Actually you are very close. The concept is the same and yes, the schematic very closely follows early Bally/Stern. Fortunately, a company I had never heard off previously, WDC, has basically 'remade' these old PIAs and they are now available brand new! They are pin for pin compatible with 6820/21's and the spec sheet even says as much and mentions all of the manufacturer's of 6820's. Interestingly they are available ONLY in DIP40 and not made as SMD at all.
Another bonus was this company is quite happy to deal directly with us, they said, because we are the manufacturer, we are the "final user" of their product.
So we are able to build these new boards using all brand new parts which is quite nice! It also means people with older 6821's in stock would be able to use them as test or spare parts if ever needed.
(eagle eyed people will observe in the picture of our board set above that we have used original 6821's - these test boards sets were built before the new chips had arrived).
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