Using the LowerROM Board on a 6128+

[Bryce]

Hi all,
     while testing the LowerROM Board, I've discovered a few effects on the 6128+ that need to be mentioned. On a standard CPC the LowerROM Board disables the internal ROMs, but the electronics are made that only ROM 0 can be disabled, ROM 7 can't be disabled, no matter what you do (despite some people claiming that it can). On the Plus it's a different story. The LowerROM Board disables the cartridge completely. Not a problem on the 464+, but on a 6128+ ROM 7 and Burning Rubber are no longer available. This means two things:

1) If you use the LowerROM Board on a 6128+ you will need to have AMSDOS installed on a ROMBoard / MegaFlash behind the LowerROM Board. Otherwise you won't have disc access.

2) You can't use The Plus OS / BASIC on the LowerROM Board, as it seems to be calling something either in the Burning Rubber ROM or ROM 7, neither of which are present because the cartridge has been disabled.

Maybe someone who knows their way around the Plus Firmware can check exactly what is being called and make a patched version that could be used on the Plus?

Bryce.

[beaker]

That means I've only tried my LowerROM on my original 6128 and haven't used the other EPROM Bryce did for me for my 6128+  How bloody lazy am I? And I was in the first batch Bryce made 

Oh well, guess I'll leave it for the moment until we know it's working 

[TFM]

Maybe someone who knows their way around the Plus Firmware can check exactly what is being called and make a patched version that could be used on the Plus?

A patch will not help, since the Cart get disabled. But you can use the 6128 ROMs for LowerROM + MF. Problem solved.

[Bryce]

It could be patched to NOT jump to external addresses in the cartridge couldn't it. Just like a standard 6128 LowerROM does.

Bryce.

[gerald]

Hi Bryce

I've added the lowrom replacement capability to my Rom/Ram/CF extension, and I do not have any issue with the 6128+.
I've been using the 6128 OS and 6128 basic (Both ROM have been patched to show a different message).
I can use them independently, and the internal amsdos is OK. My extension currently cannot replace the ROM7.

How are you asserting the ROMDIS signal on the lowerROM board ? Are you sure you are not asserting it all time ?

Gerald

[gerald]

About Burning Rubber
- If you replace the lower ROM by a non plus one, you go directly to basic. This is the plus FW that ask if you want to start the game or the basic
- If you keep using the plus Basic rom, you still can start Burning Rubber with |GAME. Plus BASIC is able to start Burning Rubber
- If you replace the both FW and Basic with non plus one, you have no way to start it.

[Bryce]

The LowerROM Board holds ROMDIS high the entire time. The aim of the LowerROM Board was a low-cost LowerROM replacement. To achieve this, it does absolutely no address decoding, it hold ROMDIS high and places the external EPROM in its place. External ROMBoards behind the LowerROM Board can disable this Lower ROM when they need to.

Of course I could have done lots of address decoding so that it only asserted ROMDIS when required, but that would have meant adding ICs, PCB space and of course cost.

Bryce.

Edit: I'll put the schematics up on the Wiki page later and you can see exactly how it works.

[gerald]

Ok, that explain the issue you can see.

The only decoding needed is A15=0, A14=0, ROMEN=0. You may do this with diode OR to the EPROM CS and a single transitor for inverting and driving ROMDIS.

[Bryce]

Yeah, I could have done that, but I went for the ultra-simple solution.

Bryce.

Edit: I just uploaded the schematic here: LowerROM - CPCWiki  and also added some notes about the lack of decoding and its effects.

[TFM]

Thanks' for all the informations / update.