By Mike Deliman
Working on a customer problem once, we had an interesting phenomena. Upgrading a system with a large VME cage and several boards, the customer replaced older processor boards with what were then "new" boards. The old boards ran at (I think) 33 MHz, the new ones at more like 133MHz.
The overall system included motor control functions and sensor feedback, there were D-to-A, A-to-D, and other boards in the system. The problem was, with a straight-and-simple rebuild of software for the new boards, the system was giving anomalous readings. Checking the registers for values from the A-to-D boards, there were numbers showing up that were not possible. Readings indicated the motors were on and moving their load when in fact the motors were not connected. Switching back to the old computer boards eliminated the problem. We added some hardware to examine VME bus activity, to see if the new CPUs were reading the bus wrong. We had to put the test board between the CPUs and the sampling boards.