68K-MBC

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Sparrowhawk
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68K-MBC

Postby Sparrowhawk » Mon Apr 12, 2021 2:29 pm

I saw this today and given that it's based on a 68008 thought that it might be of some interest here.

https://hackaday.io/project/177988-68k- ... w-computer

I actually have the Z80 version of this, and it runs CP/M 2.2 and 3.0 really nicely.


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Re: 68K-MBC

Postby RichardCGH » Mon Apr 12, 2021 3:15 pm

I wonder why they went for a 68008 instead of a 68000 ?


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Re: 68K-MBC

Postby Pr0f » Mon Apr 12, 2021 3:41 pm

Probably the ease of connecting an 8 bit bus rather than a 16bit one, and also the pic is emulating the boot rom forthe 68K part


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Re: 68K-MBC

Postby Brane2 » Mon Apr 12, 2021 7:42 pm

RichardCGH wrote:I wonder why they went for a 68008 instead of a 68000 ?


It was a screwup and mismanagement. Clive thought he could do QL with 64K RAM as a fot in the door and with substantially cheaper 68008.
But when it came out, 68008 was _more_expensive than 68000. Also, 128K in QL used 16 DRAM chips.
And as it turned out, QDOS + SuperBAsic needed 2 ROMs anyway.

So, they could do 68000 machine in that budget with 2x better graphics etc.

Also, Clive found a sponsor ( British Telelcom ?) that wanted a machine in the format that he thought he could shoehorn QL into.

Same story as with microdrives. Ccheap imported crap that turned out to be as expensive as a floppy.

If he wanted to go that route while thinking out of the box, he could use cheap audiotape mechanism with ordinary cassete and autoreverese head and
a bit of smarts about data format and modulations. Thing could be cheap, big and fast, at least compared to any floppy of the time.
Last edited by Brane2 on Mon Apr 12, 2021 10:29 pm, edited 1 time in total.


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Re: 68K-MBC

Postby Pr0f » Mon Apr 12, 2021 9:14 pm

Brane2 wrote:
RichardCGH wrote:I wonder why they went for a 68008 instead of a 68000 ?


It was a screwup and mismanagement. Clive thought he could do QL with 64K RAM as a fot in the door and with substantially cheaper 68008.
But when it came out, 68008 was _more_expensive than 68000. Also, 128K in QL used 16 DRAM chips.
And as it turned out, QDOS + SuperBAsic needed 2 ROMs anyway.

So, they could do 68000 machine in that budget with 2x better graphics etc.

Also, Clive found a sponsor ( British Telelcom ?) that wanted a machine in the format that he thought he could shoehorn QL into.

Same story as with microdrives. Ccheap imported crap that turned out to be as expensive as a floppy.

If he wanted to go that route while thinking out of the mob, he could use cheap audiotape mechanism with ordinary cassete and autoreverese head and
a bit of smarts about data format and modulations. Thing could be cheap, big and fast, at least compared to any floppy of the time.



I think you may have answered the wrong question - original poster was referring to the small 68K board with PIC - not the QL.

An interesting rant however...


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Re: 68K-MBC

Postby RichardCGH » Mon Apr 12, 2021 10:10 pm

Pr0f wrote:Probably the ease of connecting an 8 bit bus rather than a 16bit one, and also the pic is emulating the boot rom forthe 68K part


OK. Thanks for this.


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Re: 68K-MBC

Postby Brane2 » Mon Apr 12, 2021 10:28 pm

Pr0f wrote:
Brane2 wrote:
RichardCGH wrote:I

I think you may have answered the wrong question - original poster was referring to the small 68K board with PIC - not the QL.

An interesting rant however...


Yes, but as it turns out, it seems to hold true in his case also.
His main argument for 68008 is false.
I've posted comment on hackaday.

WRT CLive's Microdrive effup- quick calculation suggests that he should be able to push easily more than 100MB on good but still relatively cheap CrO2 ordinary 60 minute audio cassette with transfer speeds of more than 80 KB/s.
With far greater realiability than floppy ( let alone Microdrive). And tape mechanisms were cheap then - no need for inventing everything from ground up. QL could easily have two such mechanisms, just it had two microdrives.

As a Quantum leap in storage, he could repeat the joke with VHS cassete a few years later.
Last edited by Brane2 on Tue Apr 13, 2021 3:39 am, edited 3 times in total.


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Re: 68K-MBC

Postby bwinkel67 » Mon Apr 12, 2021 10:31 pm

So no way to verify this:

https://retrocomputing.stackexchange.com/questions/15570/were-there-really-any-cost-savings-in-sinclair-ql-because-of-it-being-8-bit-desi

But the top replier (posted July 12, 2020) claims to be David Karlin:

Yes, there was a significant cost saving. For a start, Motorola charged us something like 30-40% less for the 68008 (I forget the exact amount, but it was substantial), because the price was so much lower than they were charging their other 68000 customers. Secondly, the pin count issue was key: for the custom chips, going anything over a 40 pin PDIP literally doubled the cost. The alternative would have been to add several HCT245 buffers at around £0.50 each plus more PCB space: for Clive, adding £2 to the cost would have been anathema.

The design would have been much easier and the result much faster. But it would have added perhaps £50-80 to the retail price (remember, retail price is 4x components). So it was out of the question.

David Karlin (I actually designed it!)


If you click the link there is a Q&A that follow with David.

Added for ease:

Q: This is the best that could happen to questions here -- when the actual designers and engineers explain their decisions. Thanks a lot!

A: Say thank you to peteri on twitter, who sent me the link to this post!


Q: Would it have been practical to design an 68008 machine to optimize the case of consecutive accesses to two bytes within a word? Perhaps by adding a 74LS373 between the less-significant byte (upper address) DRAM data outputs and the bus, so that when fetching an even byte, the corresponding odd byte would get latched into the latches, and when performing an immediate follow-on address to the corresponding byte, the DRAM could complete its cycle? Or would there have been no nice way to distinguish "other half" accesses from any other kind, or would the timing advantages have been tiny?

A: Not sure I understand exactly what you're suggesting. It would conceivably have been possible to have a 16 bit memory bus with the 8 bit processor, which would have won a lot of bandwidth - at the expense of PCB space and TTL. It's not something I considered at the time.


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Re: 68K-MBC

Postby Brane2 » Mon Apr 12, 2021 11:41 pm

bwinkel67 wrote:So no way to verify this:

https://retrocomputing.stackexchange.com/questions/15570/were-there-really-any-cost-savings-in-sinclair-ql-because-of-it-being-8-bit-desi

But the top replier (posted July 12, 2020) claims to be David Karlin:
<SNIP>


So he basically confirmed this. They would have got good price with 68000 but everlasting Poker Genius chose 68008 to save literally few pounds.
For a machine that would otherwise outperform Atari ST in many regards. And cost less than 100 pounds more ( probably 50ish).

It woudl be interesitng to know how much was Ferranti charging them for ULAs , how much NRE cost were introduced with each one and what were their
upgrade options. I found datasheet for one of their models, but have no idea about the rest of them and available densities.


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Re: 68K-MBC

Postby bwinkel67 » Tue Apr 13, 2021 12:52 am

Brane2 wrote:So he basically confirmed this. They would have got good price with 68000 but everlasting Poker Genius chose 68008 to save literally few pounds.


So David Karlin notes that the 68008 was close to 40% cheaper than the 68000 in mass quantities and he was there when those contracts with Motorola were made.

...but this thread was asking why a 68008 for the new board and presumably for similar design simplifications and maybe cost...dunno about the later.



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