The Card That Made the Apple II Serious
4 hours ago
- #Videx VideoTerm
- #Apple II
- #FPGA Emulation
- The Videx VideoTerm was an 80-column expansion card that transformed the Apple II into a business machine by enabling compatibility with word processors, spreadsheets, and CP/M software.
- Emulating the Videx card on an FPGA involves reverse-engineering the MC6845 CRTC chip and understanding the unique hardware architecture of slot 3, which controls expansion ROM ownership.
- The Apple II's native 40-column text was insufficient for serious work; the Videx card provided sharper 80-column output via its own 2KB VRAM, bypassing the limitations of NTSC composite video.
- The MC6845 CRT Controller generates display timing signals but no pixel data; the FPGA emulation replicates its functionality along with the character ROM and firmware using minimal logic resources.
- Slot 3 is special because it manages the INTCXROM mechanism, which routes expansion ROM space to internal firmware; Videx must be in slot 3 to properly handle C8-space ownership for compatibility.
- The Videx card responds to three address regions: device I/O for CRTC registers, slot ROM for firmware, and expansion ROM with a VRAM window that allows bank-switched access to the full 2KB video RAM.
- The character ROM stores pixel patterns for 256 characters; FPGA optimization stores only 128 characters and uses XOR logic to generate inverse video, saving valuable BSRAM blocks.
- Software like CardCat identifies the emulated Videx card as hardware because the FPGA faithfully reproduces the firmware ROM bytes and VRAM architecture, making it indistinguishable to diagnostic tools.