replaced with serial communications employing differential signaling based upon the RS-422 standard. Differential signaling, with sufficient balance, allows electromagnetic interference of the type prevalent on the factory floor, to cancel itself during the data communications reception.
But what about fiber optic cable as the Transmission Medium, doesn't this have great interference protection? Good point! If fiber optic cable is employed in the industrial environment concerns about interference can vanish. This Transmission Medium is simply not affected by the electromagnetic interference plaguing the factory floor. Furthermore, there is a side benefit. It was mentioned that data transmission rate requirements are usually modest. However, this may not always be so. Using fiber optic cable eliminates the concern about future bandwidth needs.
Fiber optic cable as a Transmission Medium has been slow in coming to the industrial environment. This has been principally due to cost. However, this is changing as the price of fiber optic cable steadily decreases.
There are two possible ways by which fiber optic cable based data communications may be introduced into a given manufacturing facility. In the first way, a fiber optic cable based network may be introduced from the ground up. In other words, it is installed where no network previously existed in the facility. In the second way, fiber optic cabling may be patched into a network already installed, a pre-existing network that was based on UTP copper cable.
Today, if you are considering installing a network from the ground up then you are talking about installing an Ethernet LAN with a fiber optic cable Transmission Medium. In the past, token ring LANs were quite popular in factory settings. They guaranteed maximum transmission delays and were matched to polling techniques. However, lately Ethernet has come to dominate even the industrial environment. Furthermore, there is the advantage of being able to bridge the factory floor LAN to other Ethernet based LANs in your organization.
If you are installing fiber optic cable by patching into a pre-existing UTP copper based network then you must deal with the different types of data interfaces that may exist in that network. These data interfaces may include RS-232, RS-422 and RS-485. Electrical representations of data from/to these interfaces have to be converted to/from light pulses traveling down fiber optic cable.
5.3 Fiber Optic Data Communications Products that Can Help
Telebyte offers a number of different products that are well suited to providing data communications in the industrial environment. These products are particularly well suited to the second approach described above, the case where a fiber optic cable capability is being patched into a previously existing UTP copper cable network. Several of these will now be described now.
The fiber optic cable multiplexer discussed in Chapter 3 and the Ethernet LAN Extenders
discussed in Chapter 4 can be also be used to implement
data communications on the factory floor. A multiplexer can be used to allow the Master Computer to reach to different automated tools/sensors/monitors with a single fiber optic cable. However, the cost saving that they can realize depends upon how the tools/sensors/monitors are clustered. The LAN Extenders can be used to realize a total Ethernet LAN approach to the problem of data communications in this environment.
Model 271 Fiber Optic Auto Powered Line Driver
Figure 5-3: Model
271 Fiber Optic Auto Powered Line Driver
The Model 271 Fiber Optic Auto Powered Line Driver is a short haul modem that employs an RS-232 data interface and transmits the data onto a fiber optic cable. This modem provides full duplex, asynchronous, data communications over two fiber optic cables. The length of the fiber optic cable can be up to 2 km and the data rate as high as 56 KBPS. Performance of the unit is optimized for 62.5/125-fiber optic cable. However, the modem can also be used with fiber optic cable having other dimensions.
The operating power for the Model 271 Fiber optic Auto Powered Line Driver is derived from the transmit data line. This is a real convenience when an electrical outlet is not readily available. The Model 271 is equipped with a DTE/DCE switch that reverses pins 2 and 3 of the RS-232 connector. This allows the modem to support terminals, printers, computers or any other RS-232 based device. The fiber port of the unit employs ST connectors.
One application of the Model 271 is illustrated in Figure 5-4. Notice while this application deals with the factory environment there is no card cage shown. Rather, the application deals with the situation where there is the need for a data communication link between a mini-computer located in the front office of a company and a PC located on the company's factory floor. Both the front office and the factory floor are in the same building.
Figure 5-4: Example application for the Model 271
Data communications carried out strictly in the front office may be quite reliable over UTP copper cable. However, in this application the data link traverses the boundary to the factory floor. Consequently, there is a need for the extra reliability provided by fiber optic cable.
RS-422 to Fiber Optic Converter
The Model 272A provides the capability of performing an
interface conversion between full duplex, RS-422 signals
and their equivalent for fiber optic transmission. For
applications where the transmission medium must be
protected from electrical interference, lightning,
atmospheric conditions or chemical corrosion fiber
optics is the perfect solution. The Model 272A RS-422 to
Fiber Optic Line Driver handles full duplex data rates
to 2.5 MBPS. The electrical interface to the RS-422 port
is fully differential for transmit and receive data and
is implemented in an industry standard DB25 connector.
The fiber optic ports are implemented using the industry
standard ST connectors. The design has been optimized
for 62.5/125 micron fiber cable, however other sizes may
be used. The optical signal wavelength is approximately
850nm. The optical power budget for the Model 272A is 12
dB. In normal applications the distance between a pair
of Model 272A's will be at least 2 km (6,600 ft). Power
to operate the Model 272A is supplied by a small, wall
mounted, 9 Volt AC transformer and line cord.
Figure 5-5: Model
272A RS-422 to
Fiber Optic Converter
One application of the Model 272A is illustrated in Figure 5-6. This is a simple case of a single data communications link being required on the factory floor. To avoid complexities there is no card cage although the extrapolation to one is quite easy for the reader to see. The link is between a PC and an Intelligent Machine Controller. Previously, the link was using RS-422 signaling for protection. Consequently, the data interfaces of both the PC and the Intelligent Machine Controller have RS-422 implemented with DB25 connectors. The Model 272A is placed at both ends of the link and allows the data communications to proceed using fiber optic cable with its much greater protection from interference.
Figure 5-6: Example application for the Model 272A
Model 276A -
RS-485 to Fiber Optic Converter
The Model 276A RS-485 to Fiber Optic Line Converter accepts half-duplex data at rates up to 1 MBPS through an RS-485 interface. It then transmits this data onto a fiber optic cable. Likewise the unit is able to receive data from a fiber optic cable and send it to a device through an RS-485 interface. The RS-485 interface
used by this model is balanced and implemented in a female DB25 connector.
Figure 5-7: Model
276A - RS-485
Fiber Optic Converter
The network architect specifies the control of data flow in any RS-485 based communications facility. The Model 276 RS-485 to Fiber Optic Line Driver provides the network architect with the greatest versatility by enabling the RS-485 transmitter when data is detected at the fiber optic receiver.
In the Model 276A RS-485 to Fiber Optic Converter the fiber optic ports are implemented using ST connectors. Performance is optimized for fiber optic cable having dimensions 62.5/125 and for an optical signal with an 830 nm wavelength. However, fiber optic cable of other dimensions can be employed.
The unit provides reliable communication over a distance of 2 km.
One application of the Model 276 RS-485 to Fiber Optic Line Driver is illustrated in Figure 5-8. This is a situation in which a PC on the factory floor is controlling an environmental control unit and a number of different automated tools. Control is exercised by communicating commands and receiving responses through an RS-485 polling network. However, there is the complication in that the PC only has an RS-232 interface. The environmental control units and the automated tools have RS-485 interfaces. The enhanced interference protection provided by fiber optic cable is required.
Figure 5-8: Example Application for the Model 276A
In this application the PC is connected to the Telebyte Model 290
RS-232 to RS-422/RS-485 Concentrator - Wiring Hub. This allows conversion of communications from an RS-232 interface to a grouping of both RS-422 and RS-485 interfaces. We are only interested in the RS-485 ports of the Model 290. Data from/to the PC is converted and is presented on these RS-485 interfaces. Each of these interfaces is then connected to a Model 276A RS-485 to Fiber Optic
Converter. The Model 276A then sends this data out on a fiber optic cable or receives the data from a fiber optic cable. On the far side of each of these fiber optic cables is another Model 276A. This takes the data from the fiber optic cable and provides it either to the environmental control unit or to one of the automated machine tools. Likewise, it takes data from these and transmits it back along a fiber optic cable to the PC.
RS-232, RS-422, RS-485
to Multimode Fiber Optic Line Driver
The Model 277 Multi Interface Fiber Optic Line Driver is pictured as a stand-alone unit in Figure 5-9. Also shown with it
Model 8277. The Model 8277 is the same as the Model 277 except that it is DIN Rail
Figure 5-9: The
Model 277 and the Model 8277. Both units are the
same except the
Model 8277 is DIN Rail mountable.
The Model 277 Multi Interface Fiber Optic Line Driver is a unique asynchronous fiber optic modem. The optical interface can operate either by a point-to-point or daisy chain ring, multi-drop, configuration. The electrical interface can also operate in either a point-to-point or multi-drop configuration. The network architect selects the configuration.
This unit is appropriate for factory floor networks where there is an existing mixture of point-to-point and multi-drop, UTP copper cable based links. It can easily convert them to fiber optic operation with the added protection this provides.
For a point-to-point configuration, two Model 277's are connected back-to-back, to form a high speed, full duplex, fiber optic link.
In an optical ring configuration, three or more Model 277's, in all 4-wire modes are daisy chained in a ring. The ring will consist of a Master Model 277 and two or more slave Model 277's. Master/slave modes are switch selectable. The slaves pass the received optical data along with the transmit data from their own electrical interface to their optical transmitters. The Master does not pass the received optical data. A ring of up to 10 Model 277's at a data rate of 1 MBPS can be formed. To extend the optical distance a pair of Model 277's can be inserted into the optical interface to act as a line extender.
This unit can support fiber optic links as long as 1 mile with a transmission rate as high as 1 MBPS. The design is optimized for transmission over multi-mode cable at a wavelength of 850 nm.
The Model 277 electrical interface is switch selectable between RS-232, RS-422 and RS-485. As a result, this unit is well suited to assisting in the evolution to fiber optic cable of existing UTP copper cable based factory networks. Switch selection enables data to flow from the electrical interface the optical transmitter or to be controlled by the Request To Send (RTS) line.
Full duplex, four wire, or half-duplex, four or two wire, may be selected when the RS-422 or RS-485 interface is selected. The RS-422 or RS-485 interfaces of the Model 277 may operate in a multi-dropped or point-to-point environment.
In the half duplex mode, the Model 277 controls the transmit data line on the electrical interface.
The Model 277 is shown in an application in Figure 5-11. Here several Model 277's are being employed to extend link length past 1 mile.
Figure 5-10: The Model 277 shown in application to extend the link length
RS-232 Fiber Optic Auto Powered Line
RS-232 Fiber Optic Auto Powered Line
features a standard DB9 interface for maximum
performance and reliability of data transmission over
glass fiber, eliminating the need for serial to nine-pin
adapters. In addition, it brings effective data
communications to manufacturing environments. It can be
installed in applications requiring very high data
transmission rates, offers resistance to Electromagnetic
Interference (EMI), and isolation from lightning-induced
current surges and ground loops. The unit employs an
RS-232 data interface, can achieve 56 kbps
asynchronously and operates in either half- or
full-duplex modes over dual fibers up to 2 km in length.
Figure 5-11: The
Fiber Optic Auto Powered Line Driver features a standard
9271's ability to take/direct data from/to this
interface without any conversion eases implementation. A
highly flexible solution, the Model 9271 has been
optimized for 62/125 fiber cables, and is compatible
with other sizes as well. It features industry-standard
ST cable port connectors, plus a DTE/DCE switch to
reverse Pins 2 and 3 of the RS-232 connector to
accommodate equipment with different data output
Operating current for the Model 9271 is derived from the
transmit data line, with a power budget of 12 dB when
using 62/125 cable. For applications requiring a
dedicated power source, the unit can be ordered with a
wall-mounted power pack (available as the Model 9271A).
The Model 9271 incorporates clips in the outer casing so
that the unit can be securely attached to a DIN rail,
wall, table or desk in an organized manner.
This is an appropriate point to discuss DIN Rail mounting in greater detail. DIN Rail mounting is a cabling system that was developed specifically for factory automation. Only recently has it been discovered for use with data equipment. This system is simple and straightforward. It uses a steel channel called a DIN Rail. The DIN Rail has slotted holes for mounting and is normally mounted in a horizontal position. DIN Rail products like the Model
9271 are then placed on the Rail by snapping them in place after which the wiring is completed.