Todd's ELF-ish to get an FDC #Homebrew #microboards


Back in July, in the crazy topic "Okay, let's get really crazy...",, there was a discussion about the RCA CDP18S651 Microboard Floppy-Disk Controller.  This got me interested in a the idea of building a clone of that card for my ELF-ish project.  However, as was pointed out then, a large number of the components are dedicated to the data separator, which also required some rather critical adjustments, and there are dedicated chips to do this sort of thing.

Initially, I looked at the FDC9229BT which does the data separator and write precomp function and directly supports the FDC765.  But then I found the FDC9266 , which is basically both of these combined. And conveniently for me, a UK based eBay source.

This radically reduced the component count, removed the critical adjustments, and brought the project into the scope of something reasonable to build.

Another aspect of the CDP18S651was the support for the various floppy interfaces of the time.  But in the post-PC world, the interface is now fairly standardized unless you want to go for some rather vintage drives.  So I decided to cut out all of the unrequired drive interface support stuff.  The simplified drive interface is based upon the example FDC design on page 24 of the 1979 application note on the uDP765, which I found as uPD765_App_Note_Mar79.pdf on bitsavers. There is support for up to four drives using two PC-style 34-pin IDC interfaces.

I replaced the single motor-on flip-flop with a 74HC175 Quad D-Type Flip-Flop to give separate motor-on control for each drive.  Although I added a jumper to allow both drive 0 and 1 to be controlled by the same motor-on bit to be compatible to the original.

I also dropped the full-support of the RCA two-level I/O system and just hard-wired Group-8 (or IO4 in my ELF-ish world).  I used the now spare 1/2 4013 flip-flop to latch the state of BUS3 during an OUT 1.

I also allowed a jumper option to connect to the IR7 level of the CDP1877 Programmable Interrupt Controller back on the CDP-IO chassis.

I also like the idea of being able to read the control latches, so I added a 74HC244 3-state Octal Buffer controlled by the previously unused INP 7.

And I use a DIL-8 8Mhz can oscillator rather than a crystal.

The complete schematic has been uploaded to as ELF-BB-FDC-II_20191124.pdf.  It's "-II" as it's my second attempt at a design, the first having been closer to the RCA original.

Here's the planned layout.

Now I just have to build it.  I'll keep the forum posted on how I get on.

I would appreciate reading any comments, thoughts, or suggestions on the design.

Share and enjoy,

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