The VERY first thing you should do when ever trouble shooting any digital logic is make sure that the path to ground is still clear and sweet.
Years of operation have a tendency to vibrate loose grounding straps and what not.
A poor ground can have inscrutable effects upon the logical state of any controller. (floating ground)
Of course, a poor ground is the LAST thing suspected.
Unless someone's been inside the coding, actual chip failure/ logical corruption is extremely remote.
The failure mode described is consistent with the ground path going in and out of tolerance. (heat expansion causes the path to 'get right' a cold machine is not quite 'right.'
Other troubles: dust is contaminating some contacts. This may take the form of random failure.
Dust also means static electricity -- and its associated B fields. Again, you need a sweet ground path and clean components.
Every time you open up the lid you provide an opportunity for foreign matter to drift into the circuitry. Be sure to blow out the controller with (canned) dry air.
The above maladies can have surprising impacts as the VDC used by modern controllers keeps dropping.
(3.3 VDC is a common rail voltage for fast memory. Some chips use sub- 3.0 VDC logic.)
Lastly, the classic mind bender is a failing power supply -- starting at its transformer. This failure mode has sent many a tech to the nut-house in frustration and befuddlement.
Until you've addressed all of the above, don't waste any time trying to figure out the ladder logic, if any. In digital systems a change in the logical flow requires human intervention or an engineering casualty. (A hammer dropped on the box)
Last edited by Tesla; 06/12/14 02:44 AM.
