Connecting VGA Monitors

VGA monitors can be connected to the A4000; however, since the special circuitry in the Amiga video output can mistakenly identify a monitor as a genlock and thus cause problems, a special cable or adapter is the best way to hook them up. All this cable really does is buffer the horizontal and vertical sync signals by double-inverting them through a TTL gate. Commodore's DB23-to-HDD15 adapter (supplied with most A4000s) used a 74HCT08 for this, but you can use a 74LS04 or other low-power TTL-level inverters (or other gates wired as inverters or buffers, of course).

Remember that most VGA monitors won't sync at 15.75 kHz, so you'll have to set the A4000 to use the Double NTSC (or Double PAL) or Multiscan video modes. Even using these modes, the video output may not work with some VGA monitors, since some of the modes use horizontal sync frequencies as low as 23 kHz, and normal VGA starts at 31.5 kHz. Loading the VGAOnly monitor driver in addition to the other drivers will bump these frequencies up a bit, perhaps enough to make them usable with picky monitors.

A4000 VGA Video Cable

  A4000 DB23                                        VGA HDD15

  Red   (Pin 3) ----------------------------------- Red (Pin 1)

  Green (Pin 4) ----------------------------------- Green (Pin 2)

  Blue  (Pin 5) ----------------------------------- Blue (Pin 3)

  Grounds (Pin 16-20) ----------------------------- Grounds (Pin 5-8,10,11)
                                                    and ground for inverter.
                                                    Also use a .01 uF ceramic
                                                    decoupling cap between
                                                    +5V and ground at the
                                                    chip power supply pins.

                               |\     |\
  Horizontal Sync (Pin 11) ----| >o---| >o--------- Horizontal Sync (Pin 13)
                               |/     |/

                               |\     |\
  Vertical Sync (Pin 12) ------| >o---| >o--------- Vertical Sync (Pin 14)
                               |/     |/

  +5V (Pin 23) ------------------------------------ Power supply for
                                                    inverter or buffer chip.