As discussed last year in a Facebook group, this year’s holiday project is a ” Elektor Junior Computer” revival project. This means I will redesign. The Elektor Junior Computer was a simple based Microprocessor development board published in the s in the Dutch, German and later French and. Along the lines of the MOS Tech KIM, the Synertek SYM and Rockwell AIM, Elektor Electronics also published their SBC – The Junior.

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This elekotr certainly NOT the case. We set out to design a compact computer that would be inexpensive and simple to build, yet have the capabilities of much larger systems. Although small in size, the JC has plenty of programming power, which makes it ideal for use by amateurs and professionals alike. We, however, have a different opinion and set out to justify it by designing the Junior Computer. In principle, a microcomputer is really quite simple.

For jjunior construction, little more than basic electrical know-how is required. It is simply a matter of putting little black boxes into the correct holes — anybody can do a jigsaw!

The important aspect of any electronics system is what it can do, rather than how it does it. So the challenge here lies not so much in the electronics involved, as in learning how to use the instruction set to tell the eoektor what you want it to do. As can be seen from the block diagram in figure 1, a micro computer consists of three basic sections.

A bus is quite simply a common line or collection of lines that are connected to more than one device. The compter block diagram of a computer consists of three blocks and three buses. The latter provide the connections between the blocks. A computer works with information, or data, which is in a form it can understand specifically digital pulses.

As its name suggests, the data bus carries this information to or from elekfor various sections of the computer. The data bus consists of eight conductors and is therefore capable of transferring eight bits junjor data at a time.

A bit is one piece of digital information or BInary digiT.

org • View topic – [64] The Elektor Junior Computer

A group of eight bits is commonly called a byte. Comphter largest computer in the universe is totally useless unless it is able to communicate with the outside world.

For humans to be able to understand what the computer has to say, and vice-versa, some form of translation medium has to be incorporated. This is usually carried out by means of a keyboard and video terminal, or clmputer other form of display. However, this is not to say that communication is limited to these.

On to hunior memory. The memory is simply a store where the computer holds all the relevant information instructions, data etc. A computer has to be told explicitly what to do and in what order. Date is stored in individual compartments in the memory. These compartments are usually referred to as locations and each has its own unique address. Via the address bus, the computer can pinpoint the exact memory location, and therefore the data, that is required.

The address bus elektot also used to select the various input or output devices needed by a particular program. Last, but by no means least, is the control bus. The control bus regulates various internal functions as well as telling the computed bus which way to allow data to flow, whether to transfer data into or away from the CPU. First, the three buses. The address bus is formed by sixteen lines and is independent of the other two buses the data bus and the control bus.


With 16 lines, the CPU is capable of addressing up to 2 16 or 65, different memory locations. The data bus consists of eight lines, but is bidirectional.

This means that information can be moved in two directions, to or from the microprocessor. Of course, data can only be transferred in one direction at a time.

The direction of data transfer is determined by the control bus. If the computer is told to read information then the control bus allows data to be transferred from the memory or any other source to the CPU. Conversely, if it is told to write information, the control bus allows data transfer from the CPU to the memory or any other device. The control bus does this via bidirectional data bus buffers which, according to control bus signals, allow data to pass in the proper direction only.

Memory comes in two main types, which is why there are two memory blocks in figure 2. There are also two types of data: Where a system monitor program is used it is invariably stored in ROM. RAM is used to store such things as intermediate results and programs which are under development.

Random access memory is therefore often called work memory. It seems reasonable to point out at this time that when information is read from memory, that information is not lost. Similarly, the act of reading this page does not remove the text. The information is read and transferred to the brain CPU and further processed.

RAMs do however forget when their power supply is cut off. When the information contained in RAM is to be saved for extended periods of time it is usually transferred to a more permanent form of data storage, cassette tape or floppy disc for example.

This is more convenient and safer than leaving it in RAM. If the mains should drop out, all information in mains powered RAM would be lost. A detailed version of the block diagram in figure 1, this time with a specific computer in mind: Just as with RAMs, bidirectional data transfer is necessary. The 8 bit data bus is passed out as two bundles of eight conductors each, through port A and port B. The active port is determined by the address bus. Each individual line can operate independently of the other 15 at any given moment, as an input or as an output.

This facility can be used when the CPU has to do something else at the same time as data is being transferred through one of the ports.

Information from either direction may be stored in this memory, but only from one direction at a time.

The address bus informs the PIA which port, which direction, and whether or not to hold outgoing or incoming information. There are also three buses that go to the outside the three elekgor pointing right in figure 2 but these are for future expansion of the system rather than communication with the outside world.

As far as the JC is concerned, the outside world is everything beyond the keyboard and display. Briefly, the function of the CPU is to control the operation of all the other units and to process data. The CPU contains a number of registers which are used to temporarily store address, data and instruction information for decoding and manipulation purposes, etc.


The CPU also contains the program counter which simply counts the steps in the program. Its output can be fed onto the address bus in order to have access to the memory for the retrieval of program instructions. To determine the next successive address in a particular program, the program counter and the instructions already executed are analysed. Exactly how all this is carried out, however, is outside the scope of this chapter.

RES compiter the reset signal and is virtually self-explanatory. Information then comes from the outside to tell the computer what to do next. This feature can be useful when the computer is used with a relatively slow device a human for instance.

The computer can manage over half a million operations per second whereas the human may only be able to manage 3 or 4 during the same period. Once the interrupt is finished the computer will continue with the main program from where it elekhor off. In the event that both of the interrupt functions are used at the same time, they are given a priority which is determined by the program.

It should be noted that the interrupt request can be controlled by the program whereas, as its name implies, the non-maskable interrupt cannot.

A programmable timer can jjnior be seen in the PIA block in figure 2. More attention will be given to this in chapter 5 Book 2. The keyboard and display complete the package. They are shown in figure 3.

1. Getting acquainted with the ‘Junior Computer’

The keyboard consists of 23 key-switches and 2 toggle switches. Sixteen of these switches are used for entering information in hexadecimal form into the computer.

The remaining keys are assigned various control functions. Computfr display consists of six seven-segment LEDs and shows address and data information, again in hexadecimal form. The keyboard and display are connected to the computer via ports A and B. Port A is designed for bi-directional data transfer whereas port B is unidirectional.

More will be said about these when discussing operation. The sixteen lines from ports A and B are also connected to a 31 pin connector for future expansion. A further development of the diagram in figure 2: The address, data and control buses are accessible via a 64 pin expansion connector. The reason for these sockets is quite simple. Looking at it realistically, the hexadecimal keyboard as the standard input source, and the hexadecimal display as the output indication, are the simplest and least expensive methods of interacting with the computer.

The 64 pin expansion connector may be used to expand the memory capacity of the system so that longer and more complex programs can be run. The circuit diagram of the Junior Computer is shown in figure 4. Readers unfamiliar with the various types can be assured that the is a fast, high quality device.

The juniir bus consists of lines A0 … A15 while the data bus consists of lines D0 … D7. The electrical signals on the address and data buses are coded juniro information. What is this code? Imagine a numbering system with only two numbers 0 and 1 as opposed to the normal ten: