Model 366 Application Notes



Data processing equipment in the office, on the factory floor and across the campus may come in a variety of 'flavors,' embracing terminals, badge readers, alarm equipment and all types of manufacturing and laboratory instruments. The serial EIA's RS-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 these devices. 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 RS-232 interface is the most widespread interface, it is not ubiquitous. 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 RS-232 interface with a device having something else like RS-485 or RS-422.

Of increasing interest is the problem of interface conversion when the conversion is between RS-232 and RS-485. Networks based upon communication between RS-485 interfaces commonly operate with a polling protocol and employ a multi-dropped topology. One of the data devices is designated 'master.' The others are designated 'slaves.' The master communicates with the slaves in a query-response type of mode under the control of software. Networks based on RS-485 are found in many environments. These include locations that have Point of Sale terminals, like supermarkets. These also include factory floor locations, where a computer may be controlling automated tools. However, many of the data devices which have to operate in an RS-485 network, may only have RS-232 interfaces - a 'master' minicomputer for the factory floor is an excellent example. Hence, the need for RS-232 to RS-485 interface conversion.

The Model 366 allows conversion between EIA's RS-232 and RS-485. This unit can support up to 32 RS-485 stations on a multi-dropped network. The Model 366 can even handle contention between RS-485 network users. This is accomplished by dip switch settings and the use of the control signal pair RTS/CTS. This signal pair is used to control the transmitter and the receiver.

The Model 366 even includes a display. This provides the user with important status information with regards to the operation of the device and is extremely useful during installation and checkout and whenever there are suspected problems. The display presents the user with the live status of the data signals TD/RD and the control signals RTS/CTS/DSR/DCD/DTR.

The illustration above shows the Model 366 in a typical application. This is a multi-dropped polling network. Here we have the master computer on the left communicating with various other data devices. These include a plotter and two PCs.
  

 

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