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KeyWedge for RS232 dumb terminals

KeyWedge is a trademarks of Altek Instruments Ltd

In this discussion we describe the way a barcode scanner can be interfaced to a terminal. However the same principle can be used for virtually any device with a RS232 interface. For example electronic weighing machines and scales, magstripe readers, credit card readers and numerous scientific and technical instruments, all can be connected in a KeyWedge configuration using an Altek Multiplexer.

Most accept the principle that the KeyWedge Technique is the best way to interface a barcode scanner to a computer. This is easy if you are using a PC because virtually all scanners can be bought with a wedge interface specially designed for PCs. However this type of scanner often fails to work with dumb terminals because they do not have 100% PC compatible keyboards.

Even though popular terminals may claim to have PC compatible keyboards they are often only compatible at the user level. In other words the keyboard 'looks' similar to a PC but at the engineering level they are different and the regular KeyWedge scanners will not work properly.

Using an Altek Multiplexer any RS232 terminal may have a barcode scanner which looks and behaves just like a KeyWedge scanner regardless of the type or functionality of the keyboard.

How it works

Computer Terminal
A computer terminal is usually configured as shown in the illustration on the left.

When keys are pressed on the keyboard the information is sent directly to the computer. Notice that at this time the data is not yet displayed on the screen.

When the computer receives the data it transmits a copy back to appear on the terminal screen. (In technical language the computer is said to 'echo' the data back to the terminal).

The transmission of data to the computer and back again happens so quickly it seems to the user as if the keyed data is going directly to the screen.

This may seem an over complicated way to get information on the screen but it has an important benefit. If a fault occurred on the data link between the computer and terminal it would immediately be apparent by missing or corrupted screen characters. A fault might not be noticed if the keyed characters went directly from keyboard to screen.

Computer Terminal
The Altek Multiplexer intercepts the link from the keyboard to the computer. Keyed data is passed through without interruption to be received by the computer exactly as before. Any data arriving from a device connected to either of the auxiliary inputs on the multiplexer is injected into the link and passed to the computer.

The computer cannot differentiate between keyed or injected data. In both cases it echoes the data back to the terminal where it appears on the screen.


An Altek Multiplexer provides a keyboard wedge system for RS232 dumb terminals that works in an identical manner to that adopted as the standard for barcode scanners on stand-alone PCs.

The keyboard wedge method has the important advantage that it is usually possible to connect the additional device(s) without any changes being necessary to existing applications software and without any computer reconfigurations. It is simple to set up and understand. The user intuitively knows how the new device interacts with the existing software application.

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KeyWedge is a trademarks of Altek Instruments Ltd

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