Hardware Bugs

Known Hardware Issues and Bugs

  • The limit of 128 colour palette as opposed to the desired 256 was allegedly due to not having enough spare physical pins on the ASIC’s die to accommodate it. As it stands there are two pins for each colour (Red, Green and Blue) and a BRIGHT output meaning that there are three bits for each colour, top two bits as per the colour pins output and the least significant bit set by the BRIGHT setting.
  • When the reset button is pressed the SAA1099 sound chip is not reset so notes being played will continue.
  • The ASIC does not send an 8Mhz reset signal to the Sam Drive disc controllers when the Reset button is pressed which can cause corruption to a floppy disc left in the drive - hardware fix is the DPU by Edwin Blink
  • The NMI button is not de-bounced causing multiple NMI’s to be generated - hardware fix is the NMI Debounce by Edwin Blink. The Messenger included a separate NMI board to get around this issue.
  • The MIDI ports allegedly do not work as explained by Steve Nutting in Fred 2
  • There is a lot of reflection on the screen when using an RGB cable, there is a Reflection fix from Colin Piggot
  • The serial port on the Comms Interface has a hardware bug as listed in Format Vol.5 No.8 where you get transmission errors after 100-200 characters regardless of baud rate. This is due to an electrical supply issue in the board which needs smoothing. To fix, solder a fat tantulum capacitor between the 0v and +5v across the supply lines near R3.
  • The Sam Mouse interface can also be erratic due to the same power issue - the HCT chips in it don’t like the SAM’s glitchy supply so shove a couple of the biggest electrolytic capacitors you can find and a tantalum capacitor inside your SAM across the +5v and 0v rails. from Simon Cooke on the Sam Users Mailing List October 1994.

See also ROM bugs

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