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 Post subject: how to switch OS when burned in 27c256 [SOLVED]
PostPosted: Fri Aug 31, 2007 11:24 am 

Joined: Thu Feb 05, 2004 10:34 am
Posts: 259
Location: NL
Hi!

Important to know: I can build technical schemes, but I don't understand a thing of it. If you can help me with this question, please provide clear info :)

I have burned a 27C256 eprom with 2 OS roms in it. When I put it in a stock XE it uses one of these 2 OS roms.

What components do I need to use, and where do I have to solder them, to make a switch on my Atari?

I heard something about pull-up resistors... but I don't understand how this works, or where I have to solder hem.

I hope someone could help me.
Thanks
Marius

p.s. Thanks to Fox-1 and Beetle I found the solution for this. It's without resistors, and simply putting the VCC or the GND on pin 27 (A14) ... thanks!


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PostPosted: Sun Sep 02, 2007 3:05 am 
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Best is to put a 4.7k resistor between Vcc and A14.
Switch A14 to ground to select the first 16k.

This to avoid high current peaks at powerups....

Cheers,
Sijmen.


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PostPosted: Sun Sep 02, 2007 3:49 am 

Joined: Thu Feb 05, 2004 10:34 am
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Thanks!

I'll do that.

but one question: I used pin 28 of the rom itself for VCC, I was supposing this pin 28 was already protected for current peaks... so I could safely take vcc from there? Or is the chip self on pin 28 protected for these, and isn't A14?

thanks
Marius


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PostPosted: Mon Sep 03, 2007 4:07 am 
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Sysop*Marius wrote:
I used pin 28 of the rom itself for VCC, I was supposing this pin 28 was already protected for current peaks... so I could safely take vcc from there? Or is the chip self on pin 28 protected for these, and isn't A14?


I don't know about what pins are protected or not but if you don't use a pull-up resister, but just switch A14 between VCC and ground, then during switching the state of A14 is undetermined (not connected to VCC or ground at all). To avoid undetermined state, you use a pull-up resistor so you are sure the state on A14 is always VCC or ground.

Robert



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PostPosted: Mon Sep 03, 2007 8:24 am 

Joined: Mon Jun 21, 2004 7:53 am
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mr-atari wrote:
Best is to put a 4.7k resistor between Vcc and A14.
Switch A14 to ground to select the first 16k.

This to avoid high current peaks at powerups....

Cheers,
Sijmen.


Using the same theory, how would you do a 27512? I have been using a scheme that uses two SP-DT switches, but does not use pull-up(s), so I presume it would have the same issues. Always looking for a better way...
Thanks,
Larry


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PostPosted: Mon Sep 03, 2007 11:52 am 

Joined: Thu May 17, 2007 8:48 pm
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Location: Waterloo, ON Canada
thewhiz wrote:
Using the same theory, how would you do a 27512? I have been using a scheme that uses two SP-DT switches, but does not use pull-up(s), so I presume it would have the same issues. Always looking for a better way...
Thanks,
Larry


Basically, the same way. Two 4.7K resistors, A14 and A15 to VCC. Then from those two pins through two switches to GND.
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PostPosted: Tue Sep 04, 2007 2:39 am 
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Can someone elaborate a little more? I was wondering how to switch to 2 OS's too. How does it get wired?


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PostPosted: Tue Sep 04, 2007 9:18 am 

Joined: Thu Feb 05, 2004 10:34 am
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I'm not a technical drawer, but I tried to make it clear.

+5 (VCC) is on pin 28 and GND is on pin 14, if you want to take that from Eprom itself.

Only pin 27 (a14) is out of socket. In case of 27C256.

Marius


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PostPosted: Tue Sep 04, 2007 9:35 am 

Joined: Thu Feb 05, 2004 10:34 am
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4 OS switch


Thanks
Marius

p.s. with 2 switches you have 4 options, so that is 4 OS'es. You need to merge 4 16K OS's in one file, and burn this 64K file in a 27c512.


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PostPosted: Tue Sep 04, 2007 4:18 pm 
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Thanks!!

So you said "+5 (VCC) is on pin 28 and GND is on pin 14"

Where is a good place to hook up to VCC? So the toggle is for GND and no GND? Thats how that toggles the two OS's?

Looks cool will have to try that on my 800XL sometime.


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PostPosted: Wed Sep 05, 2007 3:53 am 

Joined: Thu Feb 05, 2004 10:34 am
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I think it doesn't matter where you get it from.

Just get it from a near IC. I left the Eprom as much as possible alone, so it is easy to remove it again for any reprogramming.

If you don't think you will reprogram it again (so it will be there for a much longer time) why not taking pin 14 and 28?

Strange thing btw. First I had the 27256. I was happy... then I thought: hmmm 4 OS's is really what I want... (I thought: 4 OS's must be enough for everyone forever*) so i did put a 512 in the XE ... and now I think: hmmm a 1Mbit eprom is cool too, and it is not so hard to put that in.... 8 OS's hmmmmm great.

I'd say
1. Default XL/XE OS
2. Qmeg OS
3. MyIDE 3.1
4. MyIDE 4.3
5. Mux OS (do you know the multiplexer...?)
6. SP+ OS
7. free for future use
8. free for future use.

I'm getting greedy by this idea.

Perhaps I should consider that 32-in-One upgrade...

Marius


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PostPosted: Wed Sep 05, 2007 6:23 am 

Joined: Mon Jun 21, 2004 7:53 am
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Sysop*Marius wrote:
I think it doesn't matter where you get it from.

Just get it from a near IC. I left the Eprom as much as possible alone, so it is easy to remove it again for any reprogramming.

If you don't think you will reprogram it again (so it will be there for a much longer time) why not taking pin 14 and 28?

Strange thing btw. First I had the 27256. I was happy... then I thought: hmmm 4 OS's is really what I want... (I thought: 4 OS's must be enough for everyone forever*) so i did put a 512 in the XE ... and now I think: hmmm a 1Mbit eprom is cool too, and it is not so hard to put that in.... 8 OS's hmmmmm great.

(snip...)

I'm getting greedy by this idea.

Perhaps I should consider that 32-in-One upgrade...

Marius


What kind of switch(es) would you use? Rotary? I've never seen how those rotary switches are wired to a 27C1000. Can you sketch part of it?
-Larry


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PostPosted: Wed Sep 05, 2007 6:38 am 

Joined: Mon Jun 21, 2004 7:53 am
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Sysop*Marius wrote:
I think it doesn't matter where you get it from.

Just get it from a near IC. I left the Eprom as much as possible alone, so it is easy to remove it again for any reprogramming.

(snip...)

Marius


P.S. FWIW, yes, the eprom legs are an issue with these. I cut most of the legs off A14 and A15 on a 27C512 leaving just stubs to solder wires to, but to re-program it, I just take a short piece of resistor wire and solder on new legs to the pin stubs. Works fine for me. The biggest hassle is setting up my laptop and burner and eraser (and soldering iron, sometimes).

Flash memory is great. So I think that Steve's new internal flashable MyIDE should be pretty neat. True, it takes a little time to re-flash, but I probably only change OS once a week or so. But an easily reprogrammable 32-in-1 would be very nice, indeed...

-Larry


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PostPosted: Wed Sep 05, 2007 7:24 am 

Joined: Mon Jun 21, 2004 7:53 am
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Sysop*Marius wrote:
I'm not a technical drawer, but I tried to make it clear.

+5 (VCC) is on pin 28 and GND is on pin 14, if you want to take that from Eprom itself.

Only pin 27 (a14) is out of socket. In case of 27C256.

Marius


I had not seen (or tried) one wired up like this previously. So a few comments/questions:

1) It seems like what you are doing is either supplying current to the chosen pin or grounding out the pin/VCC through the 4.7K resistor. I presume when the pin is grounded, then that bank is *inactive*?

2) The 27512 diagram seems a little more difficult to understand. Same theory, but now there are 4 banks. Either switch will select one bank or the other, but it seems like there is a conflict in the bank selection here, and only 3 banks could be selected. (?) Can someone make a truth table (or similar) that shows what banks are selected depending on the switch positions?

-Larry


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PostPosted: Wed Sep 05, 2007 8:42 am 
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thewhiz wrote:

I had not seen (or tried) one wired up like this previously. So a few comments/questions:

1) It seems like what you are doing is either supplying current to the chosen pin or grounding out the pin/VCC through the 4.7K resistor. I presume when the pin is grounded, then that bank is *inactive*?

2) The 27512 diagram seems a little more difficult to understand. Same theory, but now there are 4 banks. Either switch will select one bank or the other, but it seems like there is a conflict in the bank selection here, and only 3 banks could be selected. (?) Can someone make a truth table (or similar) that shows what banks are selected depending on the switch positions?

-Larry


Hi Larry,

All you are doing here is supplying either logic 0 (ground) or logic 1 (Vcc) to the additional address lines of the chip:

0 0 - Bank 1
0 1 - Bank 2
1 0 - Bank 3
1 1 - Bank 4

In one of my systems I have a 27C2001 with 16 OS's burned on to it. I select the OS with a Hexadecimal rotary switch. Much easier than 4 toggle switches!


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