[Bbraun] has an old Macintosh SE computer. He was looking for a way to view the video output from the SE on a newer, modern computer. He ended up working out a pretty clever solution using a stm32f4discovery board.
First, the SE’s logic board was removed from its case and placed onto a desk for easier access. The discovery board was then hooked up to the SE’s processor direct slot (PDS) using normal jumper wires. The discovery board acts as a USB COM port on a newer Mac OSX computer. The discovery board watches the SE for writes to video memory. When it sees that the R/W pin goes low, it knows that a write is occurring. It then waits for /AS to go low, which indicates that an address is on the bus. The discovery board reads the address and verifies that it falls within the range of the video frame buffer. If it does, then the discovery board writes a copy of the data to a local buffer.
The OSX computer runs a simple app that can make a request to the discovery board via USB. When the board receives the request, it sends its local frame buffer data over the USB connection and back to the host. The OSX computer then displays that data in a window using CGImage. The demo video below was captured using this technique.