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


Hello Paul,

The wiper of the switch does go to the active low write enable pin(s) on the RAM in both the original PE diagram and Paul's diagram. The poles are connected to +5v (write protect) and pin 35 on the 1802 (active low memory write).

Not much of my Elf is "period correct", having a 6116 and laser-cut and engraved panels, but the general layout started like that of the original (and quickly changed). I might one day build an unexpanded original with authentic parts but that's not a priority. Having fun with it is the priority to justify the time I'm spending on it. An alphanumeric LCD is a probable addition, and a hex keypad. I fancy building an AFSK oscillator too, 1275/1445Hz, so it can talk acoustically to an RTTY app on my Android tablet at 50bps. I love that warbling sound!


On Wed, 10 Apr 2019 at 22:20, David Madole <david@...> wrote:
The 22K shouldn't go to the wiper, if it's wired like Paul's diagram. It should go to the terminal that connects to the RAM.

Maybe your circuit is a little different though.

The MAX232 is not "period correct"... don't do it! Just kidding. Kind of. 


On April 10, 2019 5:08:23 PM John <easydogxray@...> wrote:

Thanks again for all the help. I can confirm;

There is no difference between the original circuit diagram and the one I called "Paul's design" 
The DIP switch I added does exactly the same as leaving the MP switch in the active condition (why didn't I spot that?)
Adding the 22k pull-up between the switch wiper and +5v has fixed the problem. I can now flick the MP switch up and down without any unwanted writes to RAM.

Adding the address LEDs was worthwhile too. I can see where I am. I will sleep well tonight.

Next job is to wire the other DIP switches to A8, 9 and 10, make the new back apron with D connector cut-outs (maybe tomorrow), add the MAX232 and some ports.

I love this machine, and I only found out about it by chance while planning a SC/MP-based project. 



On Wed, 10 Apr 2019 at 20:47, David Madole <david@...> wrote:

Ah ok, I was just replying to what John called it. I took a quick look at the schematic but I really didn’t read the history or other documentation with it to understand that.


It’s been 40 years since I’ve seen the magazine article so I have no idea any more what the original design looked like :)





From: <> On Behalf Of Paul Schmidt
Sent: Wednesday, April 10, 2019 3:36 PM
Subject: Re: [cosmacelf] Mar1e gets (some) address LEDs


For what it is worth (and I am gratified that folks are using the ELF documentation I prepared), there is no "Paul's Design" in regard to the ELF schematic. What I show is just the same as the magazine article schematic, merely cleaned up and made neater, with more descriptive comments and such. I did not change anything that I can recall in regard to the way "MP" works.....



-----Original Message-----
From: David Madole
Sent: Apr 10, 2019 2:15 PM
To: ""
Subject: Re: [cosmacelf] Mar1e gets (some) address LEDs

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: <> 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?





Paul Schmidt


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