Cables built and installed, ready for more testing

GOT SLIGHTLY WRONG CABLES BUT TURNED THEM INTO PROPER PARTS

I received a link to AliExpress items to finish the EDUC-8, which I expected were premade cables, ribbon cables with connectors on both end, in the 2, 3 and 5 pin widths. What I received were half cables - connectors with a short ribbon cable with bare wire ends. They were also quite short, so even if soldered together they wouldn't have had the 10" or so length to fit in the computer.

These cables are all additions that were added to the original EDUC-8 design in order to increase the memory size. The first couple of articles in Electronics Australia where the project was described had only a 32 byte memory, but by the third article where construction began, they had expanded it to 128 bytes with wiring in place for a future update to 256 bytes. 

This was due to the rapid decrease in memory cost and increase in memory chip size. At the time of the articles starting in August 1974, memory cost was very high and thus the tiny memory was a practical accommodation. 

Thus the memory address bits 7, 6 and 5 were added in mid stream. If it had been part of the original design work, the signals would have been assigned on the connectors of the motherboard. However, as a last second change, they were routed as discrete wires soldered between boards. The recreation boards that I used have put those wires on connectors for neatness, rather than directly soldering them into holes. 

Thus, several cables have to run between boards to deal with the added memory addresses. If the design had been refactored when the new boards were designed, the signals could have been put on the upper side of the boards. The original design had copper only on one side, thus fingers on the board were only on one side and the connector pins were unused for the other side of each board. 

However, the goal of the boards is to get as close as possible to the original article version of the EDUC-8. The only change to the designs were using the back side of each board instead of soldering jumper wires across the boards, and to put in connectors instead of soldering individual wires for the board to board cabling. 

I had some spare ribbon cable which I cut and soldered between the cable fragments I received from AliExpress, with heat shrink tubing on the connections. That gave me all the cables I needed to plug the computer together. 

TESTING PLAN FOR TOMORROW

The initial testing per the article had only a subset of boards installed on the motherboard - the timing, decoder, accumulator and program counter/adder boards. A temporary set of wires are tacked on the front panel to connect the eight toggle switches to the memory buffer lines, since the memory board is not installed at this time. The switches introduce data that the logic treats as the output of memory. 

I injected all the instruction op codes and modes while I watched the EDUC-8 perform properly. However, the incrementing of the program counter was not working properly, which I attributed to the lack of the cables between boards. I will test again to see if the incrementing and other adder functions are working properly.

Once I pass the tests with the subset of boards, I will add in the Memory and IOT (input-output control) boards and do a more comprehensive test of the machine. This will include memory operation, of course, but also all the edge cases with data and addresses.