Work on the internal disk drive of IBM 1130 - part 1

RESTORED 1130 POWER RAIL LEVEL LEADING TO NEW SYMPTOMS

When working on the disk drive attempting to make the adjustments for seek behavior, I checked the voltage at a pin that the Field Engineeering Maintenance Manual (FEMM) suggested be tested. The expected level was 8V but I found it at 6.2V. I verified the incoming power rails - +3V, -3V and +6V - only to find that the last was only at 5V. 

I adjusted the linear voltage regulator in the 1130 to get the rail back to +6V. Once that was done, I powered up while watching the same pin on the disk drive. I saw the voltage start near 8V and slowly decline to just over 6V. I had made quite a few changes to two potentiometers on the disk drive circuitry trying to follow the adjustment procedure in the FEMM, so that might be the cause.

I spun up the drive, with the cable from the 1130 logic disconnected, so that I could use the manual movement switches on the rear of the drive. However, when the drive went ready, the arm stayed at cylinder 0 and it would not move when I used the manual movement switches. 

I manually released the detent that holds the arm in position. These are solenoid operated chisel shaped teeth that fit into V shaped notches in a rack that moves with the disk arm. There is a detent for even cylinder numbers and another for odd cylinder numbers. The rack has notches for each cylinder - spaced 10 mil (.01") apart. 

Holding the even detent out of the notch for cylinder zero, I tried to slide the arm outward but I felt that the voice coil magnet was strongly energized pulling the arm back to the home or cylinder 0 position. That should not be. The voice coil should only energize to move the arm during a seek operation, taking either one (10 mil) or two steps (20 mil distance). 

One of the adjustments I had previously tried to adjust is called the dead band - it determines how much of a drive signal level is needed to start the arm accelerating or decelerating. Set too low and the circuit could oscillate with small random fluctuations in drive current. I set it much higher just to be safe, but the coil remained pulled at full force to the rear. 

The other adjustment is the dynamic balance of the tachometer amplifier. A tachometer attached to the arm produces pulses as the arm moves - the faster they arrive the higher the output from the amplifier. However, it should be set so that a net zero voltage is produced when the arm is not moving. Nothing I did to the pot for this setting would move the net voltage off of zero, however, that doesn't make sense. It is possible that the amplifier is producing a strong movement signal. 

Still, the movement signal should be gated by the logic circuitry on the disk drive so that it only attempts to move in two cases - either it gets a movement signal from the 1130/manual switches, or it is doing its initial power-on reversal to ensure the arm is always at the home cylinder as it powers up. 

The two sides of the voice coil are driven by a power amplifier which gets its drive signals from the card where I was adjusting potentiometers. It is designed to put 48V across the coil in one of two directions, depending on whether we want to move to higher cylinder numbers or back towards 0. A quick check of the voltages on two of the drive control signals showed them at about 2V and about 5V. That is not a valid combination of voltages so I have some debugging to do. The drive previously worked until I began with the adjustments, but some part may have failed. 

I ran out of the short window I had at the shop, since I had somewhere to be. I will be collecting information and building a debugging strategy to figure out what is failing to cause the arm to be retracting strongly. 

Testing my new seek logic for the Virtual 2315 Cartridge Facility - part 13

ASSEMBLED NEW VERSION OF 2310 INTERFACE BOARD

The Virtual 2315 Cartridge Facility (V2315CF) has a component that is a board that interconnects the cables from the IBM 1130 disk controller logic, the internal disk drive, and the main V2315CF unit, as well as a few other functions. I adjusted the design to include electronic switches that connected the signals between the internal disk and the disk controller logic when the V2315CF is in real mode, the norm. The alternate virtual mode does not use the internal disk thus all signals flow only between the disk controller logic and the V2315CF. 

ADJUSTING THE DISK DRIVE SEEK SETTINGS

I sat down with the maintenance manual, a scope, a voltmeter and tools in order to make all the adjustments to the internal disk drive. Simply unplugging the cable from the V2315CF from the disk drive electronics put it in Customer Engineer (CE) mode where I could operate it without using software on the 1130. There are CE switches to move the arm in and out, in both 10 mil (1 cylinder) and 20 mil (2 cylinder) steps, either once per switch press or continually while the switch is held on. 

The first step is to perform a dynamic balance of the tachometer amplifier output, setting it so that there is zero output voltage while the arm is not moving. The next adjustment is for the voltage at which the mechanism stops accelerating the arm and turns off the go latch. This drops the +Access Ready signal, flips the voice coil polarity to begin slowing the arm and starts to engage the appropriate odd or even detent to lock the arm in at the target cylinder position. 

There are different target voltages for 10 mil and 20 mil, since the arm has to move farther for a 2 cylinder step and that requires a faster speed. The target voltage is compared to the voltage on a capacitor which is (dis)charged by the tachometer output, e.g. by the speed. 

A third adjustment sets the dead band, the level below which the servo does not attempt to move the arm. This is done to avoid oscillation while still providing enough oomph to properly accelerate and decelerate the arm. 

The last of the electrical adjustments is for the timer which keeps the +Access Ready signal low from when the trigger voltage stops arm acceleration until the detent should be fully engaged and the arm not moving. The target is 10 milliseconds. 

STARTED TO CHECK DISK DRIVE SETTINGS AND DISCOVERED VOLTAGE ISSUE

When I began to adjust some of the potentiometers in the disk drive I saw weird behavior. The balance was always zero, no setting of the pot would produce anything else. The oscillations never occurred when changing the dead band. These settings are all on the Amplifier card at slot F2 and F3 of the disk drive card cage. The manual suggested voltage checks at certain pins, one of which should be 8V but came up at only 6.2V. 

A QUICK CHECK OF THE INCOMING POWER RAILS TO THE DISK DRIVE FOUND ISSUE

The 1130 generates the DC voltages for the disk drive, the three main power rails as well as 48VDC. The power rails for Solid Logic Technology (SLT) are +3V, -3V and +6V. The 6V rail was only generating 5V, which is way too low for good operation. I checked back at the main logic gates of the 1130 and found that the voltage was low at that point as well. This is a definite problem for the entire system.

ADJUSTED LINEAR VOLTAGE REGULATOR TO RESTORE RAIL TO +6V

I adjusted the potentiometer on the regulator to restore the voltage to the correct level. I have no idea how this got so far out of spec. It might explain some of the odd results or intermittent issues I observed in the past month or so. 

The 1130 has a circuit that verifies the presence of all three primary voltage rails, shutting down the machine or blocking power-up if they aren't good. However, it isn't highly precise, since 5V was high enough to pass the check while still being far too low for proper operation. It is intended more to detect the lack of one of the rails, not that they are correct. 

Testing my new seek logic for the Virtual 2315 Cartridge Facility - part 12

TRYING OUT A TERMINATOR AT THE 1130 BACKPLANE TO IMPROVE SIGNAL

The +Access Ready signal that is generated by the disk drive in real mode of the Virtual 2315 Cartridge Facility (V2315CF) or by the V2315CF in virtual mode. It is feedback to the disk controller logic in the 1130 as it moves the disk arm to seek to different cylinder locations. There was some ringing and glitches on the signal which did cause issues for the disk controller and for the V2315CF which needs to monitor the disk movement to keep track of the current cylinder. 

I have tried multiple things to improve the signal as received at the pin on the 1130 in gate A, compartment C1 where the disk driver logic resides. I changed the impedance of the terminator installed in the V2315CF, which is on the sending end of the signal to the 1130 and receiving end for the signal from the disk drive. I removed the terminating resistors entirely. I shorted the +Access Ready signals from the 1130 and the disk drive together on the 2310 Interface Board so that the V2315CF is electrically uninvolved other than additional cable lengths. 

All of these changes made some improvement. The last one gave me excellent agreement between the disk drive arm and the intent of the XIO Seek commands I issued. However, the V2315CF failed to completely shadow the movements, missing a couple of cylinders on longer seeks. 

I therefore decided to add termination right at the destination pin on the 1130 backplane. The IBM design did not have terminators there, other than whatever pullup resistor they used on the backplane after the signal arrived. I couldn't find any resistor or source for a pullup. The signal is generated by a single shot timer in the disk drive itself. 

I have no component level circuit schematic for that gate so I can't tell if it has pullup or not. In any case, having the pullup on the source side which is typical for the 1130 logic would not protect the input pin from ringing caused by inductance and capacitance in the cabling over to the SLT backplane. 

I therefore decided to mount termination resistors right at slot L4 of the backplane, where the +Access Ready signal is connected at pin D04. I used a 330 ohm pullup to +3V which is available on pin D02 of the slot and a 470 ohm pulldown to ground which is available on pin D08 of the slot. 

I had to find a good way to install this in that area. I chose to solder the two resistors together with very short leads at the connection. I then installed wirewrap wires to pins D02, D04 and D08, with their other ends loose. Once I find a way to mechanically secure the resistors in place between the rows of pins on the backplane, I will cut the wires and tack solder them to the common point and other ends of the resistors. For the time being, I did a quick and dirty installation.

OBSERVATION OF SIGNALS AND BEHAVIOR WITH TERMINATOR RESISTORS IN PLACE

The signal looked almost exactly like the signal leaving the disk drive at the backplane pin of its electronics, thus I was not getting any distortions based on the run of the signal through various cables, boards and traces on its way to the 1130 backplane pin. 

The drive would still fail to seek properly on some longer seeks, stopping a few cylinders short of its intended destination. It would also lock up and buzz at a point short of cylinder 202 but well along in its travel. Something is still wrong with the drive that I have to resolve. I need to go through the entire sequence of settings per the manual.

Below is the image I took of a seek of 193 cylinders.

Yellow is the +Access Ready signal as it is delivered to the 1130 disk controller backplane pin. Green is the -Access Go command from the 1130 disk controller that triggers each step. Purple is the -10/+20 Mil signal which makes the first step 10 mil and the remainder be 2 cylinders. Blue is the source +Access Ready signal at the disk drive electronics backplane pin. 

The expected waveforms and timing are shown in one of the maintenance manuals, which I will be checking while the drive performs some steps. 

There are four adjustments I will check and change if necessary that control the timings shown above.

1. Tachometer amplifier static balance - no output when carriage is not moving
2. Dead band of servo - adjust to eliminate oscillation but achieve proper movement
3. 10 mil and 20 mil stop voltages - duration of acceleration for each step size
4. 10 millisecond timer duration - time that +Access Ready stays low

I will also verify the 2.6 ms single shot is the proper duration to allow the detents to retract before arm movement starts. Watching voltages and timing of other signals above help identify if any components are out of spec and causing issues.

The FE maintenance manual has multiple methods of adjusting the drive values above, including a very promising method that observes the continuous movement of the arm for erratic detent operation and a means to tweak the adjustments for the most reliable and consistent stepping. 

Testing my new seek logic for the Virtual 2315 Cartridge Facility - part 11

STILL FOCUSING ON SIGNAL INTEGRITY FOR V2315CF SEEK

I was not happen with the glitching and malformed signals going into the 1130 for the +Access Ready feedback signal. I am determined to clean this up and not just hope that the Virtual 2315 Cartridge System (V2315CF) works in spite of the signal distortion. The first thing I did was to remove the terminator resistors for that signal, to see what the signal looked like. It was still distorted.

The new version of the 2310 Interface Board, fabricated and current being shipped to me, adds electronic switches to short the signals between the disk and the 1130 while in real mode, rather than trying to generate the signal as a copy of the incoming one. I installed a short bit of twisted pair to bridge the B07 pin from the disk drive to the B07 pin heading into the 1130 disk controller circuitry. The result was considerably better.

The blue trace on the bottom is the +Access Ready signal generated by the disk drive and the yellow trace is the same signal at the entry to the disk controller SLT cards inside the 1130. The green trace is the -Access Go signal that goes from the 1130 to the disk to start a seek. Finally, the purple trace is the -10 Mil Step signal, which initially requested a single track move and then rose so that the additional steps are two tracks each. 

The temporary bridge connecting the signal straight through

Above is a trace of a seven cylinder seek. The -10 Mil Step signal is low for the first step and high for the next three. It does look good, although with a time scale of 5 milliseconds per division any glitches wouldn't be visible there. 

V2315CF AND 1130 IN PERFECT SYNC, DISK SOMETIMES IS OFF

I pounded away at the disk trying small, medium and long seeks in both directions. I was pleased that the disk arm position is now exactly in agreement. The only issue is that the V2315CF shadowing of the arm position sometimes drops a few cylinders from the count. 

I am hopeful that the new PCB with its electronic switches will further improve the fidelity, as it will manage seven signals that run between the disk and the 1130 controller circuitry. By the end of the week I should have built the new version of the board and installed it. I can troubleshoot further if the issue is still present with the new board version. 

VERIFIED THE POWER FAIL PROTECTION

I wired together the elements that give the V2315CF the ability to detect a power failure in the 1130 and conduct a full writeback of the RAM contents into the mini cartridge leveraging a 12V motorcycle battery. 

The power distribution unit board, in the center of the photo above, routes power between the 1130 12V rails, the motorcycle battery, a smart trickle charter, a timer module, the power supply for the V2315CF and then the V2315CF unit itself in the lower left. At the top is a bench power supply that stood in for the 1130, allowing me to flip it off to verify that the timer powers the unit during a 40 second interval, and that the V2315CF properly detected that the 1130 12V power dropped. It is ready to be wired into the 1130 system. 

WILL MAKE ADJUSTMENTS OF THE INTERNAL DISK DRIVE SETTINGS

The internal disk drive has a set of six adjustments that are important to proper behavior. These include the thresholds for the 10 and 20 mil seeks, the timers for the various actions during a seek, and the sensitivity of the seek movement circuitry. Those, coupled with the mechanical stops at cylinders 0 and 202, will ensure the most reliable operation. 

Testing my new seek logic for the Virtual 2315 Cartridge Facility - part 10

NEW TERMINATOR BOARD IMPROVES BUT DOES NOT FIX V2315CF SEEK TRACKING

The Virtual 2315 Cartridge Facility (V2315CF) watches the control signals between the 1130's internal disk controller and the internal disk drive (named 2310 or 13SD or RAMKIT). It shadows the position of the disk arm as commanded by programs issuing XIO Seek commands, since the data to be returned when the software reads from the disk depends on which of the 203 cylinders the read/write heads are positioned over. The V2315CF generates the data signals instead of the disk heads. 

The current situation is that requests to move the arm are obeyed correctly by the disk drive. The physical arm is in the correct location after every move. However, the location as shadowed by the V2315CF is off, short by 1 or 2 cylinders from the actual final position. The error is cumulative.

The programmer requests a movement of some number of cylinders relative to the current arm position. The disk controller implements that as a series of steps of 1 or 2 cylinders at a time, the first being 1 cylinder only if the requested movement count is an odd number; all others are 2 cylinders at a time. 

Glitches on the signals between the 1130 and the disk drive do not appear to affect the disk drive's operation but confuse the V2315CF logic. I can work on two fronts here - eliminating the glitches entirely and/or ensuring that V2315CF is invulnerable to the glitches. 

OSCILLOSCOPE OBSERVATION OF SIGNAL ISSUES USING NEW TERMINATOR BOARD

I set up the oscilloscope to watch +Access Ready inside the 1130 at gate A, compartment C1, where the signal enters the logic card. A second probe was connected to the V2315CF main box where the +Access Ready signal is emitted. A third probe was attached at the pin on the 2310 Interface Board where the cable is attached running to the 1130 logic where our first probe is placed. The final probe monitors the signal as it is produced by the disk drive. 

I triggered on the falling edge of +Access Ready, using my test code in the 1130 to issue an XIO Seek of 3 cylinders forward which is one that we have seen malfunction consistently. I captured the waveforms and looked to see what was happening to the signal at several points in its routing and hoping to understand what is producing the glitchy behavior. 

I can see a very strong drive from the output chip on the V2315CF and then substantial ringing of the signal where it enters the 1130 logic circuits. Part of the problem is ground bounce between the 2310 Interface Board, the V2315CF, the 1130 and the disk drive. I will add some wire braid for high frequency ground paths, although that is not the issue here. 

NEW LOGIC IN FPGA TESTED TO SEE IF IT MATCHES DISK ARM POSITION

I modified the seek shadowing logic in the V2315CF and installed it to the flash memory that initializes the FPGA. I ran some seeks of various sizes, noting for each whether the arm position matched the intent of the seek and whether the V2315CF reported cylinder matched the other two. 

We were down to one issue with the shadowing. The V2315CF seemed to miss the first seek, that error in positioning remained for all subsequent seeks. I tracked that down to an error in my Verilog, where I look at the state of the Home signal and don't bump the cylinder when it is on in real drive mode. That means we don't count the first step away from cylinder 0.

I updated the logic and tested again. The disk drive and V2315CF are in complete agreement with the seek amounts I executed. I ran a series of forward seeks in a small program, issuing the next seek as soon as the previous one caused an operation complete interrupt. I moved 40, 20, 10, 7, 6, 3 and 1 cylinders with the short program. It completed in a fraction of a second with the disk arm and V2315CF showing the current cylinder location as 87. 

Testing my new seek logic for the Virtual 2315 Cartridge Facility - part 9 - Fixed cylinder 202 issue

ADJUSTING THE FORWARD CRASH STOP ON THE INTERNAL DISK DRIVE

Crash stop when arm is not at 202

Arm against crash stop just past 202

In order to be safe, I changed the stop to a gap of .008", giving the maximum margin for acceleration while still blocking the ability of the arm to move to cylinder 203. With an attempt to move 20 mil, the arm should stop and the detent should still fall back into the notch for cylinder 202. 

Just as importantly, it should not do anything bad if it gets a 10 mil step thus engaging the odd detent instead of the even detent. Looking closely at the positioning rack, I can see there is a notch at the 203 location which the odd detent can enter. 

The way you read the location is to look where the right detent is pointing. The numbers are etched every 40 tracks, with a large vertical bar at the 20 track intermediate point. The fine vertical lines on the positioning rack mark each 4 track step, with a notch in between at the 2 track point. The left detent (odd) will fall in to indicate that the location is the number you read from the right detent plus 1. 

The picture is of the arm when I had a .006" feeler gage between the crash stop and the arm, the arm not quite far enough along to let the even detent fall into 202. However, you can imagine the rack slightly to the right so the right (even) detent enters the 202 notch. This is where the arm stops at the last legitimate location. 

If we attempt a 1 track (10 mil) step forward, the drive flips which detent is used to stop, now engaging the odd (left) detent. Since there is a notch there that the odd detent can enter, it will try to hold the arm there. That means the drive is trying to be at 203 but the mechanical stop is trying to keep it from moving past 202. 

It may be slightly mispositioned to read or write on the cylinder but it is a reasonable state to stop in. As long as the circuitry in the disk drive turns off the acceleration of the arm so it doesn't lock up buzzing, this is a decent outcome. It will back up correctly. A step of 10 mil will bring it back to 202 with the even detent engaged, a 20 mil step will move it to 201 still using the odd detent. 

The IBM maintenance manual blithely instructs the field/customer engineer (CE) to run the drive, use the CE switches to move the arm out to cylinder 202, then adjust the crash stop to a specific gap. The stop is a bolt that is threaded into a big block holding the arm mechanism together, with a nut that locks it in position. 

The issue is that the slot in the bolt that one turns to adjust the crash stop is directly against the 2315 cartridge case that is in the machine. There is no room to fit a tool in there to turn that once the nut is loosened. If I remove the cartridge, the drive will not spin up nor load the heads, thus it wouldn't allow arm movement or perform seeks. 

However, I can manually push the microswitch that detects a cartridge is inserted, to allow the drive to spin up and load the heads. They won't really load for two reasons - no platter to ride on, but more importantly my modifications have stopped the heads from being pushed down so they will remain safely apart 

I did find a way to use pliers to turn the threads of the bolt after loosening the nut, thus could adjust this with the arm actively sitting at 202. The gap was much too small, so I adjusted it out to a suitable amount that would stop the seek action but not move past 202. 

TRYING TO MOVE THE ARM PAST 202 AFTER MY ADJUSTMENT

The disk drive is designed so that if the cable that plugs into the drive from the controller (or from the 2310 Interface Board in our case) is removed, the Customer Engineer (CE) switches and buttons can control arm movement without having to issue XIO commands or involve the 1130 disk controller logic. 

I moved the arm to cylinder 201 from zero by setting the Step switch to 20 Mil and held the switch at Continuous movement until it got close. I then set the Step switch to 10 Mil and advanced one step at a time until I verified that the arm reached that point successfully. This would have the odd detent in the rack.

I then moved the arm 1 track, with Step set to 10 Mil and using a single advance, which should put the arm at 202. That was verified by looking at the arm . The even detent should be engaged.

Now, the moment of truth. I issued a seek of 2 tracks farther, with Step at 20 Mil and a single movement request, which should leave the the arm at 202 and the drive should recover from the attempt to move past. The even detent should remain engaged. Worked great.

I cycled the drive to get the arm at the home cylinder and used 20 Mil steps to advance the arm to 202. I moved 1 more track, with Step set to 10Mil, wanting to see what happens with the detents and the arm. I looked at the detent and the arm position, as well as the V2315CF. If the odd detent is now engaged, a reverse step of 10 mil would actually stop the arm at cylinder 200 instead of 201, which is not desirable. If the even detent remains engaged, I need to understand the mechanism.

After testing and observation, it does engage the odd detent, but has bounced back to 201. I hooked up the system and did plenty of seeks, with the pathological behavior gone. This problem is now resolved. 

Testing my new seek logic for the Virtual 2315 Cartridge Facility - part 8

FINISHED MODIFYING TERMINATOR BOARD WITH NEW RESISTOR VALUES

The terminator board that fits into the Virtual 2315 Cartridge Facility (V2315CF) main unit is the same one used successfully with the unit in its original role as George Wiley's RK-05 Emulator connecting to a DEC computer to emulate a disk drive that is a derivative of the IBM drive. The terminator implemented the same resistance values as the M903 terminator board built by DEC. 

The impedance of the terminator connections is just over 122 ohms, which may match the impedance of the cabling in the DEC computers. The RK-05 emulator kit offers an option A1 RK-05 to RK8-E that has two IDC 40 pin ribbon cables to make the connections to the controller card, since the RK-05 drive needs 36 signal lines for the interface between disk and controller, deploying ground lines between each signal line on the ribbon cables. 

Ribbon cable characteristic impedance for single ended signals (not differential pairs) is 80 to 100 ohms with interspersed ground wires. Using the DEC terminator board yields a big mismatch in impedance, which will lead to reflections of about 20% of the signal level. If the rise time of signals is faster than 6 nanoseconds the signal will ring and appear to have spurious transitions. 

I determined resistor values to drop the terminator impedance to about 100 ohms, which would minimize the issues even with the current sharp rise times. The Lattice FPGA does not support the means to slow the signal transitions to 7ns or slower instead typically rising at 1 to 1.5 ns.

I removed all the original resistors - a mix of 1206 size and 0603 size for the pullup to 5V and the pulldown to ground respectively - and cleaned up the board.

The original terminator board has a few unpopulated resistor locations - the crude circles show you were to concentrate to see them. 

I installed my 179 ohm pullup and 249 ohm pulldown resistors onto all the positions where I have signals connected to the ribbon cables. The new terminator board was tested and then plugged into the V2315CF to do more testing. 

DETAILED VIEW OF EFFECT OF SIGNAL GLITCHES ON SEEK BEHAVIOR

A glitch on the +Access Ready signal causes the 1130 disk controller logic to malfunction, thus not forming correct step requests to the drive. The disk drive responds to brief glitches in -Access Go that are induced by the +Access Ready glitch, so it does move to the requested cylinder. The V2315CF fails to recognize the malformed sequences thus it does not reach the same cylinder number as either the drive or the intent of the XIO Seek commands. 

The dance of signals we should see is the controller drop -Access Go with -10 Mil Step low, holding it until the drive drops +Access Ready about 5 milliseconds later. When +Access Ready drops, the signal -10 Mil Step will rise from low to high since only the first seek should be a 10 mil step. 

Also at the time when +Access Ready drops, if the count register (Bits 7 to 14 plus the -10 Mil Step which is bit 15) has all bits on, the -Full Word Count signal is dropped which terminates the seek in the 1130 disk controller logic.

The disk drive, meanwhile, moves the arm for another 9-10 milliseconds then raises +Access Ready to indicate the seek step is complete. If the count register is not all ones yet, another cycle of -Access Go dropping with its +Access Ready responses will occur, iterating until -Full Word Count drops. 

Above, we will examine a successful seek of 1 cylinder. We can see the signals conforming exactly to the protocol. After -Access Go is dropped with -10 Mil Step low, we see +Access Ready drop 5 milliseconds later. At that time, the controller changes -10 Mil Step to high, which turns on -Full Word Count, stopping the controller from trying any more steps. At 15 ms from the start of the step, +Access Ready returns to high indicating the move is complete. 

When I did a move of 3 cylinders, however, the glitches arrived and the process breaks down. The arm moved three cylinders but the V2315CF only saw a single cylinder step. Lets look in detail below, zooming in to the time when the glitch first arrives:

At the 5 ms point, when +Access Ready should drop for 10 ms, we see a very short blip downward, circled in red. This turns off the -Access Go and makes -10 Mil Step go high as pointed by the green arrow.

We see another -Access Go, but very brief, indicated by the blue arrow. Since -10 Mil Step is now high, this asks for a 20 mil step. The short drop of +Access Ready just to the left of the blue arrow causes the disk controller to add 2 to the count register, making it reach the all-ones state and turning on -Full Word Count pointed to by the pink arrow. The seek is over so no further step requests. 

The drive did respond to the additional -Access Go, in spite of the prior seek not being finished. It is only a 20 nanosecond pulse, borderline for the Solid Logic Technology (SLT) edge detectors to see but obviously long enough. Somehow the drive managed to do the second step correctly, as the arm did end at the proper spot. 

Since the V2315CF logic is generating the correct protocol dance - keeping +Access Ready low for 10 ms starting 5ms after the falling edge of -Access Go. It doesn't watch for the next -Access Go falling edge until after the 5ms and it misses the defective short mistimed pulse that the disk drive responded to. 

The drive even responded to a 5 cylinder seek with a correct movement. The signal pathology is even uglier, as seen below:

We see that a single step is observed as far as the V2315CF, based on the 15 ms dance expected. Zooming in on the glitch area, we can see how it managed to move 10 mil, 20 mil and 20 mil to achieve the 5 cylinder goal. 

The glitchy +Access Ready is see to the left of the green arrow, resulting in the -10 Mil Step signal going high where the green arrow points. The first step we began is 1 track, but the subsequent moves will be 2 tracks each. A glitch from the controller shown with the first red arrow drives the arm another 2 tracks. Later we see the -Access Go drop for a longer interval, driving the last 2 track move. This adds 2 to the count register, completing the count of 5 and turning on -Full Word Count to complete the XIO Seek.