Initial testing good, with a few mistakes to clean up

WIRING THE EIGHT TOGGLE SWITCHES TO THE MEMORY BUS FOR TESTING

The memory card is not yet installed, but by wiring the toggle switches to the memory bus I can set up the data as if it came in from a memory read. That is the way that the author/designer of the EDUC-8 recommended testing the functionality. 

FIRST ISSUE - METAL LOCKWASHER AND THE GROUNDED SIGNAL

The connectors that the cards plug into are screwed down on the motherboard. I used some metal screws, nuts and lockwashers. When I began testing with the Timing, Decoder and Accumulator cards plugged in, I found that switch 4 was not working - there was always a ground as the state of the memory bus in that position. 

When I pulled the boards, the short to ground was still there. I pulled the suspect connector off the motherboard after removing the hardware, and the short was gone. I soldered down the connector, but just the pin that had the problem, so that I could keep checking to be sure the issue didn't come back. When I reinstalled the screws and nuts, the short was back! 

That led me to find a trace (for the switch 4 signal) passing very close to the mounting hole on the motherboard. The lockwasher scraped the solder mask off and shorted the signal trace. I switched over to nylon screws and nut, after which the problem was gone. The connector was fully soldered in place and I got back to my testing.

SECOND ISSUE - MOMENTARY SWITCHES INSTALLED UPSIDE DOWN

There are five DPDT switches that are momentary on, used for the functions Run, Halt, Examine, Deposit and Load Address. I installed them with the idle position down, thinking that I would lift the switch to activate its function, e.g. Examine. However, the actual wiring of the front panel board (and design of the EDUC-8) expects that you push these down momentarily to activate, rather than push up. 

I removed the five and installed them properly. 

The magazine article that defined this system suggested a sequence of tests, which I followed successfully. I set each of the eight op codes into the first three bits and hit Run while in Slow and Single Instruction modes. I saw it perform the fetch cycle and then illuminate the proper op code LED during the execute cycle. 

Indirect addressing in the machine is indicated by the fourth bit. I tested this and verified it injected a third cycle, Defer, between the Fetch and Execute cycles. Further, the op codes IOT and OPR use this bit for other purposes, so I verified that there was NO Defer cycle with IOT and OPR even with bit 4 turned on. 

NEXT UP - ADD PROGRAM COUNTER/ADDER CARD AND TEST

In order to use this card, I need to make up a few cables that run between various cards. When I get back to the shop I will make these and install them, then move on to the next set of tests to perform before insert the memory and IOT cards. 

DISCOVERED ONE MISTAKE IN THE BOARD VERSION I RECEIVED

The front panel has a toggle switch for Fast and Slow mode, using a DPDT On-On component. The traces on the front panel are wired so that with the switch up, the machine is in slow mode. However, the magazine article and original EDUC-8 reverse that, so up means fast mode and it must be switched down to run in slow mode. I cut some traces and added wires to make this work as intended. 

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