BBC Model B No2. Will it live again!

Any 8bit repairs, images, cool things, upgrades etc.
BennehBoy
Posts: 219
Joined: Sat Apr 24, 2021 12:35 pm

Re: BBC Model B No2. Will it live again!

Post by BennehBoy »

Here's a pinout comparison between 27c256 & 28c256

27-to-28C256-300x175.png
27-to-28C256-300x175.png (38.82 KiB) Viewed 2660 times

Looking at this the reason that the 28c256 needs the rom writing to the upper half is that Vpp which is normally on pin 1 of a 27C256 is A14 on the 28c256 so presumably will be supplied with vcc on the beeb? (it's much higher for programming).

I think you just need to make sure Pin 27 (a14 on the 27c256) is tied high to get the upper address range? (lift leg and solder blob to pin 28, or just check 27 with a multimeter first - worst case it won't boot if it's grounded or floating)

Hopefully someone with more of a clue will be along shortly to ratify/debunk this :lol:
User avatar
rubber_jonnie
Site Admin
Site Admin
Posts: 10479
Joined: Thu Aug 17, 2017 7:40 pm
Location: Essex
Contact:

Re: BBC Model B No2. Will it live again!

Post by rubber_jonnie »

@BennehBoy Yeah, A14 I have been told must be tied high, which isn't a problem and now I have the mask ROM and 27C256 side by side on a bit of paper, the only other difference is pin 1.

The mask ROM is NC, but the 27C256 is Vpp. Everything else is identical. I guess I could try with both A14 & pin 1 tied high.

Vpp shows as 5.5v on the datasheet, so should be fine.
Collector of many retro things!
800XL and 65XE both with Ultimate1MB,VBXL/XE & PokeyMax, SIDE3, SDrive Max, 2x 1010 cassette, 2x 1050 one with Happy mod, 3x 2600 Jr, 7800 and Lynx II
Approx 20 STs, including a 520 STM, 520 STFMs, 3x Mega ST, MSTE & 2x 32 Mhz boosted STEs
Plus the rest, totalling around 50 machines including a QL, 3x BBC Model B, Electron, Spectrums, ZX81 etc...
User avatar
stephen_usher
Posts: 5583
Joined: Mon Nov 13, 2017 7:19 pm
Location: Oxford, UK.
Contact:

Re: BBC Model B No2. Will it live again!

Post by stephen_usher »

In the socket /WE is high so it doesn't matter to an (E)EPROM what Vpp does. On the Beeb that's high too.

So, A14 on the 27C256 will be high, Vpp is too low to matter. If in doubt lift pin 1 on your 27C256. Just write the image in the upper half of the chip, or write two copies, one in each half.
Intro retro computers since before they were retro...
ZX81->Spectrum->Memotech MTX->Sinclair QL->520STM->BBC Micro->TT030->PCs & Sun Workstations.
Added code to the MiNT kernel (still there the last time I checked) + put together MiNTOS.
Collection now with added Macs, Amigas, Suns and Acorns.
User avatar
rubber_jonnie
Site Admin
Site Admin
Posts: 10479
Joined: Thu Aug 17, 2017 7:40 pm
Location: Essex
Contact:

Re: BBC Model B No2. Will it live again!

Post by rubber_jonnie »

stephen_usher wrote: Thu Jun 03, 2021 9:05 pm In the socket /WE is high so it doesn't matter to an (E)EPROM what Vpp does. On the Beeb that's high too.

So, A14 on the 27C256 will be high, Vpp is too low to matter. If in doubt lift pin 1 on your 27C256. Just write the image in the upper half of the chip, or write two copies, one in each half.
Thanks for that info, good to know, I'll give them a try based on that.
Collector of many retro things!
800XL and 65XE both with Ultimate1MB,VBXL/XE & PokeyMax, SIDE3, SDrive Max, 2x 1010 cassette, 2x 1050 one with Happy mod, 3x 2600 Jr, 7800 and Lynx II
Approx 20 STs, including a 520 STM, 520 STFMs, 3x Mega ST, MSTE & 2x 32 Mhz boosted STEs
Plus the rest, totalling around 50 machines including a QL, 3x BBC Model B, Electron, Spectrums, ZX81 etc...
User avatar
rubber_jonnie
Site Admin
Site Admin
Posts: 10479
Joined: Thu Aug 17, 2017 7:40 pm
Location: Essex
Contact:

Re: BBC Model B No2. Will it live again!

Post by rubber_jonnie »

Top marks @BennehBoy and good info @stephen_usher

I dropped the diag ROM in and it worked like a charm, no mods or adapter needed :) Thanks to both of you for the assistance.

Of course this is also good for me, because being an ST fellow, I have quite a few spare 27C256s lying around that could find their way into the Beeb if needed. I have some 27C512s as well so I assume that if I write 2 copies to each half of the chip this would also work.

Either way, nice one, both the diag and OS ROMs worked perfectly :)
Collector of many retro things!
800XL and 65XE both with Ultimate1MB,VBXL/XE & PokeyMax, SIDE3, SDrive Max, 2x 1010 cassette, 2x 1050 one with Happy mod, 3x 2600 Jr, 7800 and Lynx II
Approx 20 STs, including a 520 STM, 520 STFMs, 3x Mega ST, MSTE & 2x 32 Mhz boosted STEs
Plus the rest, totalling around 50 machines including a QL, 3x BBC Model B, Electron, Spectrums, ZX81 etc...
BennehBoy
Posts: 219
Joined: Sat Apr 24, 2021 12:35 pm

Re: BBC Model B No2. Will it live again!

Post by BennehBoy »

Nice.

A batch of 10 28c256's just dropped from China, and I'm surprised to find they're all genuine Atmel parts. I'm going to try get my head round kicad so I can modify the schematic for the 2732 to 27256 adapter so it will also fit 28256's, should just need another trace and resistor so it can operate without jumpers I hope.
User avatar
rubber_jonnie
Site Admin
Site Admin
Posts: 10479
Joined: Thu Aug 17, 2017 7:40 pm
Location: Essex
Contact:

Re: BBC Model B No2. Will it live again!

Post by rubber_jonnie »

BennehBoy wrote: Sat Jun 05, 2021 3:26 pm Nice.

A batch of 10 28c256's just dropped from China, and I'm surprised to find they're all genuine Atmel parts. I'm going to try get my head round kicad so I can modify the schematic for the 2732 to 27256 adapter so it will also fit 28256's, should just need another trace and resistor so it can operate without jumpers I hope.
Cool, be good to hear how you get on. I'm currently playing games on the Beeb.

IMG_20210605_153139.jpg
IMG_20210605_153139.jpg (253.65 KiB) Viewed 2583 times
IMG_20210605_155028.jpg
IMG_20210605_155028.jpg (231.24 KiB) Viewed 2583 times

And I'm now using the RGB cable, so good to know that works too.
Collector of many retro things!
800XL and 65XE both with Ultimate1MB,VBXL/XE & PokeyMax, SIDE3, SDrive Max, 2x 1010 cassette, 2x 1050 one with Happy mod, 3x 2600 Jr, 7800 and Lynx II
Approx 20 STs, including a 520 STM, 520 STFMs, 3x Mega ST, MSTE & 2x 32 Mhz boosted STEs
Plus the rest, totalling around 50 machines including a QL, 3x BBC Model B, Electron, Spectrums, ZX81 etc...
User avatar
stephen_usher
Posts: 5583
Joined: Mon Nov 13, 2017 7:19 pm
Location: Oxford, UK.
Contact:

Re: BBC Model B No2. Will it live again!

Post by stephen_usher »

:dualthumbup:
Intro retro computers since before they were retro...
ZX81->Spectrum->Memotech MTX->Sinclair QL->520STM->BBC Micro->TT030->PCs & Sun Workstations.
Added code to the MiNT kernel (still there the last time I checked) + put together MiNTOS.
Collection now with added Macs, Amigas, Suns and Acorns.
User avatar
rubber_jonnie
Site Admin
Site Admin
Posts: 10479
Joined: Thu Aug 17, 2017 7:40 pm
Location: Essex
Contact:

Re: BBC Model B No2. Will it live again!

Post by rubber_jonnie »

I know the kbd surround is a bit nasty, but someone on stardot is trying to get replacements made.

The case colour doesn't bother me too much, and my other one is a much better color.
Collector of many retro things!
800XL and 65XE both with Ultimate1MB,VBXL/XE & PokeyMax, SIDE3, SDrive Max, 2x 1010 cassette, 2x 1050 one with Happy mod, 3x 2600 Jr, 7800 and Lynx II
Approx 20 STs, including a 520 STM, 520 STFMs, 3x Mega ST, MSTE & 2x 32 Mhz boosted STEs
Plus the rest, totalling around 50 machines including a QL, 3x BBC Model B, Electron, Spectrums, ZX81 etc...
User avatar
JezC
Posts: 2083
Joined: Mon Aug 28, 2017 11:44 pm

Re: BBC Model B No2. Will it live again!

Post by JezC »

When you've got the hang of the analogue joysticks, my favourite memories are of Elite (obviously!), Aviator & Revs...others might not have aged quite so well (but Defender, Magic Mushrooms, Monsters, Arcadians & Chuckie Egg to mention just a few others) were all played to death.

Might have to fire up the Beeb emulator later tonight... ;)
Post Reply

Return to “8 BIT CORNER”