LowerROM Swapper

[Bryce]

Hi all,
      I finally found some time to do some hardware stuff and used the time to put together the first LowerROM board. As you can see in the picture below, it is approximately the same size as the MegaFlash board. It needs to be plugged between the MegaFlash and the CPC, but I provided a pass-through connector so that no further messing about with cables is required. It will of course work without a MegaFlash, but it does disable the internal ROM 7, so if you want to use BASIC 1.0 on a 6128 and still have disc commands, then you will need to have AMSDOS / ParaDOS installed and enabled on the MegaFlash. I've done all the tests and you can still program the MegaFlash through the LowerROM board and all other expansions (RAM, Speechsynth, Lightpens, etc) will also work as long as they are plugged behind the LowerROM board. It also works on both Classic and Plus CPCs.
     The EPROM contains two versions of OS/BASIC, so you can use the same device to convert a 6128 to BASIC 1.0 or a 464 to BASIC 1.1. The jumper next to the EPROM is used to choose which OS/BASIC you want, the jumper beside the LEDs is to enable/disable the external OS/BASIC (ie: disable the device). The red LED lights when the board is enabled and green when disabled. I also added a reset button (beside the green LED) for those who complained that it was missing from the MegaFlash

I have only ordered a few prototype boards to supply those who asked for one, but I have maybe 3 or 4 extras in case someone decides they suddenly want one.

Bryce.

[Cholo]

Sweet, ill get one just for the reset button 

Nice that it comes with the passthru-cable as i fear what would happen if i had to make one myself 

[qbert]

Hi Bryce !
I do confirm I have need of one of these.
Concerning the edge card connector for non-german CPCs, we all know it is out of stock for a long time. An unique "edge card"-to-centronics adapter by person would be more reasonable, as it will not create a penury.

[Bryce]

Hi qbert,
      Eliot still has some edge connectors on offer in his CPC Parts shop, but regarding an Edge Connector to Centronics adapter, I probably still have one here somewhere, send me a PM if you are interested in having it.

Bryce.

[TFM]

Hi Bryce! Wonderfull !!! Amazing !!!! Great !!!!!

After an eternity of time the last frontier has been broken! Now we can patch our lower ROM finally!!! :-)))

[Bryce]

Unfortunately your suggestion to make it writeable from the CPC just wasn't practical from a cost point of view, but I put the EPROM in a socket, so it is possible to swap EPROMs and re-program it as you wish.

Bryce.

[TFM]

Oh, that's absolutely fine. And since the EPROM has 64 KB, I should even be able to program it at the CPC with the Dobbertin 4003. Well, it only can do 32 KB, but I can add a wire and a switch to make it a 64 KB version (I assume). Let's see... 

[Bryce]

Hmm, how do you mean? If I had known that that was a useful option I would have added a jumper to do that. Does the dobbertin 4003 use the same addresses as the LowerROM but just go up to FFFF? Or how does it work?

Bryce.

[TFM]

Hmm, how do you mean? If I had known that that was a useful option I would have added a jumper to do that. Does the dobbertin 4003 use the same addresses as the LowerROM but just go up to FFFF? Or how does it work?

Bryce.

Oh, I should tell things more clear. First a link:

http://www.cpcwiki.eu/index.php/Universeller_EPROM_Programmer_4004

The Dobbertin 4003 is an Eprom burner, that burns Eproms up to 32 KB (27256 for example), and since 27256 and 27512 are pincompatible: I thought I can burn a 27512 like a 27256, but have the need to supply an additional bit to one pin of the Eprom burner.

[Bryce]

Ah, ok. You could still burn a 27C512 with the 4003 by connecting pin 1 (A15) to the 5V line (with a 10K resistor) to program the upper 32K.

Bryce.

[TFM]

Ah, ok. You could still burn a 27C512 with the 4003 by connecting pin 1 (A15) to the 5V line (with a 10K resistor) to program the upper 32K.

Bryce.

Ah Thanks!!! So a connect A15 to a switch. And this switch relys A15 either to GND or to 10K Ohm resistor (resistor connects to +5 V). Right?

[Bryce]

You can make it even simpler, by permanently connecting A15 to the 5V supply through a 10K resistor and a simple switch that connects A15 to ground without disconnecting the 10K resistor.

Bryce.

[Executioner]

Ah, ok. You could still burn a 27C512 with the 4003 by connecting pin 1 (A15) to the 5V line (with a 10K resistor) to program the upper 32K.

What's the 10K resistor for? I would probably just have connected it to 5V?

[TFM]

What's the 10K resistor for? I would probably just have connected it to 5V?

If I got that right, then the resister sets the A15 constantly to 1, but as soon as the switch is connected (GND on) the signal goes from 1 to 0, because the GND is stronger then the +5 Volt running throug the resistor.

It also may have a protective means, in case + 5V directly is too strong. But Bryce will for sure be able to explain that stuff in a better way.

[00WReX]

Mmmm, very nice. Looks great. 

Cheers,
Shane

[Bryce]

TFM has explained it perfectly. The 10K pull-up resistor "holds" A15 at VCC (5V) but limits the current to avoid any damage to the EPROM and this also lowers the current that the circuit is using. When the switch is closed A15 goes to logic 0 but there is no short circuit because the 10K resistor is between 0V and 5V. This is the standard method for "biasing". This is also the method I used on the DualOS description here: http://www.cpcwiki.eu/index.php/DualOS
The resistor can be a tiny 1/4W type because it will only be drawing 0.5mA = 2.5mW (for those who like to do the theoretical stuff)

Bryce.

[Executioner]

The resistor can be a tiny 1/4W type because it will only be drawing 0.5mA = 2.5mW (for those who like to do the theoretical stuff)

Ah, I didn't get the fact that it switched between GND and +5V. Handy if you're using a jumper instead of a dual pole switch.

[Bryce]

Exactly, you can use a simpler switch or even a simple push button if it's a signal that only needs to to be changed for a few seconds.

Bryce.

[TFM]

The really cool thing about the LowerROM is that we can even use emulators to create our own ROM couples (Lower + BASIC) and test them there. As soom as they work, we just have to burn the EPROM once. And that's it :-)

[Bryce]

Because there are so many different variations, that's not a bad idea. As well as the different BASIC versions, people are also going to want different language versions too.

Bryce.

[Bryce]

While experimenting with the LowerROM board I just noticed a nice little side-effect that I hadn't even thought about:

On a standard 464, only ROMs 7 to 0 are scanned on start-up, so the MegaFlash only offers the 464 the first 8 ROMs, however with the LowerROM connected with BASIC 1.1 enabled, I get the first 16 ROMs as I would on a standard 6128 and my 464 can make much better use of the MegaFlash. This may have been obvious, but I hadn't really thought about it.

Bryce.

[Pentagon]

HEHEHE great news, i didnt thought about that too, thats a great and most useful "effect".

cheers
Tom

[TFM]

Well, I just think about the Booster ROM for the 464 which will provide all 32 ROMs of the MegaFlash (when using the LowerROM).

[00WReX]

While experimenting with the LowerROM board I just noticed a nice little side-effect that I hadn't even thought about:

On a standard 464, only ROMs 7 to 0 are scanned on start-up, so the MegaFlash only offers the 464 the first 8 ROMs, however with the LowerROM connected with BASIC 1.1 enabled, I get the first 16 ROMs as I would on a standard 6128 and my 464 can make much better use of the MegaFlash. This may have been obvious, but I hadn't really thought about it.

Bryce.

Yes, I noticed this (and it can be seen in the photo's I posted of my MegaROM)...my 464 with the 6128 firmware chip installed shows the 16 ROMS...then the picture with the MC0099A motherboard only shows the 7...

Same effect as your lower ROM board...handy side effect 

Can't wait to get mine 

Cheers,
Shane

[qbert]

Alas, the booster ROM hangs on a 464, whatever position you load it (even at position 7 replacing AMSDOS).
I know that problem very well because I am a 464 user... maybe there's a possible software patch,  my dear TFM    ?

Well, I just think about the Booster ROM for the 464 which will provide all 32 ROMs of the MegaFlash (when using the LowerROM).