Kickstart ROM adapter for A2000/A500

[Eriond]

I realized how difficult it is to get hold of 27C400/27C200 PROMS compared to the much more common 27C4002.
To remedy this I designed a small adapter board, which will be ordered asap. (Board designed to fit Olimex 10mil DRU).



Of course, the design is freeware, and comes with no warranties whatsoever

I'm also including the custom librarys used in this design.

Have fun!
/Eriond

[rkauer]

Neat!

Any chance to create an adaptor to use a pair of 27C801 instead?

[Eriond]

I'll do better than that; I'll give you two reasons

1) The space to fit two large 40-pin DILs is an issue. In the A500 the height must be considered so that the device fits beneath the RFI shield. This would result in different adapters depending on the target platform.
A way around the problem may be to use PLCC ROMs (smaller footprint) and a SMT pin adapter mounted on the bottom side...

2) This design does what I need... And it took me some six hours to rout. Maybe some day in the distant future, when I have eons on time at my disposal, I'll give it a shot.

Regards,
Eriond

[Eriond]

Today the boards arrived from Olimex, so I took a few minutes in the company of my soldering iron... I'll do a test loading the 3.1 kickstart on sunday at SUGA.


And when assembled:


I whish You all a nice weekend!

/Eriond

[Eriond]

Well, I held this post back on purpose. It's never fun to admit defeat.
So far my tests result in a non-booting A2000 with flickering purple video output.

Really discouraging! I will do some more fault-finding, but there's really nothing more than PCB tracks in this design.
What could possibly go wrong? (famous last words...)

I'll keep you posted, but don't build this adapter for the time being!

[cpiac64]

connect pin1 Vpp to pin 40 Vcc and tray

[pandy71]

@Eriond - with current pins You corrupting DIL socket on Amiga - use something like this https://www.elfa.se/elfa3~eu_en/elfa...4-19&toc=20459 or https://www.elfa.se/elfa3~eu_en/elfa...8-51&toc=20459 https://www.elfa.se/elfa3~eu_en/elfa...3-98&toc=20459

[Eriond]

Actually, I'm using the second one You suggested. The thing is that I made the board with too small holes. I thought the adapter would have equally thin pin diameter on both "sides" (like the first link in your post) , but it turns out that one side is much thicker than the other.

To make the adapter fit in the board I had to use the thin pins, which left the thick ones towards the Amiga socket. And, yes it ruined the low-cost socket on the Amiga mobo. I will try to repair/replace it sometime in the future.

And with ruined, I mean that it will now only accept thick-pin devices If I manage to make the adapter work, then it doesn't really matter that I can't use "real" DIP PROMs anymore, right?

@cpiac64: Once in the Amiga, the Vpp souldn't matter anymore. It's only used during programming AFAIK. But yes, I make a temporary connection between pin 1 of the EPROM and pin 31 of the socket during programming.
During normal read operation, the 27C402 uses the Vpp pin to decide if it should work in Byte or Word (8/16 bit) mode, while the 27C4002 always work in Word mode.

According to the 27C4002 datasheet, Vpp should be set to either Vcc or Vss during read. I will give that a try.

Please correct me if I'm wrong here!

/Eriond

[pandy71]

Ok, then yes, no problem with pin size.

[Counia]

Hello.
I already do an adaptator for 27C4002, it is an ROM-switch too.
27c4002 with KS3.1 and Original-ROM in 1.3
It is in use since 1996 in my A2000, recently photo when add an Indivision-ECS.

[Eriond]

Great! That means there's still hope

Did You use an especially fast EPROM? Mine is 100ns, but I have seen them in both 85ns and even 45ns! I read somewhere that access speed could be an issue when replacing ROMs...

[Counia]

Sorry, i no remember all info , but 15 years ago i no think 27c4002 existe in very fast time-acces or in expensive version, i have 150ns or 120ns in my souvenir.

But if 68000 go at 7MHz, T=142ns between "on cycle instruction" and next instruction.
Moreover "MOVE Rom,D0" is an multi-cycle instruction if i remember rigth.
For me your 100ns is good.

I remenber an anecdote in my first test, i dump the ROM space in an file ; save it in an 720Ko PC floppy, for program the EPROM.
The echec is because 8bits-high and 8bits-low are inverted by this programmator, i code an litle program that swap bytes, and the second test is good.

I search if i have info for this.

[Eriond]

Yeah, I've seen in a different thread here at EAB that byte swapping is needed. I too thought that was the answer, but I have now tried both variants (swapped and non-swap) and still the same issue. Unfortunately, my socket in the A2000 is ruined, so I can't restore the original ROM to see if the computer is permanently damaged, or if the issue is related to the EPROM adapter.

I have however added the adapter (permanently) to my A600, and will give that a go tomorrow.
If You're wondering if this is a wise decision or not, I have successfully written to, and read back an Amiga kickstart using this adapter. The programmer (an Elnec Smartprog2) was set up to use a M27C402 from OKI, and after adding the Vpp cable, it had no issues programming and reading.

So I come to the conclusion that there is nothing wrong with the adapter as such. Still, If my Amiga won't boot... well, then something must be wrong.


Click to enlarge (The dark board above is the ACA630...)

/Eriond

[Counia]

The ROM socket is very capricious in my A2000 too, when it walk, i no touch it.
I recover my old IFF file if you would compare pin-cabling ?

[cpiac64]

the prommer read directly the rom original?
insert the adapter in the prommer with your eprom mounted and verify alias original rom

[pandy71]

cpiac64 have right - read original ROM in your programmer - all faults in design (if any!) not important from this point - binary copy will be exactly the same (this is old trick mixed adress lines/data lines - sometimes easier to route on PCB sometimes for security by obscurity) - Amiga probably not use tricks with mixed adress/data lines but datasheet's not always tells You the whole truth...

[Jope]

The Amiga does not use such tricks.

[pandy71]

Originally Posted by Jope

The Amiga does not use such tricks.

Probably not.

[cpiac64]

i have compared the schematics with eagle and datasheet of 24c4002, eagle report broken pin number, the pin out in eagle are for 27c4002 tsop not dip40, dip40 is completly different.

[Eriond]

Sorry for the late reply. Life keeps me busy!

I can now confirm that the adapter works perfectly fine in my A600! My conclusion is that the large pins I used have caused some permanent damage to my A2000 ROM socket. I will replace the socket with my adapter, and I guess that will do the job.

The design has given me an urge to do something more 21:st century -style... Maybe a flash-based dual ROM (internally upgradeable kickswitch).

However, I have a completely different (Amiga-)project at hand, that takes all my free time right now. I will post info about it once we (me and my fellow co-constructors) come to a point we find suitable to present to the public. But rest assured; It will blow you away!

/Eriond