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Old 07 June 2021, 19:17   #1
ScottC2010
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Floppy disk A500+ Rev 8A and 1MB ROM

Hi,
I have successfully made a 1MB ROM with extra libs for my 1200 and works great using 27c800's

Can i use the 27c800 in the 500 Plus and do the same as i have tried and it won't work (Standard 512kb ROM with the 1MB_ROM file in Remus + 512kb Extended ROM with the correct header and then the files such as workbench.library and icon.library) - is there something else i need to do like jumpers?
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Old 07 June 2021, 22:58   #2
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As far as I can tell from my A500 Rev8A here this should Just Work (TM) in theory.
I.e. you should not have to set any jumpers. I don't have anything to test this, mind.
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Old 07 June 2021, 23:11   #3
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On second thought I think the chip select for the ROM may not be enabled for the extra addresses. In that case it won't work.
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Old 08 June 2021, 02:47   #4
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Quote:
Originally Posted by ScottC2010 View Post
Hi,
I have successfully made a 1MB ROM with extra libs for my 1200 and works great using 27c800's

Can i use the 27c800 in the 500 Plus and do the same as i have tried and it won't work (Standard 512kb ROM with the 1MB_ROM file in Remus + 512kb Extended ROM with the correct header and then the files such as workbench.library and icon.library) - is there something else i need to do like jumpers?
Well, the A500 needs a jumper wire (only 1 wire is actually needed for the
27C800) but the A500+?:

https://leblogdecosmos.blogspot.com/...mo-a500-i.html

https://leblogdecosmos.blogspot.com/...o-a500-ii.html

Last edited by SpeedGeek; 08 June 2021 at 14:07.
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Old 08 June 2021, 03:01   #5
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Fab, thanks for this!
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Old 08 June 2021, 03:10   #6
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The instructions in the link though are for A500 - i have the 500+ with 42PINs so do i just connect the two wires the same or just one and do i need to bend any pins?
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Old 08 June 2021, 05:53   #7
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No bending of leads or jumper wires should be needed since the 500+ has the extra address lines connected to the socket already
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Old 08 June 2021, 14:10   #8
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Quote:
Originally Posted by ScottC2010 View Post
The instructions in the link though are for A500 - i have the 500+ with 42PINs so do i just connect the two wires the same or just one and do i need to bend any pins?
It appears that you need no jumper wire or bent pins for the A500+. Your problem is then most likely an incorrectly built 1MB ROM.
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Old 08 June 2021, 15:54   #9
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Thanks I will try a few more rom builds
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Old 08 June 2021, 16:28   #10
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I've tried again and the 3.1.4 1MB i have made works in winuae - with A500+ quickstart settings.
I made sure i added the 1MB file to the standard 512kb part and the correct header in the extended 512kb rom and made those with the normal setting. i then built the 1MB ROM with the arexx script in remus for 1MB ROM and performed the byteswap.

It just has blank screen on my real A500+.
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Old 08 June 2021, 17:41   #11
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Tried several ROMs and still no luck - there is jp2 with 3 soldered points next to the chip - i wonder if this is something that needs looking at - i can't find any info on this anywhere - just 1mb roms for a1200 - i have done some for my a1200 too and they work perfectly (3.1.4 plus workbench.library and icon.library on the extended rom)
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Old 08 June 2021, 18:47   #12
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JP2 does not affect the ROM, it is for the Agnus highest address bit.
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Old 08 June 2021, 19:10   #13
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Quote:
Originally Posted by ScottC2010 View Post
Tried several ROMs and still no luck - there is jp2 with 3 soldered points next to the chip - i wonder if this is something that needs looking at - i can't find any info on this anywhere - just 1mb roms for a1200 - i have done some for my a1200 too and they work perfectly (3.1.4 plus workbench.library and icon.library on the extended rom)
If your pretty confident that your 1MB ROM was built correctly, then it's back to a hardware problem (again).

Try to find the A500+ schematic and determine if pin 1 of the 42 pin ROM socket is actually connected a high order address line (probably A19). It makes no sense at all for Commodore to use the 42 pin socket on the A500+ and then leave this address pin N/C (but it would not be their first design blunder).

If you can't find the A500+ schematic, then get a DMM and do some real continuity testing (which may be a good idea anyway, even if you do find the schematic).
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Old 08 June 2021, 23:20   #14
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Floppy disk

Thanks - i have a schematic now so will take a look - i am wondering if anyone else has made a 1MB ROM for a 500+ but i couldn't find any details
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Old 08 June 2021, 23:35   #15
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Quote:
Originally Posted by SpeedGeek View Post
If your pretty confident that your 1MB ROM was built correctly, then it's back to a hardware problem (again).

Try to find the A500+ schematic and determine if pin 1 of the 42 pin ROM socket is actually connected a high order address line (probably A19). It makes no sense at all for Commodore to use the 42 pin socket on the A500+ and then leave this address pin N/C (but it would not be their first design blunder).

If you can't find the A500+ schematic, then get a DMM and do some real continuity testing (which may be a good idea anyway, even if you do find the schematic).
Enclosed is a grab of U6 from the Rev8 schematic if this helps
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Old 09 June 2021, 00:33   #16
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Quote:
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Enclosed is a grab of U6 from the Rev8 schematic if this helps
The schematic shows an A19 connection on U6 pin 1 (so Commodore got it right this time).

Another test, I forgot to mention is the mirror image read test. You simply plug in the stock 512KB ROM (which is usually a mask ROM) and read the $E00000 address space with a monitor program. You should have an exact mirror image of the 512KB ROM @ $F80000.

If the mirror image test is good, then it's either a ROM build problem or a high order address bit problem, but if the mirror image test is bad, then it's a completely different hardware problem.

Last edited by SpeedGeek; 09 June 2021 at 00:45.
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Old 09 June 2021, 01:15   #17
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Thinking about A19 - i have looked at the datasheets for M27C800 and Pin1 is A18 - no A19 on the chip at all
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Old 09 June 2021, 01:25   #18
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I've had a closer look at the diagram - PIN1 is A18 and PIN42 is A19 on the A500+ mobo - this would match my 27c800 that has PIN1 as A18, but the PIN 42 is NC on the 27C800 - could connecting a wire between PIN 42 on my 27C800 and PIN 35 on GARY which is A19 possibly solve the issue?
I might try this...
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Old 09 June 2021, 01:44   #19
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Please do not confuse the CPU address with the ROM address.

The CPU address starts with A1 but the ROM address starts with A0! The end result is a 1 bit offset on 16 bit ROMs and a 2 bit offset on 32 bit ROMs...
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Old 09 June 2021, 02:14   #20
ScottC2010
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Floppy disk

I see - i have had to order a new gary which should arrive in a few days before i try anything else now as i managed to break the pin 35 on it.
Would the offset mean i would have to connect pin1 on the rom to the pin35 on the gary?
I think this is looking like it is not possible to do now - i have some 27c400 so maybe just run out a 3.1 - i would like to somehow use the ide.device file i have to be in ROM and then somehow get it to call the ide (It's a custom ide interface)
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