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Hardware mods>Amiga 500 Rev.5 stable EPROM solution
buggs 11:40 25 May 2016
I ran into serious stability issues while trying to upgrade a Rev.5 Amiga 500 to Kickstart 3.1. As the problem with EPROMs in A500s seems to be quite common, I'd like to share my experiences in getting EPROMs stable in those A500 revisions. My EPROM (M27C800) was rated properly with 100ns access time, got me to the boot menu and also booted the Workbench in most of the cases. But the system quickly crashed.

A little reading around the Amiga-centric forums led me to believe that a solution to unstable and/or unbootable Amiga 500 machines might be something worth sharing.

The only thing needed to get Rev.5 Amiga 500 boards working with Kickstart EPROMs is the installment of two resistor packs on the address lines.
Of course, the usual fix for the famous "oops" bug concerning A17 is still required by bending up pin 31, connecting it to VCC and routing the socket pin 31 (A17 on Rev.5 boards) towards pin 1 (A17 on 512kB ROM/EPROM chips).

I included a picture outlining the pins that require pull-up resistors on A500 Rev. 5 boards, in this case for a 512 kB EPROM.

To install the pull-ups, a number of options come to mind. One way would be to solder the resistor packs to the board below the ROM socket. The space between board and bottom case should be sufficient. My experimental setup was to bend the pins of the resistor packs and put them into the socket (believe it or not, while that attempt was anything but mechanically stable, the machine did run happily for hours). In the end, I chose to set up a PCB, where I've put EPROM, support logic (3x Kickstart switch in my case) and resistor packs.



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idrougge 13:44 25 May 2016
Great!
Foul 20:17 03 June 2016
on your upper picture... resistors are connected from pin 1 to pin 9 (A17-A0) and on your photo, it's pin 2 to pin 10... !?!?
Arnie 20:33 03 June 2016
Originally Posted by Foul:
on your picture... resistors are connecter from pin 1 to pin 9 and on your photo, it's pin 2 to pin 10... !?!?
Looks like 2-9, pin10 is an earth.
buggs 10:10 04 June 2016
Originally Posted by Foul:
on your upper picture... resistors are connected from pin 1 to pin 9 (A17-A0) and on your photo, it's pin 2 to pin 10... !?!?
Good catch. My apologies if I caused some confusion. What I didn't mention was that my EPROM carries two 256k images with some extra logic (not shown) towards pin 1. Therefore _my_ test setup didn't need the extra pullup on pin 1.

I'd also like to point out (again) that my photo is just meant to illustrate how I tried the idea out. I would never ever recommend to to it this way as a long-term solution.

My final setup includes a 3 Kickstart switching logic along with the resistors and looks like this:

Foul 10:14 04 June 2016
Excellent
fordav1 18:58 31 July 2017
On an EPROM that is double the normal size (and holding two different kickstart images), the highest address line can be tied high or low using a SPDT switch to toggle between the two kickstart images. Nothing else is required. So I'm wondering... what's the logic chip for?
buggs 09:55 01 August 2017
Originally Posted by fordav1:
On an EPROM that is double the normal size (and holding two different kickstart images), the highest address line can be tied high or low using a SPDT switch to toggle between the two kickstart images. Nothing else is required. So I'm wondering... what's the logic chip for?
Sure thing. But I did a little more. I've put two 256k images (1.2,1.3) and a 512k image (3.1) into my 1MB EPROM. In that case, you need to have A17 tied up/down for the 256k images or patch it through it from the 68k, respectively.
FicHitA 21:08 08 June 2019
Hi mates, and sorry for reviving this old chat.

I found this in EBAY and at first looks like your pull-up fix plus the Oops Bug but with a few diferences in the squematic.

will this one works too ??? and in the one from EBAY where is the COMMON signal from the 9+1 resistor packs ??? please con someone help me to understand this ???

PD: Sorry for my bad english.
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Eriond 12:05 09 June 2019
Maybe this link will shed some more light:
http://retropcb.com/2018/09/07/the-a...start-upgrade/
SukkoPera 18:07 09 June 2019
I have also made my own version some time ago, 100% working in A500 (all revs) and A2000:

https://github.com/SukkoPera/OpenKickstartSwitcher

There is also a version for the CDTV, same circuit but different shape:

https://github.com/SukkoPera/OpenKickstartSwitcherCDTV

It's Open Hardware, I have also a couple of PCBs left if you're interested.
buggs 20:07 09 June 2019
Originally Posted by SukkoPera:
I have also made my own version some time ago, 100% working in A500 (all revs) and A2000:
https://github.com/SukkoPera/OpenKickstartSwitcher
Nice one. I was just going to fire up Kicad while reading about the EBay-only version of the A500 Rev.5 EPROM Kickstart fix. Thanks for sharing your PCB layout.
danko 11:34 08 August 2019
Hi,i have a A500 Rev.5 with kick 1.3 which versions of kickstart are compatible without mod?
Jope 15:32 08 August 2019
1.2 and 1.3 mask roms.
danko 16:17 08 August 2019
Originally Posted by Jope:
1.2 and 1.3 mask roms.

Thanks Jope,for Kickstart v2.04 rev 37.175 or other versions need adapter like this:

Jope 18:18 10 August 2019
Up to 2.05 comes in mask ROM versions. I had a 37.175 mask ROM in my A500 rev 5 as a kid, no stability problems, no additional resistor networks. The only thing needed was a 1-31 jumper when using a 512k mask ROM.

EPROMs are a different story, as we know.
rmzalbar 10:39 23 August 2019
I did the socket of my Rev. 5 a while back, from under the board. Worked out great, following buggs' diagram. I went slightly overboard for a neat result.

I isolated pin 31 from the address line, jumping around it, and rerouted the address line to pin 1. I then ran VCC from pin 21 to (the now isolated) pin 31.

Ran all my wires up underneath the resistor networks; they made neat little channels. Everything fit easily under the board, and no messing around bending legs on the chip, ever; you just plain use it like a normal Rev 6A socket. No problems with the original KS 1.3 maskrom nor any 256k/512k EPROMS.
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buggs 19:22 02 June 2020
Originally Posted by Jope:
Up to 2.05 comes in mask ROM versions. I had a 37.175 mask ROM in my A500 rev 5 as a kid, no stability problems, no additional resistor networks. The only thing needed was a 1-31 jumper when using a 512k mask ROM.
Yes, indeed. I remember that I had to cut said jumper wire when I used the Kick 2.04 ROM I've bought in 1991 on the Commodore fair in my Rev.6A.

So in a nutshell: 1.2,1.3 and 2.04 (maybe 2.05) Mask ROMs will work in Rev.5 boards without additional hardware.

Other solutions (especially 16 Bit EPROMs) will require modifications. At least the "OOPS" Bug (Pin 1 vs. Pin 31 in the stock Rev.5 ROM socket) needs to be taken care of. It might be possible that EPROMs work sufficiently well after doing the above fix. The A500 design relies on the internal Pull-Ups of the NMOS 68000 CPUs. If a particular CPU doesn't provide enough current to EPROMs with relatively high capacitive load on the address lines (in addition to the load by the rest of the system), then the A500 will get unstable. In my experience, this effect depends not only on the board revision but also the installed CPU and EPROM manufacturer.

I've heard about Rev.5 boards working with EPROMs just by doing the "OOPS" fix. Likewise, I've had a Rev.8 board which also got unstable with the 120ns 27C400 EPROM I've installed into it.

So for Rev.5 the fix is as outlined. I chose to keep the board itself original but rmzalbar's method is also applicable, of course .

Should later revisions of the A500 become unstable in conjunction with Kickstart EPROMs, then the workaround gets a little easier. From Rev.6a onwards, there are provisions to install RP105,RP106 and RP107 directly onboard. When these three locations are populated with star-type resister networks in the range of 4.7k-10k, then the same pull-up fix I've proposed for the adapter PCB will be in effect.
Giants 16:49 11 August 2021
Hi !
I have also a Rev5 motherboard with 1.3 Kickstart, work good but, indeed don't work with my Diagom on 27C400

Question : Do you know guys if, with this hardware modification (pull-up and co, see on top of this thread), my original Kickstart 1.3 will continue to work ?
If I do this modification it would be only to use occasionally my Diagrom on my 27C400, and after, return to kickstart 1.3
lyzanxia 09:42 17 August 2021
I just bought a kernel switcher for my rev5. (based on openkickstartswitcher) Works great, have diagrom and 1.3 in one rom.

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