Monochrome display adapter for the display of the Maltese character set on a raster-scan CRT display system

Abstract

The aim of this project was to design and build a monochrome display adapter, capable of displaying the full Maltese character set in addition to a number of selected Greek characters, on a CRT raster scan display system. This adapter was to be designed for operation with an 80286 microprocessor based system, and the characters in the character set could be displayed in both the alphanumeric and graphic mode of operation. The following paragraphs aim to give a brief explanation of all the work that was involved in this project going back to the time when the projects were assigned to us. Although we were given the project titles early in 1990, work could not really start then because we still had to cope with the heavy schedule of lectures and the workload involved in the fourth year of our course. Therefore, this time served to try to define clearly what was actually required of our project. This obviously involved some extensive reading, and information had to be sent for, so that we could find the best way of satisfying these requirements as best we could. At first there were various options, apart from the one actually adopted, which were considered, including one involving an interrupt driven system and another involving software techniques. The first option was abandoned because of the delays that would be introduced in the system, undesirable since we were to operate in real time. The second option was abandoned because we wanted to have the character set hardwired into the system, and this could only be done by building a display adapter (as was in fact done) and integrating this adapter into the system as a whole. The desired character set could easily be programmed for future use in the memory on the adapter. Therefore, proper work on the different requirements of the system and the actual design started in the summer months, after the exams were over. Further reading was still obviously involved until a tentative but good idea of what we were to do could be established. Once the circuit was more or less designed, then work could start on building and testing the various sections making up the whole. These individual sections were all built and tested, where possible, on bread-board and vero-board. The actual circuit layout for the printed circuit board was then drawn up. This also involved alot of work, since the card had to fit at the back of the computer and therefore there were limitations on its size. When the final design had been drawn up the boards could then be etched. Meanwhile, the individual chips had been selected and obtained. The main problem here was that the CRT controller, the MC6845SP, which can be considered to be the heart of our project since it provides the interface between the CRT and the microprocessor, was not available locally and therefore bad to be sent for from abroad. This chip, was not received until December, and therefore it was not possible to test the circuits until that time. Luckily, we were able to obtain the data sheets of the CRT controller some time beforehand and therefore this enabled us to include the chip in the circuit design. Problems were encountered, and after a thorough fault-finding analysis, the faults were identified and modifications bad to be carried out. Since these modifications could not be carried out on the boards themselves this meant that the design had first to be modified and the circuits rebuilt once more. The main problem at this point here was the time, or the lack of it, since the thesis had to be banded in, and exams were coming up. But the work went on. I feel that I have gained a great deal of experience, both in the sense of the engineering involved in the design and implementation of the project, as well as in all the other aspects, including the various problems involved and the factors to be considered when undertaking such a project. The following chapters present a detailed account of the different aspects and considerations of our project. It includes a short introduction on CRT displays and the display adapters needed to drive them. The hardware and software considerations involved in the design, as well as the actual implementation, of the adapter are then also dealt with and explained in detail. Finally a number of appendices are enclosed, giving other information considered relevant.