It is currently Tue Oct 17, 2017 6:42 pm

All times are UTC - 7 hours



Forum rules


1. NO BLATANT PIRACY. This includes reproducing homebrew less than 10 years old, with the exception of free software.
2. No advertising your reproductions, with the exception of free software.
3. Be nice. See RFC 1855 if you aren't sure what this means.



Post new topic Reply to topic  [ 5 posts ] 
Author Message
PostPosted: Sun Jun 18, 2017 6:52 pm 
Offline
User avatar

Joined: Sat Jul 04, 2015 9:58 am
Posts: 529
Location: -29.794229 -55.795374
Hi.
I just got some pirate and dead SNES cartridges that I'm rebuilding to myself.
Could the use of 74xx, in the case a 7400, become a problem in the future?
Attachment:
File comment: Board Mess
20170618_182744.jpg
20170618_182744.jpg [ 659.06 KiB | Viewed 470 times ]

Attachment:
File comment: Cartridge
20170618_223114.jpg
20170618_223114.jpg [ 597.07 KiB | Viewed 470 times ]

Most of the time I find the epoxy ROM IC to be the problem.
Is it normal?
Maybe I'm not testing the support circuit properly...


Last edited by Fisher on Mon Jun 19, 2017 3:29 am, edited 1 time in total.

Top
 Profile  
 
PostPosted: Sun Jun 18, 2017 7:00 pm 
Offline

Joined: Sun Apr 13, 2008 11:12 am
Posts: 6276
Location: Seattle
I would arbitrarily guess that the 74'00 is there serving as a demultiplexer, enabling one chip-on-board ROM or the other depending on cart /ROMSEL and probably A20 or A21.


Top
 Profile  
 
PostPosted: Sun Jun 18, 2017 8:19 pm 
Offline
User avatar

Joined: Sat Jul 04, 2015 9:58 am
Posts: 529
Location: -29.794229 -55.795374
Exactly that!
No problem in using a non ls/hc part?
I'm also a little confused with the demux, since I got a mixed bag, with some games with it on /RD, others on /Cart...
Even the ROM control pins plugged on the demux sometimes are /CE, others /OE...
What's the correct way of doing it?


Top
 Profile  
 
PostPosted: Sun Jun 18, 2017 8:29 pm 
Offline

Joined: Sun Apr 13, 2008 11:12 am
Posts: 6276
Location: Seattle
Each ROM already has two enables. They're usually called /CE (which "should" be connected to /ROMSEL) and /OE (which "should" be connected to /RD) but plenty of games, even from Nintendo themselves, swap them.

The ROM consumes power/generates heat, but doesn't do anything else, if only its /CE is low. It only drives the data bus if both /CE and /OE are low.

So it's not really any less functional to demux /RD depending on an extra address line.

Conceptually, however, the /ROMSEL signal is generated from A15 and A17..A23, because that's the "who" signal and /RD and /WR are the "doing what" signal. So it's conceptually clearer to demux /ROMSEL.


Top
 Profile  
 
PostPosted: Sun Jun 18, 2017 8:46 pm 
Offline
User avatar

Joined: Sat Jul 04, 2015 9:58 am
Posts: 529
Location: -29.794229 -55.795374
Thanks!
I've been "fighting" for weeks with a board I'm trying to rebuild with an 74ls139 and 4 ROMs.
I'll post the results as soon as I got something new.
This one was simpler. The demux was behaving weird, getting the outputs activated ramdomly. Looks like I was right about the problem...


Top
 Profile  
 
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 5 posts ] 

All times are UTC - 7 hours


Who is online

Users browsing this forum: No registered users and 1 guest


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Search for:
Jump to:  
Powered by phpBB® Forum Software © phpBB Group