Comments for page: Building SBC6120

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Posted by Garrett February 05, 2014 - 05:10 pm
Great, thanks, Steve! I spent way too much time agonizing over that, comparing your stuff (which I've used heavily, by the way) and Bob's latest documentation. I found photos of a few boards on the web, of course, and it seemed that there were some one way and some the other.

Posted by Steve L. February 04, 2014 - 09:14 am
Hi Garrett, Investigating R6 and R8 closely, I find that they are not swapped in the parts list (PL) developed here. However, there has been some ambiguity in the values for those. Both are non-critical pullup resistors and either value (10K or 4.7K) will work fine for either. At the time our PL was created, the existing second-edition manual parts list had the values reversed from those in the schematic. Since R8 is for the IDE interface, and that standard mentioned a 10K value, we went with that. For R6, we went with 4.7K, because it would provide faster response. So our PL matched the PL in the second edition manual and not the schematic.

In the third edition manual, the values have been swapped, and now match the schematic. Since either set of values will work, we will eventually change our PL to match the order in the third edition manual. A close study of the IDE spec reveals that, in fact, it calls for the pullup resistors to be in the DRIVES and not in the host at all! Lowering R8 to 4.7K to match the third edition manual would be going further in the wrong direction. Nevertheless, it will work fine, as the drivers have plenty of capability to sink the extra current.

The bottom line: The values we have are theoretically better but the previous ambiguity in the documentation has been removed in favor of the opposite order. Since either order is fine, we will eventually change to match the new SBC documentation. Thank you for your question.

Posted by Garrett Nievin February 02, 2014 - 07:31 pm
Is it possible that you swapped R6 and R8 on the SBC?

Posted by Steve L. August 30, 2013 - 09:14 pm
Hi Don, Yes, I tested the chip and it works fine, booting OS/8 and running Focal programs. This Hong Kong eBay vendor, SunElectronic has over 5000 transactions, with a 99.4% satisfaction rating, so I had little doubt. As much as folks love to hate eBay, gotta love the vendor ratings and customer protection system.

By the way, regarding fakes, all of the HD1-6120 chips I have seen have been in ceramic packages which look like a sandwich and are distinctively different from their plastic brethren. With its prominent Military business, Harris seemed to have quite a preference for ceramic packages. They have the advantage of being hermetic and are thus more reliable. When I worked there, it was said that there was little difference in the cost of plastic and ceramic packages (excluding "jelly bean" devices) and that other chip vendors were simply using them as a way to divide their hi-rel and commercial markets.

Posted by Don N August 30, 2013 - 07:24 pm
Have you tested and validated that your Chinese HD1-6120 is a functional, real chip? There are some unscrupulous vendors that take old random chips, or just plain mechanical fakes, and remark them as something that sells for $$$.

Posted by Steve L. August 30, 2013 - 10:54 am
Thanks for the followup, Kevin. I see how you might think that, given the comment. It's fortunate that the Chinese seem to have a significant number of HD1-6120 chips squirreled away. I just bought a backup chip on eBay at a reasonable price, from a Hong Kong vendor and there were plenty more. It arrived a week later. My experience in the vintage audio arena has been that vintage semiconductors are generally available at fair prices, at least in the modest quantities that SBC runs have been. So it's really just a question of whether and when Bob feels up to mounting another offering, making boards or making Gerber files available. I'm optimistic that we may yet see more runs of this platform made.

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