Enabling the write signal for $E000-$FFFF.

What have you built or do you want to design to go with your replica 1

Enabling the write signal for $E000-$FFFF.

Postby fsafstrom » Sep Tue 25, 2007 4:08 pm

Howdy...

What I have done is to replace the 28c64 EEPROM with a DS1225AD NVRAM and it works great. The "problem" is that it is read-only and I would like to make it read/write.

I was looking at the schematics in the "Apple I Replica creation" for the RAM and ROM.
It's pretty straight forward and I noticed that the /WE signal (pin 27) on the 28c64 is held high with a 3.3K resistor.
Would it be possible to cut this resistor and bridge the /WE signal from the RAM (pin 27 of the 62256)?
It looks like this 3.3K resistor is R10 on my Replica SE but I wonder if someone can confirm this...
User avatar
fsafstrom
 
Posts: 154
Joined: Dec Tue 26, 2006 3:57 pm
Location: San Antonio, Texas

Postby Kallikak » Sep Tue 25, 2007 9:03 pm

Not that I'm even close to be expert in this kind of thing, but it looks ok to me. It's the exact same signal that is used to make the EEPROM writable in the simple burner circuit.

I'd want to put some kind of jumper on it though, so the ROM could be held readonly unless I really wanted it to be writable.

Ken
Kallikak
 
Posts: 172
Joined: Jan Sun 29, 2006 7:42 pm
Location: Sydney

Postby fsafstrom » Sep Wed 26, 2007 1:37 am

The "Read-Only" switch is a great idea, I just don't know where to put one...

Anyhow, I'm going to start the operation tomorrow, wish me luck... ;-)
User avatar
fsafstrom
 
Posts: 154
Joined: Dec Tue 26, 2006 3:57 pm
Location: San Antonio, Texas

Postby fsafstrom » Sep Wed 26, 2007 1:18 pm

The operation went fine and $E000 to $FFFF is now Read/write... ;-)

/Bamse
User avatar
fsafstrom
 
Posts: 154
Joined: Dec Tue 26, 2006 3:57 pm
Location: San Antonio, Texas

Postby vbriel » Sep Wed 26, 2007 4:19 pm

Use caution when doing that. I originally had the rev A and rev B boards set with a unlabled jumper that you could enable write mode on that socket. The problem comes when you try and write from the Woz monitor. It probably will work just fine with NVRAM but EEPROM will probably lock up the board and may corrupt the EEPROM! So, NVRAM should be fine but I wouldn't write to EEPROM unless you have a backup copy.

Vince
User avatar
vbriel
Site Admin
 
Posts: 1184
Joined: Jul Tue 19, 2005 1:10 pm
Location: Ohio

Postby fsafstrom » Sep Wed 26, 2007 7:19 pm

I'm using NVRAM so I should be OK...

So far so good, I can always undo the changes if it doesn't work properly...
User avatar
fsafstrom
 
Posts: 154
Joined: Dec Tue 26, 2006 3:57 pm
Location: San Antonio, Texas


Return to Creation Room

Who is online

Users browsing this forum: No registered users and 3 guests

cron