Paul’s design looks like it lets the /WE input on the RAM float during the short time that the MP switch is between breaking one side of the throw and making the other. I suppose this is fine if the /WE input on the RAM floats high but that’s probably not a good assumption, especially if you are not using the same RAM he did.
For the simplest fix, I would suggest tying /WE of the RAM high through a pull-up resistor (22K should be fine) to avoid this. Once you do that, you don’t really need a double-pole switch for MP either, but it won’t hurt.
From: email@example.com <firstname.lastname@example.org> On Behalf Of John
Sent: Wednesday, April 10, 2019 1:30 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?