Hexadecimal keyboard for PCs

For some programming applications such as writing Pascal units, you have to enter lots of hexadecimal numbers. Using a standard PC keyboard for this type of work is cumbersome as the relevant number and letter keys (0-9 and A-F) are not grouped together. To make hex number entry a lot easier, the author designed an add-on hexadecimal keyboard which is connected to the PC's serial port. The hardware is controlled by a simple program of which the full listing is given in this article.