The University of Iowa's DEC PDP-8Restoration Log
Part of
the UI-8 pages
|
The University of Iowa's DEC PDP-8Restoration Log
Part of
the UI-8 pages
|
Bug 64: In preparation for an open-house, we decided to start a serious attempt at tuning the memory. We began by investigating the behavior of the memory as we varied the slice voltage. For each slice voltage we tested, we stored 32 consecutive 00008 in memory starting at address 60008 and then examined memory to see how many values were still zero, and then we stored we stored 32 consecutive 77778 to see how many values were still all ones.
| Voltage | not 0000 | not 7777 |
|---|---|---|
| 7.7 | 32 | 0 |
| 7.68 | 19 | 0 |
| 7.62 | 17 | 0 |
| 7.6 | 13 | 0 |
| 7.56 | 0 | 0 |
| 7.5 | 0 | 13 |
| 7.54 | 0 | 13 |
| 7.4 | 0 | 15 |
| 7.1 | 0 | 32 |
The slice voltage was adjusted by turning the topmost trimmer on the G008 Master Slice Control board. In adjusting this and other Bourns TrimPot trimmers on DEC flip-chip modules, we have observed that the voltage is quantized, changeing in discrete steps as the trimmer screw is turned. We wonder if the trimmers are wire-wound with a contact brush touches one turn at a time.
The voltage readings here were eyeballed off of an analog meter, so the accuracy is probably 0.02V at best.
What is clear here is that the memory only just barely works. We were hoping to find a range of slice voltages that worked and then set the potentiometer to the middle of the working range.
The working range is probably a 3-dimensional volume, with the dimensions being
The next memory tuning exercise will involve playing with the other two dimensions.
With memory working as best we could adjust it, we loaded demo code in memory for the open house on Jan 20-22:
Some of the above were hand-relocated to run in new memory locations. All were tested and seemed to work.