Open connection in a wire in a cable caused one of the PC/Adder card issues

LOADING AN ADDRESS OF 00000000 LED TO 01000000 IN THE PC REGISTER

The Program Counter (PC) is implemented by two shift registers which use serial load to clock in incremented or calculated addresses as well as parallel load to set the PC to a specific value. 

The Load Address switch engages the clear function of the two shift registers when it is depressed and then does a parallel load from the switch register (the toggle switches on the front panel) as it is released. Holding down Load Address, I could see that the PC was all zero, and my toggle switches were all set to zero, but when I released Load Address the address show was 01000000 instead of all zero.

UNIQUE ASPECTS OF ADDRESSES DUE TO EVOLUTION OF THE EDUC-8

From the time that the Educ-8 was first conceived and written about in Electronics Australia until the series of construction articles were published, the designer expanded the machine from its original limit of 32 words up to 128 words, with provision for 256 using yet unavailable larger RAM chips. 

Thus, address bits were initially only five bits, to accommodate the max memory of 32 words. The assignment of pins on the connectors was already complete by the time that the first construction articles came out, thus they couldn't be modified to add 2-3 more bits without ruining the work of readers who had begun with the initial design.

Therefore, the new bits are carried over a set of additional wires that ran between boards. The toggle switches that correspond to the three high address bits had to be brought to the PC board in order to do a Load Address. The three address bits had to be brought to the front panel to display for the memory address. The three address bits had to make it to the Memory board too. 

The article simply soldered individual wires onto the boards to carry these extra signals, but in the recreation of the PCBs, Gwyllym Suter installed connectors to allow cables that could be disconnected easily for service. 

MY CABLE CARRYING THE THREE HIGH SWITCH BITS HAD A BROKEN CONNECTION

The wire bringing bit 6 to the PC board from the front panel switch register was an open circuit, which was treated as a binary value of 1 when loading the PC shift registers. I repaired the cable and now the PC is loaded properly from the toggle switches. 


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