ADJUSTING THE DISK DRIVE SEEK SETTINGS
I wired up a number of disk drive signals to the oscilloscope. The amplifier balance and dead band adjustments were previously completed. The operation of the trigger that stops the acceleration of the disk arm so that it comes to a stop .01 or .02 inches from its prior position has to be adjusted. Each of the two step sizes, 10 mil and 20 mil, have their own trigger voltage. The level needed to move farther is higher than for the short step.
The trigger voltage for the steps - 10 mil or 20 mil - are set to correspond to a velocity of the arm at which the logic stops accelerating and flips over to decelerating the arm to a stop. Later adjustments will tweak this to achieve the most reliable stepping operation, but the initial adjustment voltages get us close. They were pretty close already.
A 10 millisecond timer is adjusted to ensure that the feedback from the disk drive to the controller logic in the 1130 is correct. When a step is requested by dropping the -Access Go signal low, a timer waits 2.6 milliseconds while the detent, a tooth that fits in a rack to hold the arm at a specific cylinder, is retracted allowing free arm movement. After th 2.6 ms, the feedback signal +Access Ready drops low and the arm begins its acceleration. The 10 ms timer determines when the +Access Ready signal returns to high signaling completion of the step.
At the end of these settings, I verified several timings, voltage levels and waveforms. There is a 2.6 millisecond single shot timer that must be correct to give the detents time to disengage from the rack teeth before the arm begins accelerating, for example. I also checked the detent voltages and timing.
DYNAMIC METHOD OF FINE TUNING THE 10 AND 20 MIL STEP THRESHOLDS
Putting a voltmeter on the threshold potentiometers for the 20 mil step size, I moved the arm continuously in 20 mil steps while rotating the pot. The method involves visually detecting when the detents no longer smoothly engage the rack, perhaps skipping teeth to misposition. The observation has to be over the entire range of cylinders, in both directions. Recording the low and high voltages where the misbehavior begins, I then set the pot to the 2/3 point between those extremes.
The idea is that this is going to be the most reliable setting. It is similar to how magnetic core memory is adjusted - finding the extremes where failure begins and then selecting the point in between that is known to work best. For memory, a diagram called a Shmoo Plot is drawn to record the extremes and select the ideal operating point.
Moving over to the 10 mil step potentiometer and the drive step size to 10 mil, I again continuously move the arm back and forth over the entire range of cylinders while changing the pot until I see misbehavior. Using the low and high voltages discovered by this process gives me the 2/3 point voltage between the extremes and that will be the sweet spot for the mechanism.
RECONNECTING DRIVE AND USING SOFTWARE TO DRIVE THE ARM BACK AND FORTH
I used a simple hand loop to move the arm rapidly back and forth between two cylinder targets, observing the drive to ensure that it does not stop at the wrong cylinder or otherwise misbehave. I changed the forward and backward seek counts to various numbers, having first moved the arm to a desired starting point. This involved several ranges of seek and performed them across various portions of the rack (ranges of cylinder addresses). The most extreme being 202 cylinders forward and back, the extremes of the legitimate cylinder addresses for this drive.
RESULTS OF THE HAND LOOP TESTING
The drive is seeking very smoothly and accurately. It always reached the intended cylinder regardless of the size of the movement. I am ready to move on to running the IBM disk diagnostic program to validate that the Virtual 2315 Cartridge Facility (V2315CF) is working as intended and ready to have the system installation completed.
TWEAKING THE ENCLOSURE THAT WILL HOLD THE V2315CF INSIDE THE IBM 1130
I made some changes to the size of the enclosure that will be mounted on top of the internal disk drive, underneath the 1130 top cover that pivots up. Barry Ward is finalizing his printed drive mechanism and mini 2315 cartridges. I did a test fit of his working version of the drive and the main V2315CF unit installed in the front plate of the enclosure.
The enclosure is assembled with 1/8" thick black plexiglas and mostly glued together, relying on 1/4" square rods to bolster the joints.
The disk drive fits in the right side of the front panel, but I don't have the current outlines for the holes since Barry is working on that part, so my CAD files for testing the assembly are missing that portion of the design. It was enough to feel confident in cutting the acrylic.
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