This invention relates to a system for displaying alphanumeric messages by means of a unitary visual display device and deals more particularly with apparatus for expanding the message display capacity of such a device. 
Multiple character station unitary visual display devices used in display systems are generally well known in the art.  Such multiple character display devices generally have a fixed number of character stations which limit the number of characters in the displayed message to the number of character display stations of the device.  One such display system employing a limited capacity multiple character station device is illustrated and described in U.S. Pat.  No.  4,381,505 assigned to the assignee of the present invention. 
Although the aforementioned display system has numerous applications, it is often found in industrial applications utilizing an automatic controller or monitoring device for a manufacturing, material handling, chemical or other industrial process.  In such uses the controller may operate to select a message unit from a memory in accordance with different conditions monitored by the controller which when displayed advises an operator of a monitored condition, provides a warning, gives an instruction or the like, or the controller may also select a stored message component from the memory to which is added real time data representing the value of real time variables or other associated real time conditions. 
Often it is necessary to display a message unit exceeding the number of character stations of the display device or to display a sequence of message units to provide additional information or instructions to an operator.  A limitation in displaying multiple-message unit messages in a unitary visual display device system is generally the expense and complexity associated with the additional equipment such as memory modules and software programming in the controller operating with such a display system. 
Another drawback often associated with the above described display systems is the additional equipment required in the controller to convert binary coded decimal (BCD) signals representative of a message unit address to binary coded signals to select messages from the display system memory.  Since the real time data is represented in BCD signal format, separate multiconductor lines for message selection signals and real time data signals are required to connect the associated controller outputs to separate BCD and binary input ports in the display system. 
Still another limitation is the additional controller equipment and associated software required to provide a reserve signal to the display system to reserve character locations in the visual display device to display real time data associated with a stored message component having a real time message component. 
Accordingly, it is desirable to have a unitary visual display system for use with controllers or the like capable of displaying multiple-message unit messages wherein the message units may be made up of entirely stored message units or message units having both stored and real time components and that avoids the drawbacks of the aforedescribed display systems. 
The general aim of the present invention is to provide an improved alphanumeric unitary visual display system having an expanded message display capability that overcomes the limitations of previously used unitary visual display systems.  The unitary visual display system of the present invention provides multiple-message unit message display capability and real time data reserve signals without the complex hardware, software, and wiring generally required in controllers used with prior display systems. 
Other objects and advantages of the invention will be apparent from the following written detailed description and from the accompanying drawings.