USA spec BBC model B

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1024MAK
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Re: USA spec BBC model B

Post by 1024MAK » Wed Sep 05, 2018 3:00 pm

If you have a multimeter, it should not be difficult to work out which link is which for each ROM socket. Use either a low resistance range (200 ohm or equivalent) or a continuity range that is compatible with semiconductors (open circuit voltage at or below 0.5V).

Then test between each ROM socket pin 27 and the middle pin of any nearby three pin links.

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AndyF
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Re: USA spec BBC model B

Post by AndyF » Wed Sep 05, 2018 6:00 pm

steve3000 wrote:
Sat Sep 01, 2018 10:12 pm
My US beeb kind-of works but has lots of graphic issues, that may be related to a link set incorrectly, but I can find no documentation of the correct settings - and all links are numbered differently to the UK beeb :(
I'd suspect originally there would of been some "addendum" or at a push a US-Spec service manual, the former seems more likely with just a list of differences maybe. :)

Perhaps one day said documentation will appear, it seems completely illogical that the model would of been sold "over there" without any form of tech backup, even if it was held for "tech/dealer/repairer" use, not for public consumption as they would not of expected to ship machines with faults back to the UK, they would of been repaired in the country of sale...
Last edited by AndyF on Wed Sep 05, 2018 6:00 pm, edited 1 time in total.
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Re: USA spec BBC model B

Post by BeebMaster » Fri Sep 07, 2018 10:46 am

I think the user guide for the US BBC B came in two ring-bound volumes. I don't know how much technical detail it contains about link settings though. This is something I never clarified when I was trying to understand how the ROM sockets were mapped into the ROM table.
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Re: USA spec BBC model B

Post by flynnjs » Fri Sep 07, 2018 11:41 am

1024MAK wrote:
Wed Sep 05, 2018 2:56 pm
On the B+ Acorn provided 32K byte compatible EPROM/ROM sockets suitable for 2764, 27128 and 27256 EPROMs.
I've wheeled my .DE Beeb out for the weekend so I'll check where A14 goes.
I know for sure that ROMSEL is different on DE/US to a normal B as I've
been around it whilst fixing it, so it may well be organised to support 32K.
More this evening...

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Lardo Boffin
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Re: USA spec BBC model B

Post by Lardo Boffin » Wed Sep 26, 2018 6:57 pm

I have managed to get a 32k ROM going in one of the spare sockets today (trying to copy files from disc to econet partition but my discs are not cooperating so had to set up a MMC) and found the following by a process of changing jumpers and seeing which ROMs were available and which vanished.

Set west for 32K ROM, east for 16K ROM. If set west and a 16K ROM is present this seems to cause problems with lower order ROMs vanishing. I didn’t try setting east with a 32K ROM.

S14 relates to IC72.

S16 relates to IC73.

S17 relates to IC74.

S15 relates to IC75 possibly? A process of elimination suggested this is the case. This is the BASIC ROM in the fifth slot so didn’t experiment with this.

I assume, but didn’t try, that S12 related to IC71 the MOS slot.

It seems a bit strange that the S## don’t follow the IC## number order but I am sure there is a reason.
Atom, issue 5
BBC model B 32k issue 4, 16k sideways RAM, Watford 12 ROM board, Acorn 6502 coproc
BBC model B 32k issue 7, turboMMC, Opus Challenger 3 512k, Pi 3 coproc
USA Model B
BBC Master, Datacentre + HDD, pi co-proc, econet, NULA

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Lardo Boffin
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Re: USA spec BBC model B

Post by Lardo Boffin » Wed Oct 10, 2018 4:16 pm

Interestingly when I do *HELP it only shows one of the ROMs in a 2 ROM socket. Could this be that OS1.20 is not fully set up to work with 2 ROM sockets?
Both ROMs in a socket do work however.
Atom, issue 5
BBC model B 32k issue 4, 16k sideways RAM, Watford 12 ROM board, Acorn 6502 coproc
BBC model B 32k issue 7, turboMMC, Opus Challenger 3 512k, Pi 3 coproc
USA Model B
BBC Master, Datacentre + HDD, pi co-proc, econet, NULA

steve3000
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Re: USA spec BBC model B

Post by steve3000 » Tue Nov 20, 2018 10:45 pm

steve3000 wrote:
Tue Jul 31, 2018 8:28 am
Without a circuit diagram it's going to be difficult to know for sure unless you try. Until mine stopped working, I'd been working on the assumption the US beeb is part-way between an issue 7 beeb and a B+...
Still no circuit diagram, but I have found some interesting documentation about the US beeb, see here

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