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This article originally appeared in My little (home) shop thread on Practical Machinist.

I only worked on the CPU board schematic a few minutes today, but I finished all the address decoding logic.

The 8088 only has 20 address lines, of that, 16 of them are used for addressing segments of 64KB each. The address lines A16 through A19 are used as card selects to talk to the peripheral cards. There is a 4 to 16 line decoder that takes the upper nibble of the CPU address, then converts that to a selector that is 1 of 16 lines on the bus. Fadal repurposed a bunch of address and logic lines to make these card selects.

A 20bit address is represented as 5 hex nibbles, like 0xFBC00, that’s segment 15, address BC00. That happens to correspond to the video memory on the video card. The first F selects card 15, which is the video card (we are talking base 0 numbers here, so 0-15), the BC00 selects the base address within the F segment. The F segment also contains the system ROM, which is located at FE000-FFFFF, an 8KB chunk. The video ram is only 1KB, so it occupies addresses FBC00-FBFFF.

It would seem that the memory expansion cards occupy a card select range, though I’m not certain exactly where the memory and NC executive ROMs are mapped yet. Once I have the schematic completed and the system ROM reverse engineered, I could design my own memory expansion card.

This article originally appeared in My little (home) shop thread on Practical Machinist.

I finished my reverse engineered schematic of the Fadal 1420-1 video board. I’d estimate I have 20-25 hours into the schematic at this point.

The schematic, and observation of the control display, lead to some interesting observations about the original design.

It’s widely told that Adrian de Caussin is the one who designed the control hardware, unfortunately he passed away some time ago (a reference from 1996 indicates he’d passed by then), so this public accounting may be the only hardware level details known outside of the de Caussin family.

I did reach out to David de Caussin the younger to try and learn details about the original control development, because at this point they are considered vintage computer systems. Unfortunately I didn’t receive a reply from him, so I had to go at it myself.

This article originally appeared in My little (home) shop thread on Practical Machinist.

That’s the contents of the character generator ROM on the Fadal video card. I used a Parallax Propeller development board to make a breadboard EPROM reader.

I’m interested in making a video card that will plug in place of the Fadal card and output a video format of my choosing.

Specifically I’m talking about using a Parallax Propeller microcontroller to generate a VGA signal. The picture is actually the Propeller outputting the ROM data to a VGA monitor.

This article originally appeared in My little (home) shop thread on Practical Machinist.

Just to be clear, the control and electronics work fine on my Fadal at present, it was the iron that needed major help.

The process of reverse engineering the control started with researching the history of the Fadal control. On the VMC Electric website they give a few pieces of information about the process of creating the control.

They said they used an Imsai 8080 to develop the NC executive. If you read closely you’ll see they used the NC software to process G-code into NC tapes for their other machines. This specifically includes applying cutter radius compensation and other stuff that old NC systems couldn’t do.