Re: Mar1e gets (some) address LEDs #Homebrew #Electronics

Paul Schmidt

John, when I built the first of my two identical 'original configuration' ELFs (I have the original and my duplicate is (I think) still on display at Bletchley Park, both I (with mine) and the Bletchley volunteers (with theirs) noticed a slight glitchiness when making changes to memory. I my case, I initially attributed it to just clumsiness on my part, or brain fatigue during tedious program entry. When multiple guys at Bletchley contacted me about experiencing the same issue, I played around and found an alternate sequence of toggle switch movements that seemed to completely cure the problem. I think my online-published ELF documentation still shows the original sequence:

- MP up
- Press IN repeatedly until the byte immediately before the desired byte is displayed
- Set 8 toggles for correct byte contents
- MP down
- Press IN once
- MP up

In my correspondence with Bletchley, I have a record of the improved sequence, but offhand I don't recall what it is. My computer's boot drive took a dump over this last weekend, and although all normal data was automatically backed up, my OUTLOOK emails were not (oops!), so at the moment I cannot check to see what I told them. A data recovery service is working to recover those stored emails, but it might be a couple weeks before I (hopefully) have access to them again.

Maybe somebody else on this forum has the answer in the meantime.

Paul Schmidt

-----Original Message-----
From: John
Sent: Apr 10, 2019 12:30 PM
Subject: [cosmacelf] Mar1e gets (some) address LEDs

Hi Lee and the group,

I've made a board with eight LED and eight 3k3 resistors which plugs into the second 6116 socket to show the lower eight bits of the address bus. It confirms my fears that, when I step through a program with MP active in order to edit a location, switching MP off writes the byte from the switches to the current location. Pressing IN (as I'm supposed to) then writes the byte from the switches to the next location (as it's supposed to). My circuit closely follow's Paul Schmidt's design. 

I don't yet know whether this happens every time or just sometimes. I'll try it repeatedly later tonight. I think I can get away with it by setting the current byte for the current location on the switches (as shown by the TIL311s), switching MP off, setting the next byte on the switches and pressing IN. I don't think this should be necessary though. Something is wrong.

I'll attach a new photo of Mar1e. The 2114 sockets are gone, the 6116 sockets are on the right and I've added a 4-way DIP switch. The left-most switch holds the active-low MW pin of the battery-backed RAM chip high for switching off and on, that way the RAM contents don't get corrupted. The other three switches will be used on the 6116's A8, 9 and 10 inputs, giving me effectively eight blocks of 256 bytes of RAM which will be maintained when the power is off. I've also relocated the 1MHz can oscillator to give me more space at the top. I'll make a new back apron with holes for a 9-way D connector (RS-232 via a MAX232) and a 25-way one (for parallel interface). 

The black wire exiting stage left is for the 'scope's earth clip.

Any ideas, anyone?



Paul Schmidt

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