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Browse Prior Art Database

Character Reversal for a Two Sided Display

IP.com Disclosure Number: IPCOM000085060D
Original Publication Date: 1976-Feb-01
Included in the Prior Art Database: 2005-Mar-02
Document File: 3 page(s) / 58K

Publishing Venue

IBM

Related People

Martin, WJ: AUTHOR

Abstract

One of the techniques to reduce the cost of display terminals such as gaseous discharge display devices, is to utilize a single panel to provide two display terminals facing opposite directions with a common logic and drive/control circuitry, each display utilizing half of the panel. However, using this arrangement, conventional character generation techniques cannot be used, since if characters are generated with one set of ROS (read-only storage) modules, the set of characters on the opposite side of the display would be generated as mirror images of these characters.

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Character Reversal for a Two Sided Display

One of the techniques to reduce the cost of display terminals such as gaseous discharge display devices, is to utilize a single panel to provide two display terminals facing opposite directions with a common logic and drive/control circuitry, each display utilizing half of the panel. However, using this arrangement, conventional character generation techniques cannot be used, since if characters are generated with one set of ROS (read-only storage) modules, the set of characters on the opposite side of the display would be generated as mirror images of these characters.

To make character generation more efficient in a double-sided display of this sort, it is necessary to electronically reverse the horizontal segments of the characters in the rear display so that all characters in both displays are written in the proper direction. To do this, the position of some of the bits in the data bus are exchanged when writing in the rear display, but not when writing in the front display.

In order that regular commands to the panel not be disturbed by this exchange, exchanges are permitted only when the unit is executing a Write character command and is accessing the character generator ROS data, or when the unit is executing a Write slice command and accepting slice data from the processor. A system for providing character reversal utilizing characters generated in a 7x9 dot matrix operates as follows.

Referring to the drawing, a control signal is applied via line 11 to logical AND circuit 13, the control signal indicating whether the display is to be operated as a dual or a single display. In the preferred embodiment, a "1" bit signifies dual operation while a "0" bit indicates single operation.

Assuming that the display is operating in a slice mode, i.e., generates 7 horizontal dots simultaneously, a signal is generated from gate circuit 15 when the signal on line 17 indicates that a Write character command latch has been set, and the signal on line 19 indicates that the ROS character data is being received. Gate circuit 21 has an input on line 23 at slice data time, while a signal is applied to line 25 indicating that the write slice command l...