2009...: RF1802 FDC

RF1802 - Retro Floppy controller for RC1802 under work...

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2009-12-26: I've been working with this along the year, but quite occasionally. First version was already able to control a 5.25" drive, but when I started to implement SW for data transfer (tricky with D765), I noticed that 2MHz 1802 can't keep up with the data rate (byte every 27uS and we need several instructions in the loop), if all is done by software using the HW design I had. So I modified the circuit to use 1802 DMA for the data transfers, but doing that I broke something else as now it can't access the registers anymore correctly using the non-DMA method. Likely a small mistake, but a few evening debug sessions did not locate the problem, all seems fine. Need to work more here..

The Story

I have a tube of D765s and wanted a floppy controller for RC1802, so one evening I just jumped in and started drawing a floppy controller based on D765 (similar to Intel 8272 FDC). The chip is quite simple and does not contain all needed functionality the biggest being read data separator. I only needed single density support, so a simple one-shot triggered by clock edge should work fine (it's also mentioned in Intel's data book).

Another problem is interfacing. It's designed for Intel 8080/85 and does not naturally fit with the different control signals of the 1802 so some extra glue logic is needed.

Again I will put the schematics here when I have some feeling it does work...