Within the premises or local area environment the short haul modem is a convenient device for configuring a reliable communications link. When the distance between communicating data equipment gets beyond 100 feet signals need 'to be boosted' or they will not be received and decoded reliably. Using a pair of short haul modems in the link, one for transmitting and one for receiving in each direction, boosts the signals and gives the reliability.

However, a number of items always seem to come up when using a pair of short haul modems to deal with this rather straightforward problem.

The first issue involves the need to satisfy data transmission and speed requirements. They must meet the under application's needs. They also need to be met relative to the interference environment within which the communications is taking place. Certain environments, such as office building settings, present relatively benign environments where background noise is the only problem. Others, such as manufacturing facilities, present harsh environments. Here one may have to deal with Electromagnetic Interference (EMI) from high powered production tools, Radio Frequency Interference (RFI), power surges and other deleterious effects.

Secondly, there is the matter of isolation. The need for isolation arises when ground currents are present. In many instances the data equipment communicating must be grounded at different points. Different grounds imply different reference levels for voltages. Ground currents are generated in an attempt to achieve equilibrium between different ground points ground currents are generated. From the perspective of communications such currents make themselves known as an additional interference mode. Isolation provides a barrier between different ground points. This barrier does not allow ground currents to be generated. Isolation allows equipment being grounded at different points to be connected together without the need to worry about the interference due to ground currents.

Thirdly, there is the issue of topology. The data equipment communicating may be able to be served by just a straight point-to-point link. However, there are situations where the data equipment communicating may be organized in a multi-drop network. Multi-drop topologies are often used in polling networks. Such networks have a 'master' host computer successively poll 'slave' devices. There are even situations where the data equipment communicating is organized in a multi-drop topology, but the role of master and slave is often interchanged. There are also situations where the data equipment may sometimes be communicating by a point-to-point link and at other times in a multi-drop network.

Let us now focus our attention on the problem of data communications in the manufacturing environment. It was pointed out above that this was a harsh environment from the point of view of interference - an environment where RS-422 communications may present a possible solution, but also may not provide enough protection. It is also an environment where ground loops may present problems and isolation is needed.

Carrying out premises data communications in the manufacturing environment by using fiber optic cables presents several ready advantages. First, there is tremendous bandwidth potential. It can deal with traffic from relatively low data transmission rates to T1 (1.544 MBPS) to and well beyond. Applications that require very high data transmission rates can be easily accommodated. Secondly, there is the protection that fiber optic transmission provides against the variety of deleterious effects which plague transmission over copper cable. These include the resistance that fiber optic transmission has to Electromagnetic Interference (EMI), lightning induced current surges and ground loops. Finally, there is the protection that fiber optic transmission has with respect to 'tapping.' It is much more secure with no effective radiation of the communication occurring out of the cable.

Point-to-point data communications links are often found in the manufacturing environment. However, multi-dropped networks are also quite prevalent. There are many situations where a 'master' computer is controlling a variety of different automated tools, connected to it as 'slave' data devices. Such control is often effected as a polling network. Here the master computer transmits instructions to each tool. Each tool responds with a status message of one sort or another. Modems of choice for the factory environment may need to accommodate the multi-dropped capability.

For data communications in the factory environment, you could buy stand-alone modems, each in an individual case, and have them 'hanging off' of the respective equipment which they are directly connected to, the 'master' computer or the 'slave' PLC or whatever. However, in the factory environment modems 'dangling' this way just seem to add to the mess and clutter which is always present.You could buy PC card versions of the modems. Each card then could be inserted into a slot in a card cage close to the device to which it is connected - that is, a card cage by the master computer and cages by the slaves. The problem with this alternative is that it is a terrible waste of cage space and cages themselves may not be available. True, there may well be a communications closet close to where the master computer is and this may well be able to accommodate a cage. However, to install a cage there for a single modem PC card, or perhaps an additional card for back up, is very wasteful. There are probably no communications closets out on the factory floor close to the slave devices. But if there were, this argument of wastefulness would still hold.

Another, more attractive, alternative for the manufacturing environment is to use a DIN Rail mounted approach for dealing with the problem of modem placement. With this approach the modem has a special clip attached to it. This clip the allows the modem to be conveniently attached to a special metal rail, the DIN rail. That is, the modem can be clipped onto the rail. On the factory floor there are usually always places where a rail can be put up. For example, it can be attached to the side of the communications closet considered above, screwing it on just like a leg of an erector set. It can be attached wherever there are spare holes, and you can be sure that there will be spare holes all along the manufacturing line. By quickly putting in a DIN rail and clipping the modems to it, the problem of cabling mess is solved and the expense of a card cage is avoided.

The Model 8242 modem presents an attractive choice for data communications in the manufacturing environment. It can effectively deal with all of the issues discussed above. Its employment in a typical application is shown in the illustration below. Here is shown a fragment of a factory floor data communications network. On the upper left is a PC serving as a 'master' computer. It is a communicating with the other PC's shown. Each of these is designated as a 'Display Controller.' A 'Display Controller' is a Programmable Logic Controller - a PLC - with a display. These are commonly found in the manufacturing environment especially where process control is being carried out. Each of the Display Controllers is communicating with the master computer. As illustrated the communication is being carried out by a polling network. This is realized as a daisy chain multi-dropped network. The master computer is communicating with the Display Controller 'slaves.' The master computer is sending instructions to the slaves and receiving back status updates.

The Model 8242 is ideal for this application. It provides transmission over fiber optic cable and gains all the performance advantage this presents with respect to harsh interference environments and isolation. It can be used for multi-point communication and is thereby well suited for the multi-dropped polling network described in this application. Finally, as shown, the Model 8242 is DIN Rail Mounted. This conveniently organizes the cabling in a manner suitable for maintaining order in the manufacturing environment.