The MCS6501, a first and very rare member of the MOS Technology 65xx CPU family.
Introduced in 1975 with the MCS6502 to MCS6505, pin compatible with the Motorola 6800 and, after legal issues, not produced afterwards.
An advert in the magazine Electronics August 1975, the price is incredible low for these days at $20.
The (preliminary) datasheet of August 1975 is the only document where the 6501 is documented. PDF format, Below the pages are shown.
More photos found on the web, the 6501 is at least produced in week 34 to week 50 of 1975.
Afaik the only known system where the 6501 was used is the The Digital Group 6501 CPU board.
The 6500 Hardware manual from 1976 time had detailed information on 6501, see page 30 – 40.
The following table summarizes what makes the 6501 pin compatible with the 6800. Note the two phase non-inverting clock, Data Bus Enable
| Pin | 6800 | 6501 | 6502 |
| 2 | Halt | Ready | Ready |
| 3 | ∅1 (in) | ∅1 (in) | ∅1 (out) |
| 5 | Valid Memory Address (output) | Valid Memory Address (output, always high) | N.C. |
| 7 | Bus Available (output) | Bus Available (output) | SYNC |
| 36 | Data Bus Enable (input) | Data Bus Enable (input) | N.C. |
| 37 | ∅2 (in) | ∅2 (in) | ∅0 (in) |
| 38 | N.C. | N.C. | Set Overflow Flag |
| 39 | Three-State Control | N.C. | ∅2 (out) |
From the 1976 Hardware manual page 30-40
See also:
The late Andrew Jacobs set up a web site with relevant 6502 information. It is lost now. This site is reproduced here...
https://www.youtube.com/watch?v=FY4wjTuzkM8 1979 - Educational trainers Educational instruments division was the...
Tiny Basic Tom Pitman's Tiny basic. Small enough to fit in the 1K KIM-1, yet a real Basic interpreter Ti...
1977 01 Tiny Basic a mini language for your micro 1977 01 What's that Digital Group Really Doing ...