Forcing 128 startup in 48 mode

[spider]

Specifically with the black +2

Is there any (simple?) hardware modification I can do to this to force it to start up in 48K mode but without locking paging or anything else ?

As in , the same effect as issuing a USR0 from 128K basic, leaving me the 48K editor but with the bonus of access (via assembly) to the 128K features...

I think to do this to the toasty rack and/or grey +2 is not hugely difficult.


I did wonder if a small wire link and maybe a couple of components could do it, however the 'not convenient as no working burner' method I guess would be a custom EPROM replacement***

*** Unsure if I want to get into that as replacing a mask ROM with an EPROM may require some additional wiring as well.


Would just appreciate any thoughts on this please.

[1024MAK]

Put your own EPROM in it, so that “48K” BASIC is in control at power up / reset.

Or if you are willing to modify the existing pins of the existing ROM, or the board, or can be done without using a different ROM/EPROM.

Mark

[1024MAK]

Post above edited

[adam]

Plugging in a DivMMC interface or similar is a simple and non-invasive way of doing it. If you're worried about the extra space used, you can probably get an extension cable and wrap it up and over inside the machine.

[spider]

Put your own EPROM in it, so that “48K” BASIC is in control at power up / reset.

Or if you are willing to modify the existing pins of the existing ROM, or the board, or can be done without using a different ROM/EPROM.

Mark

I did wonder at the back of my mind if swapping the _Select lines (not sure of the exact wording sorry) between the two IC's would do this, along with probably a couple of other changes. Although vaguely (unrelated) I am reminded of the BBC's ROM sockets where you could simply attach a flying lead from one ROM/RAM chip to have it "appear" in a different location. Not a great example sorry.

Plugging in a DivMMC interface or similar is a simple and non-invasive way of doing it. If you're worried about the extra space used, you can probably get an extension cable and wrap it up and over inside the machine.

That is one way, but expensive. Unless I could maybe use an IF1 with the 48K rom in it, but I think the IF1 (assuming I can find it) won't like a +2A/B, more accurately the +2 won't appreciate the interface. I could be wrong.

[zx64]

1. Make your own ROM with 48K ROM starting at address 0. Replace internal ROM with this one.
2. Same but use your own ROM with external ROM interface. Can DIY very cheap
3. Connect A14 pin on the ROM permanently to +5V
4. Remove IC31 (LS174) and replace it with custom board. This board must replicate logic of LS174 but select ROM bank 1 (instead of 0) on reset. This is for toast rack, I didn't look at other models
5. Again, for toast rack. If you remove D32, paging will never be disabled


Do you need option to switch back to the 128K ROM at all?

[zx64]

I did wonder at the back of my mind if swapping the _Select lines (not sure of the exact wording sorry) between the two IC's would do this, along with probably a couple of other changes. Although vaguely (unrelated) I am reminded of the BBC's ROM sockets where you could simply attach a flying lead from one ROM/RAM chip to have it "appear" in a different location. Not a great example sorry.

Aren't 48K and 128K in the same ROM chip?

[zx64]

Another option. May work on Amstrad models as well.

Cut the trace going to the ROM A14 pin and insert inverter (for example transistor) between the trace and ROM pin. If ROM is socketed, can do it without cutting

[oO cozy Oo]

Pop a divmmc in and it boots to 48k mode lol

[1024MAK]

So, just to clarify, in the Amstrad black +2A or +2B models, there are a lot less chips (glue logic) because it’s all been absorbed (assimilated Borg style) by the gate array chip. That includes the functions of the 74LS174 chip used in the earlier 128 and grey +2 machines.

It’s also not particularly practical to make alterations to the gate area pins. So, if you do want to modify your machine, rather than use an alternative EPROM (a 27256 or 27C256 once burned can be plugged straight in, no wiring alterations needed), read on.

The best option is to remove the ROM (IC7 is the one, I think), bend up pin 27 (address pin A14) and then wire this pin to a +5V point, such as pin 28 on the same chip.

Mark