Newbrain memory replacement

[CDesp]

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I am planning to replace the Memory Board of one of my machines with a new design that uses a 32Kb Static Ram instead of all the 4116 DRam chips the original is using.

All the signals of the old board are replicated. I am using the RAS and CAS signals to set the signals of the 62256 Static Ram Chip.

I ll test this at home using a cpld for the 74ls ics and if everything is ok i’ll send the PCB for fabrication.

If someone knows a good replacement for the paper flatwires please let me know. I ordered several KF2510 male and female connectors for this purpose.

Schematics

 

New Board 3D

Topic

[Michael Fog]

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Hello Chris….

There are no really good substitute for the solder-in-ribbon-cables.

The space is very limited if you want a detachable connector. It simply stacks to high.

I have done a solution where I solder the normal flat-cable in the mainboard and pin-rows in the mainboard. The plastic most likey has to be removed on the pin-row, and pins cut down.

On the Flatribbon cable from the mainboard I then solder on a female-header like this RS type : 681-6826

RS components also have the oldschool-flatribbon cable : 241-1356

Really the NB should have been made with IDC connectors from the start, instead of single row headers.

Best regards

Michael

[Michael Fog]

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…yes the picture clearly shows detachable headers in both ends but it is at the keyboard end. Ther is more room there… 😉

[kokkiklhs]

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Very nice project, thank you, Chris!

I was wondering if there’s a way for more RAM to be included in this new board.

I am mainly interested in such an option, as I have a NB AD with disk controller but no RAM expansion, so I am afraid that I’ll never be able to boot CP/M with 32k…

Would an internal RAM upgrade e.g. to 64k or more be possible?

Or, if internal RAM upgrade is impossible, is there some workaround for me to build a replacement external DIY RAM expansion?

Thanks!

[CDesp]

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Sorry for the delay.

You can’t add more ram (or actually you can but it will not work) without also change the code in roms. If you want to run CP/M with more ram you need the expansion box because it contains new roms (and more ram) that support paging so you can access the extra RAM.

So the short answer adding more ram does not make it available to the system like PCs.

[kokkiklhs]

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On CDesp said

Sorry for the delay.

You can’t add more ram (or actually you can but it will not work) without also change the code in roms. If you want to run CP/M with more ram you need the expansion box because it contains new roms (and more ram) that support paging so you can access the extra RAM.

So the short answer adding more ram does not make it available to the system like PCs.

Chris, is there a chance of reverse-engineering the original Grundy RAM expansion board somehow? Have you ever tried something like that, would it be possible? I have seen similar projects like e.g. the Camputers Lynx FDD controller replica, which can give a totally new perspective to anyone who owns just a standalone rare machine of this category…

I suppose that right now you are the absolutely only one who could start such a project, so the Newbrain community hopes globally rely upon you! 🙂

[Klaus Chr Sjøquist]

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There is a possibility to get more RAM, but in Basic there is still only 32Kb. This is because the Z80 CPU can only handle 64Kb divided in the following way 28Kb EPROM/ROM (Basic and Graphics) and 32 Kb RAM in NewBrain, or as CDesp writes, connect an Expansion.

It can also be done with another RAM card and then it is a machine that is called a poor man’s CP/M, it consists of a converted RAM card with 128Kb RAM and an Add card that must be put on the Main board in place of IC 406 and the EPROM/ROM from 406 must be moved to IC 407 and EPROM/ROM IC 408 must be replaced with a 16Kb EPROM/ROM possibly 27128.

But this conversion means that an expansion box cannot be added to the machine, only a disk controller.

There was a program for the converted machine that when run gives 8Kb more so that the total RAM came up to 40Kb free RAM in Basic.

Unfortunately, I haven’t been able to find that program.

Kind regards

Klaus

[Author]

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