This is a general communications error. You will experience this when...
  • The machine has a faulty 7721 board.
  • The W401 is causing computer bus communication issues.
  • A ribbon cable is damaged.
  • A possible 5VDC supply issue.
Check the following:
  1. Set your digital multimeter to DC and please measure wire 505 with respect to wire 500. Please do this as close to the card rack as possible (where the 7803,7721, and W401 are housed). You should measure between 4.8VDC - 5.2VDC 
  2. Please inspect the W401's battery for corrosion.
  3. Please inspect the ribbon cables associated with the "card rack".
  4. Please change the location of the 7721, you could try an open slot or swapping the boards position with another.