So how do they work?  With most Operator Interfaces, there is specialized software available that is used for configuration.  Let’s examine the simple Operator Interface with the keypad and text display.  A typical configuration software will allow you to enter in pre-defined text messages such as “Valve Open” or instructions to the operator such as, “Close Valve Number 1.”  The messages you enter can be configured to operate off of memory locations from a separate controller. 

For example you may have a limit switch for a valve that indicates the valve is open using an I/O location in a PLC.  The configuration software will allow you to enter the PLC memory location for the input that is tied to the valve open limit switch.  That memory location can be tied to the text message that says, “Valve Open” using the configuration software.  In addition the keypad we mentioned previously can be configured to toggle a bit in the PLC that is programmed like an open pushbutton in the PLC.  Pretty soon you are controlling a remote valve and are aware of its position using this simple Operator Interface.  In summary keypad buttons are tied to memory locations in the PLC that allow for the manipulation of field devices controlled by the PLC.  Display messages are also tied to memory locations to allow the PLC to inform the operator of alarms or conditions.  Sounds pretty simple – and it is. 

The tricky part with this simple example and many projects such as this is all the additional details that have to be worked out.  How will it communicate to the PLC?  How does the PC based configuration package communicate with the Operator Interface? What format has to be used to allow the PLC to understand the correct memory location you are trying to address?  Unfortunately each PLC manufacturer has a different method of designating their memory locations.  All of these questions (and more) need to be answered before the Operator Interface can be successfully used.  I might add that communication problems are generally the biggest headache in control system projects where you are working with unfamiliar equipment. 

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