Disable Disc ROM

[IanS]

How do people disable the Disc Rom in a 6128? If you use an external romboard to replace rom 7 with parados or other non-standard rom, what hardware mods do people use? If the rom is in a socket, you can just rememove it, but I think most 6128 disc roms are soldered in.


Looking at the schematic, Linking pin 13 (or any of the connected tracks ) of IC210 to GND will stop the internal rom from ever being selected.


Is there an easier way? (preferably without permanent damage)

[TFM]

Use the ROMDIS of the CPC. You can look at the MegaFlash to have an example. See wiki article. 

[IanS]

No, I know what the ROMDIS signal does.

What modification have people done to their 6128 to use rom 7 in an external rombox. You need some way of disabling the internal circuitry from trying to be activated for rom 7. Otherwise you will have both the external rombox and the internal rom will try and present data at the same time.

[Executioner]

Most external ROM boards disable the internal ROM using ROMDIS when one of the external ROMs is active.

[IanS]

But you can't disable the internal DOS rom in a 6128.

From the megaflash wiki page:-

CPC6128 - Classic CPC6128 can not overwrite the internal ROM 7 with an external device, to make use of the external ROM 7 the CPC must be modified to allow the internal ROM 7 to be disabled (eg: By adding a switch to the internal ROMs Chip Enable pin)

I was wondering if there was a better way than cutting a track to the CE pin. Adding a link or switch between pin 13 of IC210 to GND can be done with just added wires, which can be removed without permanent damage.

[TFM]

Well, I own a couple of ROM expansions allowing to overwrite ROM 7 WITHOUT any cutting. No, I don't mention ROMDIS again, it was mentioned here before and I have had a long discussion with Bryce about it. 

[IanS]

What machine are you using, and what romboards? It appears to be a problem with 6128 classic (and 664, but who uses them?), I can't see how you can disable the internal disc rom exterbally without internal modification to the hardware.

[TFM]

Either a CPC6128 or a 6128Plus with either a Dobbertin Eprom card, Symbiface2 or a Inicron 512 KB S-RAM Box. I also own the MegaFlash, but I haven't had time enought to check if ROM 7 can be effectively replaced or not. But I would suggest that it would work, other users can tell you more about the MegaFlash (since all my CPC stuff is in Europen, and my biological base = body is in north america).

[IanS]

Interesting, I'd love to know what they do differently then. I've tried a honeysoft and rombo rombox on a classic 6128, and if I put a rom in No.7 the machine hangs.

Can anyone explain how the Dobbertin, Symbiface2 or Inicron do things differently?

[gerald]

Interesting, I'd love to know what they do differently then.

Me too 
From schematics, there is no way for an external device to prevent the internal rom 7 to drive the bus when selected.
Only a device with stronger pad drive than internal rom may work, with the implied shorts circuits on data bus when bit does not match.

[SyX]

It looks Bryce is in holidays 

When i asked him about disable the Disc ROM to replace with Parados in the rom number 7 in my Megaflash, he told me this:

To disable the internal ROM 7 you just need one switch. Generally what you need to do is disconnect the /CE pin (pin 20) on the EPROM from the PCB.

The switch should be a two way switch. In one position it should connect pin 20 back to the PCB where it originally was connected (internal ROM enabled). In the other position it should connect pin 20 to the 5V line (pin 28 of the EPROM) which disables the ROM by holding the /CE signal off.

So pin 20 goes to the centre pin of the switch, pin 28 goes to one outer side of the switch and the PCB where pin 20 was connected goes to the other side.

[IanS]

And that's were the thread started, I was trying to find a way to disable the internal rom without having to disconnect pins. Just add easily removable wires.


@TFM/FS, is there any chance you are mistaken and have tested this on a 6128plus, and not a 6128 classic?


On my 6128 classic you have to disable the internal rom if you want to add a rom 7 in an external romboard. The easiest way that I've found is to add a link (to permanently disable rom 7) or switch (so it's switchable) between pin 13 of IC210 and GND.


I added a switch and only needed two wires.

[TFM]

@TFM/FS, is there any chance you are mistaken and have tested this on a 6128plus, and not a 6128 classic?

No, mistakes. I worked in this configuration for years with both machines. Well, I had all that discussion before with Bryce. Sadly I can't provide an explanation.

[IanS]

Can you identify the revision of motherboard your 6128 has? The version 3 with asic could be sufficiently different.

[TFM]

Can you identify the revision of motherboard your 6128 has? The version 3 with asic could be sufficiently different.

It's a regular CPC6128, but I have used several other keyboards too. There's nothing special.

[IanS]

There's nothing special.

I think there is. You appear to be the only person who can disable the internal dos rom of an unmodified 6128 with just a standard rom board, I think that's very special.


Are there any other users on here that have managed to do the same?

[TFM]

I think there is. You appear to be the only person who can disable the internal dos rom of an unmodified 6128 with just a standard rom board, I think that's very special.

Well, me, and all others I ever met on German CPC meetings... may one of them reads here...

[IanS]

We have a stalemate, I can't see how it can possibly work (unless it's a particular variant of motherboard) and you say it just works.


As I see it, the internal disk circuitry of the 6128 is the same as the DDI-1 circuit. So to try and override the DDI-1 rom on a 464 by plugging in another romboard with rom 7 enabled makes no sense. I also wouldn't expect to be able to plug in two romboards with the same rom number enabled, and expect the system to know which one to use.


So if anyone else is still reading this thread, if your romboard is able to override the internal rom on an classic 6128, great (please tell me how it works). If not, you could try adding a link or switch between pin 13 of IC210 and GND to disable the internal rom 7.

[Gryzor]

 Or we can wait for TFM to make a video

[TFM]

Or we can wait for TFM to make a video

Patience is a virtue 

[Bryce]

I have looked at the 6128 Schematics in detail and spent a lot of time trying all sorts of tricks, including skewing the Clock signal and I have never found a way of disabling the internal ROM 7. I've heard claims (from TFM, but also others) that certain ROMBoards have achieved this. But I've looked at the schematics of those, and they definitely don't do anything different to the MegaFlash. If anyone would like to lend me one of these devices, I'd love to examine it closer, and experience it for myself.

Bryce.

[gerald]

From Campursoft ROMBOX user guide :

NOTE : PARADOS will not always work in ROM slot 7. This depends upon the version of the CPC6128 you have. On earlier version of this machine, the internal DISK ROM cannot be disabled.

Looking at cost down version schematic, ROMDIS is an direct connection to the gate array (pre-asic), which issue chip select to both firware/basic and amsdos rom (nrom1/nrom2 signal). The gate array could take ROMDIS into account to generate both ROM chip select.
On old CPC ROMDIS as a wired-or with the firmware chip select.

So TFM/FS may have a cost down version.

[Bryce]

True, I never thought of that. All ASIC CPCs should be able to disable ROM 7. The pluses certainly can.

Bryce.

[arnoldemu]

So TFM/FS may have a cost down version.

I was thinking the same.

Unless there is some other earlier revision which can also do this.

TFM/FS show us some pictures of your pcb please!!!!

[SyX]

I have checked in my 6128 preasic, disabling the internal cpc rom 7 with the MegaFlash jumper and i only get continuous resets. Do i need to do something else???