Model 290 Application Notes



The RS-485 interface is often found in environments where networks are configured to support a polling type of operation, a 'query-response' communication from a 'master' central computer to 'slave' terminals or other data devices. An example of this may be found in the factory setting where a master computer is controlling different machine tools involved in the manufacturing process, querying them on the status of a job and issuing resulting controls. Another example may be found in the retail store setting, the grocery or department store. Here the master computer may query Point of Sale terminals for status, receive back the status of just completed sale and then appropriately adjust the inventory data base.

Often in the same environments are found master computers communicating with other data devices using the RS-422 interface. Why? Generally, the electrical interference present drives the communication to certain devices to this interface since it will give them greater protection through its use of differential signaling.

There is thus a general need to configure networks for the local environment that can accommodate both RS-485 and RS-422 communications. The Model 290 was designed to do this and even more. It is a concentrator, 'hanging' off of the central computer and allowing it to communicate with either RS-422 or RS-485 data devices. However, it pushes the RS-485 networking capability well beyond what is usually encountered. Typically, the master can only communicate with up to 32 devices on an RS-485 network. The Model 290 allows this to be expanded to support up to 496 nodes on the network.

The accompanying illustration shows the Model 290 in an application. The unit has 16 ports, although only two ports are shown in the diagram. Each port can be set, that is programmed, as either a 4 wire RS-422 or a 2 wire RS-485. This allows the Model 290 to be used to configure the hybrid RS-485-RS-422 network. The illustration shows the master, centralized computer with a 2 branch network. In the upper branch, the Model 290 is allowing it to communicate with 3 PCs and another computer from a single port using the RS-485 protocol. This is nothing. Each port of the Model 290 in the RS-485 mode of operation can support up to 31 connected data devices. In the lower branch, the Model 290 is allowing the master to communicate with a computerized scale using the RS-422 interface. This scale may be in a particularly harsh environment of a store, the meat department for instance, where motorized slicing equipment may continuously generate electrical interference whose effects RS-422 communication can ameliorate.

As a further bonus all of the ports of the Model 290 are isolated and have surge protection. Consequently the hybrid networks are protected against inaccuracies from ground loops and destruction from lightning and other surges.
  

 

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