Wednesday, January 20, 2016

Work at CHM, new lift cart and continued work on SAC Interface logic

I had ordered up a cart with hydraulic lift, having seen the lifts used by some fellow enthusiasts, to handle the disk and tape drives that typically weight between 50 and perhaps 100 pounds each. I found a low price version at Harbor Freight, gambling that the sometimes shoddy quality of their products.

The box arrived all twisted up because the packaging was totally inadequate to ship a heavy metal cart like this, with the rolling wheels poking out of torn holes in the bottom of the carton. I found it missing two bolts which had obviously fallen out of the breached cardboard, but they are standard bolts I can pick up. Otherwise, it works fine.


I put in more time on two tasks - carefully thinking about the new flow of the 12 word transactional engine, and filtering some of the remaining 'false alarm' warnings from the Xilinx software. Still not ready to test on the machine until I have a decent level of comfort that the basic code should work right.

The other task I worked on was completing the local (fpga) handling of skip, space, accurate time simulation and all that goes with it, leaving the PC side with only the need to fetch print lines, translate them to ASCII and write to a PC file. It doesn't have to handle anything beyond XIO Init Write (to print a line) and receiving the number of blank lines to insert as a consequence of any space or skip commands that were handled locally in the fpga.


I stopped by the Computer History Museum at midday to fix a power supply that was giving others a problem. It was one of a pair of units that each provide 20A of -12V filtered power. The challenge was that this unit was the earlier version while we only have schematics for the successor version. Quite a bit different - 10 power transistors versus 6 on the newer unit, major restructuring of the overvoltage (crowbar) circuit, and other changes.

After I sorted out some of the differences I came to see that the voltage regulator circuit needs a reliable -6V reference voltage in order to set the output voltage to the target of -12. It also needed a good load to test. We had some power sources which gave me the -6V reference, but sinking 20A of power was a challenge.

We have a resistor box that has four 50 ohm and four 3 ohm resistors that can be individually switched in and out of the circuit. With all eight switched in, the box has a resistance of .675 ohms and will draw 17.777A at the -12V output of the power supply. Not the full 20A but not too far from it, particularly since the real application in the 1401 computer doesn't deliver exactly 20.0 amps.

After figuring this out and some work, I had it adjusted to deliver -12V under load and the output voltage didn't vary more than a few hundredths of a volt between 4 and 17.777 A of load. I put the oscilloscope on the load to test the ripple and noise from the power supply. Setting the scope down to its most sensitive setting, 5mv per division, I saw essentially zero ripple. The line wasn't ruler flat but it appeared that the fluorescent lights above were coupling to the probe cable to cause the microvolts of noise, not any AC supply related ripple.

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