1. cosmacelf@groups.io

Re: Noob question: examine arbitrary memory address? #ELF #memory

P Todd Decker
 

Tor-Eirik—Thank you for contributing your design! I am glad we now know which member did these designs.  It wasn’t obvious to me from your GitHub site. There are five of us now who have your boards and will be building it. And, I know of a sixth who is also interested. You may have also seen from other posts that I have created a more detailed BOM.  Could you take a moment to review it?  Especially the specs for the 7-segment LEDs.  

All — For the rest of us who are preparing to build this, should we switch the resistor value to 3.3K as John did?  Or, should we keep the same resistor and instead change from the 74LS10 to the 4023 or 74HC10?  Are there any other ICs or resistors that we should also change?

On Sep 26, 2020, at 6:57 AM, Tor-Eirik Lunde <teblunde@...> wrote:

Yeah... I'm a bit of an amateur when it comes to electronics, so when it comes to faults I can just about figure out the logic of things. 

Nice going on finding a fix for the errors in my ways. Will have to revisit the design once I get a semblance of a hobby workshop up and running again 😀

lør. 26. sep. 2020, 04:18 skrev David Schultz <david.schultz@...>:
On 9/25/20 8:46 PM, John Kennedy wrote:
> 
> I unsoldered and replaced D6, D7 and the 74LS10.
> Interestingly the LOW signal at pin 10 of the 74LS10 is exactly as
> expected. However, when the chip is inserted, the logic probe won’t give
> a high or low - there’s a small voltage there. Where could it be coming
> from?

The 47K resistor in the wired-OR is great for CMOS logic but TTL, even
LS TTL, draws more current. A quick check of a data sheet shows input
current  when Vin=0.4V of -0.4mA. Which will create quite the voltage
drop on a 47K resistor.

You could lower that resistance but I can't help but be horrified by the
use of LS TTL with an 1802. 74HC or HCT if you must but please not LS.


-- 
http://davesrocketworks.com
David Schultz
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