Model 285 Application Notes



There are a variety of different data processing equipment types in the office, on the factory floor and across the campus. You may encounter minicomputers, plotters, alarms, terminals, badge readers, and different manufacturing tools and laboratory instruments. The serial EIA-232 interface is perhaps the most widespread interface found on different data devices. It is perhaps the most universally found debarkation point for bringing data into and out of equipments. Data communication connections between such equipment in the premises environment can often be accomplished by merely connecting interfaces together by a cable.

One problem arises though. While the EIA-232 interface is the most widespread interface, it is not ubiquitous. It is not on every device. Data equipment may employ another interface because it is more appropriate for the communication architecture being used or for the interference/noise environment within which communications must take place. There are other interfaces, notably RS-422 and RS-485.

The need often arises to interconnect data devices having different interfaces. Usually this need is to interconnect a device having the popular EIA-232 interface with a device having something else like RS-422.

Interface conversion is a straightforward task. However, when you have a heterogeneous environment with different types of equipment having different types of interfaces the whole management problem begins to get 'messy.' The network manager would like to have one interface converter type around to use in these situations and use it for whatever type of conversion is needed. The Model 285 is a neat solution to maintaining control and avoiding a mess.

The Model 285 allows conversion between EIA-232 and either RS-422 or RS-485 in just one box. The user simply selects either RS-422 or RS-485 for the conversion. Furthermore, the user can select either 2 or 4 wire RS-485. The universality of this converter goes even further. If RS-485 half duplex is desired for conversion the Model 285 can be configured to control data flow in either of two ways. It can use RTS of the EIA-232 interface or it can turn on when TD is applied to the EIA-232 interface.

Besides doing straight forward interface conversion when needed, the Model 285 can even give you further flexibility in configuring your premises data communications network and allow you to 'make do' with data equipment that may not be exactly suited to your needs, but which are available. A simple example will demonstrate this.

It may be that you have procured or inherited all data devices with the same interface and that this interface is the widespread EIA-232 interface - what else? You put these inherited devices aside in some storage area. However, some time later you (and the emphasis is on 'you') decide that you need to interconnect these types of devices in an RS-485 type 'polling' network. You realize that your inherited devices do not have the RS-485 interface demanded. But, you also realize that the Model 365 placed at each of your data devices gives it an RS-485 interface and allows you to implement the network you want and without having to go out and buy expensive data equipment with the needed interface built-in.

This is exactly the situation in the illustration. Here we have a minicomputer on the left, a plotter in the center and two terminals on the right. They were all 'cast off' old equipment in the laboratory storage area of a known Fortune 500 company. Each of these devices only came with the 'almost ubiquitous-but not quite' EIA-232 interface. After some period of time the laboratory manager decided that for his installation a network was needed where a minicomputer would control a plotter and communicate with two terminals. The minicomputer would carry out the control and communication using a polling protocol. RS-485 networking was perfect for this situation. However, the laboratory manager did not have data devices with the RS-485 interface. He also did not want 'to bust' his annual budget by going out and procuring new devices which had this interface. Instead he went to his storage area and pulled out the inherited devices with the EIA-232 interfaces. He then went and purchased a Model 365 to attach to each data device and give it an RS-485 port, an inexpensive solution to his networking needs.

 

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