Successfully built

Floppy disk interface - Version 1.2

Next, the description of a floppy disk interface derived from Version 1.0, where I adopted a different version of the digital PLL circuit, the 74S188 PROM being replaced by a 74LS161. I named this version "1.2".

I applied the idea presented on the forum by "oamariei", who posted the schematic of a digital PLL circuit for the CoBra floppy interface which uses a 74(LS)74 flip-flop and a 74(LS)161 counter. This idea originates in a document from NEC containing an application note for µPD765, a replacement of 8272, made by Intel and UMC and older brother of 8272A which is also used in CoBra.

I did not get to build this version myself yet, to test it and see if it actually works, but fortunately a user on the Romanian Home Computer forum (Csaba) built one and confirmed it works well, even with a Gotek hardware floppy emulator. Therefore I decided to classify this version as being "successfully built".

I'm showing below the schematic as posted by "oamariei" on the forum, side by side with the document from NEC where the PLL schematic in the application note is found on page 18 of the original document (page 19 in the PDF file).

The PLL circuit presented by "oamariei" (page 1 the CoBra original, page 2 the PROM-less version)	NEC Application Note 8 - µPD765 (vezi pag. 19 din PDF)

Since so far I never tested this version, from my point of view it is purely theoretical. But considering others declared they have built it and it worked, I decided to draw the full schematics for this version together with the modified circuit board, and I also make here available the Gerber files required for fabrication for anybody interested in building and testing it.

Schematics - v.1.2
	Features of v.1.2: The schematics are modified from v.1.0. The only difference is the PLL circuit, which in this version basically has one less IC, the space formerly taken up by 74S188 being now empty.

Board layout - v.1.2

The component layer (top) is drawn in red and the opposite one in blue. The corresponding Gerber files for this circuit board can be downloaded as a ZIP archive.

Floppy disk interface - Version 1.2

The PLL circuit presented by "oamariei"(page 1 the CoBra original, page 2 the PROM-less version)

NEC Application Note 8 - µPD765(vezi pag. 19 din PDF)

Schematics - v.1.2

Features of v.1.2:

Board layout - v.1.2

The PLL circuit presented by "oamariei"
(page 1 the CoBra original, page 2 the PROM-less version)

NEC Application Note 8 - µPD765
(vezi pag. 19 din PDF)