New ROM / RAM Expansion Board for BBC B

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KenLowe
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New ROM / RAM Expansion Board for BBC B

Post by KenLowe » Tue Jan 15, 2019 12:18 am

Ok folks. I'm edging ever closer to finalising a replica IntegraB sideways / shadow / RTC board for the BBC model B, which (rather unashamedly) will be an almost identical copy of the original board, using the same or equivalent components. The board will consist of the following:
  • 8 x Expansion sockets for 8 / 16k ROMs - 4 of which can be used for 32k ROMs / write protectable battery backed RAM / EEPROM (banks 8 thru 15)
  • 4 x 16k write protectable battery backed RAM banks (banks 4 thru 7)
  • 20k Shadow RAM
  • 12k Private RAM
  • Y2K compliant RTC
  • IBOS (IntegraB OS) / Basic in a combined 32k EPROM. This will use one of the 32k ROM sockets on the IntegraB board
As with the original, installation will be fairly straight forward. No soldering required. The 6502 processor needs to be removed from the BBC motherboard, and inserted into a socket on the IntegraB board. The board then plugs into the 6502 socket on the BBC. There are also 3 wires than need to jumper from the IntergaB board onto existing jumper pins on the BBC motherboard, which allows access to the 4 ROM sockets on the main BBC motherboard (banks 0 thru 3). Additionally there are 2 power wires than need to be re-routed from the BBC to the IntegraB, and 2 wires from the IntegraB back to the BBC motherboard.

The board has been tested and will happily co-exist with DFS / Floppy drive / Gotek, Econet, VideoNula, Datacentre, 2nd Processor / Pi Copro, GoSDC / GoMMC, MMFS, BeebSCSI / Winchester HDD, speech synthesiser, various PALPROM solutions (like SpellMaster 128).

PCB design is almost complete, and I will be looking to place an order for a prototype PCB in the very near future. One off costs are quite high, but come down considerably if I order in bulk.

I would like to gauge interest these boards before I commit to placing any orders. Even in bulk, it's not going to be cheap. A fully populated board is still likely to cost somewhere in the order of £75 - £80 + shipping! Please let me know if you might be interested (no commitment at this stage), and I can look to place an order for an appropriate number of boards. As I'm never very quick at progressing these things, I'm setting myself a realistic mid year target to have these available.

Thanks
Ken.
Last edited by KenLowe on Wed Feb 20, 2019 3:22 pm, edited 2 times in total.

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jms2
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Re: New ROM / RAM Expansion Board for BBC B

Post by jms2 » Tue Jan 15, 2019 8:10 am

Wow, that's quite an achievement Ken - well done!

I already have a B with a (not quite as good) shadow ram and rom board, and a Master, so I'm not interested in buying a pcb. But I'm certainly interested to hear about the details of your project and I'm sure others will be as well.

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Re: New ROM / RAM Expansion Board for BBC B

Post by marcusjambler » Tue Jan 15, 2019 8:54 am

Great work Ken!!

I was wondering about the lack of availability of shadow RAM for the beeb just the other day.

I'd be interested in a PCB and parts list ( assuming its through hole components )

Marcus

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KenLowe
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Re: New ROM / RAM Expansion Board for BBC B

Post by KenLowe » Tue Jan 15, 2019 8:57 am

marcusjambler wrote:
Tue Jan 15, 2019 8:54 am
assuming its through hole components
Yes. Everything will be through hole.

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Re: New ROM / RAM Expansion Board for BBC B

Post by Prime » Tue Jan 15, 2019 9:27 am

KenLowe wrote:
Tue Jan 15, 2019 12:18 am
PCB design is almost complete, and I will be looking to place an order for a prototype PCB in the very near future. One off costs are quite high, but come down considerably if I order in bulk.
Might be worth considdering https://www.allpcb.com they are quite reasonable for runs of 5 boards, it would also give you a couple of spare boards to send out to testers.....

e.g. the AtomColour I recently had produced is 114x90mm and cost $28 + shipping ($19) for 5 boards, which worked out at just over £30

Cheers.

Phill.

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Re: New ROM / RAM Expansion Board for BBC B

Post by MM5XT » Tue Jan 15, 2019 10:16 am

I'd no doubt be interested in putting a board into one of my beebs.

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Re: New ROM / RAM Expansion Board for BBC B

Post by kieranhj » Tue Jan 15, 2019 11:57 am

Hi Ken - this sounds like an amazing expansion board, great work!

One question I have - how does the SHADOW RAM work; Is it possible to do 'Master style' double-buffering, i.e. have the CPU write to one memory bank whilst the CRTC 6845 video chip reads from the other?
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Re: New ROM / RAM Expansion Board for BBC B

Post by KenLowe » Tue Jan 15, 2019 11:59 am

Prime wrote:
Tue Jan 15, 2019 9:27 am
KenLowe wrote:
Tue Jan 15, 2019 12:18 am
PCB design is almost complete, and I will be looking to place an order for a prototype PCB in the very near future. One off costs are quite high, but come down considerably if I order in bulk.
Might be worth considering https://www.allpcb.com they are quite reasonable for runs of 5 boards, it would also give you a couple of spare boards to send out to testers.....

e.g. the AtomColour I recently had produced is 114x90mm and cost $28 + shipping ($19) for 5 boards, which worked out at just over £30

Cheers.

Phill.
That does seem more reasonable alternative. Thanks for the pointer.

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Re: New ROM / RAM Expansion Board for BBC B

Post by KenLowe » Tue Jan 15, 2019 12:26 pm

kieranhj wrote:
Tue Jan 15, 2019 11:57 am
Hi Ken - this sounds like an amazing expansion board, great work!

One question I have - how does the SHADOW RAM work; Is it possible to do 'Master style' double-buffering, i.e. have the CPU write to one memory bank whilst the CRTC 6845 video chip reads from the other?
Nothing that elaborate, I'm afraid. Access to shadow RAM simply is enabled /disabled through the use of bit toggling of two registers:

MEMSEL - which is located at bit 7 of &FE30 (ROMSEL)
SHEN - which is located at bit 7 of &FE34 (RAMSEL)

Main memory / shadow memory is switched in and out as follows:

Code: Select all

Control bits		Memory area selected
SHEN	MEMSEL		&3000-&7FFF
0	X		Main / Video memory
1	0		Shadow memory
1	1		Main / Video memory
Further details can be found in the IntegraB manual.

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KenLowe
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Re: New ROM / RAM Expansion Board for BBC B

Post by KenLowe » Tue Jan 15, 2019 6:10 pm

The one component I'm struggling to source is a 40 pin dil (or 2 20 pin sil) round pin header, approx 25mm long, that would solder to the underside of the board, and then plug into 6502 socket on the BBC motherboard. I can find plenty of sil headers, but these have square pins, and tend to damage the IC socket when the pins are forced in. I'll post up a photo later on of what I'm referring to, in case I'm not making any sense. Any help in sourcing something suitable would be much appreciated.

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Re: New ROM / RAM Expansion Board for BBC B

Post by marcusjambler » Tue Jan 15, 2019 6:27 pm

I use SIL pins and a stack of sockets... effective and not entirely ugly :lol:

Marcus

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Re: New ROM / RAM Expansion Board for BBC B

Post by kieranhj » Tue Jan 15, 2019 6:30 pm

KenLowe wrote:
Tue Jan 15, 2019 12:26 pm
Nothing that elaborate, I'm afraid. Main memory / shadow memory is switched in and out as follows:

Code: Select all

Control bits		Memory area selected
SHEN	MEMSEL		&3000-&7FFF
0	X		Main / Video memory
1	0		Shadow memory
1	1		Main / Video memory
Thanks Ken. So this is purely switching which bank the CPU can access - the CRTC will understandably remain wired to read the existing Main / Video memory.
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Re: New ROM / RAM Expansion Board for BBC B

Post by RobC » Tue Jan 15, 2019 6:53 pm

Great project =D> =D> . I have a non-working Integra but it's clear that it's a great expansion. The windowing system that came with it was also very good.
KenLowe wrote:
Tue Jan 15, 2019 6:10 pm
The one component I'm struggling to source is a 40 pin dil (or 2 20 pin sil) round pin header, approx 25mm long, that would solder to the underside of the board, and then plug into 6502 socket on the BBC motherboard. I can find plenty of sil headers, but these have square pins, and tend to damage the IC socket when the pins are forced in. I'll post up a photo later on of what I'm referring to, in case I'm not making any sense. Any help in sourcing something suitable would be much appreciated.
Are these Samtec APA plugs the right sort of thing? Not cheap but I bought a few 640s when I wanted to repair my Watford ROM/RAM board.

I use these on my VideoNuLA boards but they may not be long enough for you.
Last edited by RobC on Tue Jan 15, 2019 6:55 pm, edited 1 time in total.

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Re: New ROM / RAM Expansion Board for BBC B

Post by KenLowe » Tue Jan 15, 2019 7:21 pm

RobC wrote:
Tue Jan 15, 2019 6:53 pm
Are these Samtec APA plugs the right sort of thing? Not cheap but I bought a few 640s when I wanted to repair my Watford ROM/RAM board.
Many thanks for the link. The APA-640-T-P is exactly what I'm looking for. Unfortunately, this one isn't available for sale on their website, so I'll need to contact them to see if they have stock available, and what the cost would be.

RobC wrote:
Tue Jan 15, 2019 6:53 pm
I use these on my VideoNuLA boards but they may not be long enough for you.
Yes. I've got some of these already, but they're too short for this project.
Last edited by KenLowe on Tue Jan 15, 2019 7:24 pm, edited 1 time in total.

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Re: New ROM / RAM Expansion Board for BBC B

Post by RobC » Tue Jan 15, 2019 7:30 pm

KenLowe wrote:
Tue Jan 15, 2019 7:21 pm
Many thanks for the link. The APA-640-T-P is exactly what I'm looking for. Unfortunately, this one isn't available for sale on their website, so I'll need to contact them to see if they have stock available, and what the cost would be.
Should have said that I had to contact them when I bought mine years ago. They were very helpful and were happy to order a handful in for me.

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Re: New ROM / RAM Expansion Board for BBC B

Post by Prime » Tue Jan 15, 2019 7:47 pm

marcusjambler wrote:
Tue Jan 15, 2019 6:27 pm
I use SIL pins and a stack of sockets... effective and not entirely ugly :lol:
Exactly what I do for my boards using these : https://www.rapidonline.com/truconnect- ... er-22-1704

Bonus is they are 20 pin so 2 of them and a 40 pin socket for the CPU / 6847 (on the Atom) etc.

Cheers.

Phill.

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Re: New ROM / RAM Expansion Board for BBC B

Post by MartinB » Tue Jan 15, 2019 8:39 pm

Ken wrote:I'm edging ever closer to finalising a replica IntegraB sideways / shadow / RTC board for the BBC model B
Nice one Ken! =D> 8)

I’m not sure I’ll promise to take one as I simply lack the bandwidth for new stuff given that I’m eternally drowning in my own projects but I’ll certainly watch with interest how it develops and well, you just never know....... :wink:

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Re: New ROM / RAM Expansion Board for BBC B

Post by KenLowe » Tue Jan 15, 2019 8:40 pm

marcusjambler wrote:
Tue Jan 15, 2019 6:27 pm
I use SIL pins and a stack of sockets... effective and not entirely ugly :lol:

Marcus
Sorry. Missed your post earlier. I've done similar in the past, but was looking for something a little bit more 'professional'. I've emailed Toby for price & delivery.

For reference, I've taken a photograph of the connector that's used in my existing IntegraB. It looks to be exactly the same as the APA-640-T-P:
Attachments
20190115_202927.jpg

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Re: New ROM / RAM Expansion Board for BBC B

Post by 1024MAK » Wed Jan 16, 2019 7:52 am

Nice 8)

I would be interested in a board and a kit of parts :wink: :D

Mark

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Re: New ROM / RAM Expansion Board for BBC B

Post by KenLowe » Wed Jan 16, 2019 9:45 am

RobC wrote:
Tue Jan 15, 2019 7:30 pm
Should have said that I had to contact them when I bought mine years ago. They were very helpful and were happy to order a handful in for me.
That's the APA-640-T-P added to their web page, and an order placed for a batch of ten. £4.10 + VAT per unit + delivery!
Last edited by KenLowe on Wed Jan 16, 2019 9:46 am, edited 1 time in total.

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Re: New ROM / RAM Expansion Board for BBC B

Post by Stewie66 » Wed Jan 16, 2019 10:44 am

Hi Ken,

I’d be interest in a board and/or kit

Cheers
Stewart

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KenLowe
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Re: New ROM / RAM Expansion Board for BBC B

Post by KenLowe » Sun Jan 20, 2019 1:53 pm

Board layout progressing well. Still to add the RTC & battery / charging circuit to the top RHS of the board, and sort out some of the power distribution...
Attachments
IntegraB.png

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Re: New ROM / RAM Expansion Board for BBC B

Post by marcusjambler » Sun Jan 20, 2019 2:36 pm

Board layout progressing well
Looking great Ken =D>
I'm wondering if it will work with a 65C02 to mitigate some of the extra power consumption

Marcus

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Re: New ROM / RAM Expansion Board for BBC B

Post by 1024MAK » Sun Jan 20, 2019 5:26 pm

marcusjambler wrote:
Sun Jan 20, 2019 2:36 pm
Board layout progressing well
Looking great Ken =D>
I'm wondering if it will work with a 65C02 to mitigate some of the extra power consumption

Marcus
Speaking of which, are you intending to use low power CMOS versions of the glue logic, such as 74HCTxxx chips?
And also are you going to recommend using the CMOS versions of the EPROMs, like 27C256 for example?

Mark
Last edited by 1024MAK on Sun Jan 20, 2019 5:28 pm, edited 1 time in total.

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Re: New ROM / RAM Expansion Board for BBC B

Post by KenLowe » Sun Jan 20, 2019 8:56 pm

1024MAK wrote:
Sun Jan 20, 2019 5:26 pm
Speaking of which, are you intending to use low power CMOS versions of the glue logic, such as 74HCTxxx chips?
Yes. All 74 series glue logic ICs will be 74HCTs. There's not actually that many of them (7 in total). Most of the address decoding logic is done in two PALs (although I'll be using GALs).
1024MAK wrote:
Sun Jan 20, 2019 5:26 pm
And also are you going to recommend using the CMOS versions of the EPROMs, like 27C256 for example?
I wasn't going to make any recommendation regarding EPROM types, as this was never a stipulation with the original board. I've certainly never had any PSU loading issues with any EPROMs I've had installed on the board, but happy to take some guidance on this. It's worth pointing out that the board does actually accept flash / eeprom without any problem (and I've just received some f-ram that I'm about to try). I'm pretty sure they'll draw less power than some older EPROMs.

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Re: New ROM / RAM Expansion Board for BBC B

Post by 1024MAK » Sun Jan 20, 2019 9:40 pm

Well of course 27128 and 27256 EPROMs will work, but the more modern CMOS versions (27C128 and 27C256) are lower power, which means less additional heat.

As a well known TV commercial tag line goes, every little helps :wink:

Mark

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Re: New ROM / RAM Expansion Board for BBC B

Post by KenLowe » Sun Jan 20, 2019 9:41 pm

1024MAK wrote:
Sun Jan 20, 2019 9:40 pm
As a well known TV commercial tag line goes, every little helps :wink:
Mark
Indeed.

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Re: New ROM / RAM Expansion Board for BBC B

Post by KenLowe » Thu Jan 24, 2019 1:25 am

That's the board design and layout pretty much complete. I need to do a full QA check before I order up some boards, though, and I might look to thicken up some of the power lines. A job for the weekend.
I've also pulled together a BoM, and I'm not getting much change out of £70. It's surprising how the costs mount up!
I've shown the board with a couple of ROMs and the 6502 plugged in. Unfortunately, I couldn't find a 3d battery symbol, or spade connector, so those are missing from the 3d render.
Attachments
IntegraB-01.png

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Re: New ROM / RAM Expansion Board for BBC B

Post by KenLowe » Sun Jan 27, 2019 12:02 pm

Opinions sought, please. Referring specifically to the attached schematics, which track layout is best? The tracks I'm referring to are more obvious in schematics 3 & 4. These tracks are used for ROM bank selection.

Shortest path diagonal or longer path horizontal / vertical in keeping with the rest of the board layout? Vias further spaced apart, or closer together?

I'm inclined to go with the layout in schematic 4...
Attachments
IntegraB-06.png
1. Horizontal / vertical tracks with wider spaced vias
IntegraB-05.png
2. Horizontal / vertical tracks with narrower spaced vias
IntegraB-04.png
3. Diagonal tracks with narrower spaced vias
IntegraB-03.png
4. Diagonal tracks with wider spaced vias
Last edited by KenLowe on Sun Jan 27, 2019 12:04 pm, edited 1 time in total.

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Re: New ROM / RAM Expansion Board for BBC B

Post by myelin » Sun Jan 27, 2019 4:02 pm

Diagonal with wide spacing for sure. It doesn't matter all that much when the fastest signal is running at 2MHz, but for high speed designs, 90 degree turns are frowned upon, as they result in more reflections and EMI emissions.
SW/EE from New Zealand, now in Mountain View, CA, making BBC/Electron hardware projects for fun.
Most interesting: Arcflash, FX2+PiTubeDirect Tube/Cartridge adapter, USB cart interface.

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