Monday, November 28, 2016

Board checkout completed, ready to fire up test bed


I finally found the intermittent contact in the new disk driver board and repaired it. Going on through the remaining to finish all the continuity/correctness/shorts testing. Finally, all the careful checking was done.

Next, I populated the chip sockets, turned on power to the board and validated that injecting 1 and 0 values to either side produces the proper results on the converse connector pin associated with each signal,

The twelve input signals worked properly, swinging the fpga input pin between 3.3 and near zero volts as the pin on the Diablo connector was grounded. The initial test on the output signals, where I supplied 3.3 or 0 volts to the fpga output pins and watched the Diablo connector, didn't produce any voltage swing.

It was only after a few minutes of puzzlement that I remembered these are open collector drivers which depend on the terminator resistor network to pull up when the driver is not grounding. I have to add on a pull-up resistor and supply +5V before I could get the outputs to swing as needed.

With the pull-up in place, I verified that all 15 of the output signals would swing up to 5V when the input was pulled to ground, but would drop to zero if the inputs floated up. It all looked good, each circuit performing well.

Tomorrow I connect the cable to the Diablo, plug in the terminator, and hook the fpga board to the driver board. With that, I can bring up the new board and disk drive to test its operation on all the driver functions that had previously tested as working correctly. 

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