Improved Virtual 2315 Cartridge Facility behavior when drive powered down then back up

SITUATION BEING ADDRESSED

When we have loaded a valid mini cartridge into the Virtual 2315 Cartridge Facility (V2315CF) it should remain active regardless of whether we power the disk drive down and back up. In the real world, a 2315 cartridge that had been inserted in the drive will still be there when the switch if flipped to run. So too, the virtual cartridge should remain available if we spin up the disk drive.

When we have powered down the disk drive, the V2315CF state machine sits in its run level 9, waiting for the disk drive to signal File Ready after the drive believes it has loaded the read/write heads on to the disk platter inside the 2315 cartridge. Once the drive goes ready, V2315CF advances to run level 10 where the 1130 can access the disk however it wants.

Powering down from run level 10 will simply step back to run level 9, since we do have a valid virtual 2315 image loaded into the V2315CF. The next time the disk asserts File Ready after we spin it up, we will go to run level 10 and be able to use the cartridge again from the 1130. 

What was not properly modeled before was that the cylinder number that V2315CF records to match the position of the real disk arm does not go to zero when the drive is powered down (File Ready goes off). That is what the real drive hardware does, but V2315CF naively keeps its previous cylinder number instead.

SIMPLE FIX APPLIED

The same logic in the PICO code that turns the File Ready lamp on or off on the V2315CF main box will now reset or set a flag in the FPGA that forces the cylinder number to 0. Anytime the RDY lamp on the V2315CF is turned off (and the File Ready lamp on the main 1130 console is off), the cylinder will be locked to 0 (home cylinder). Turning on the lamp also unlocks the arm so that seeks from the 1130 can be recorded to set the cylinder to the appropriate number. 

Testing my new seek logic for the Virtual 2315 Cartridge Facility - part 6

NEW APPROACH FOR SHADOWING SEEK IN REAL MODE

I made changes to the logic for the FPGA in the Virtual 2315 Cartridge Facility (V2315CF) to handle the glitches I saw on +Access Ready as it was passed to the 1130 disk controller logic. That feedback signal generated by the disk drive had glitches which looked like spurious changes of state to the disk controller, resulting in failure to drive the disk arm to the intended cylinder in all cases. 

For virtual mode, where the disk drive is not involved at all, V2315CF generates +Access Ready using a timer to match the specification for what the 2310 internal disk drive would do if it were hooked up natively. When an -Access Go from the 1130 requests a movement, the disk waits 5 milliseconds, then drops +Access Ready for a further duration of 10 ms. At the time that it first drops the feedback signal, the logic records how the cylinder number (disk arm position) would have changed based on the step size and direction that accompanied the -Access Go falling edge. 

I now generate the same +Access Ready signal in real mode (where the disk drive is running and performing the seeks being requested) as we do in virtual mode. Thus, we don't care about +Access Ready coming from the disk drive, we just assume that it is performing the seek as requested. This ensures that the V2315CF will interact properly with the disk controller logic and that we record every arm movement that was requested by the 1130.

Testing my new seek logic for the Virtual 2315 Cartridge Facility - part 5

BETTER LOGIC ANALYZER RECORDINGS AND THEIR RESULTS

I recorded several seek movements - 1, 2, 3 and 193 cylinder movements. What I observed in all of these are glitches or spurious changes of +Access Ready which caused the 1130 disk controller to malfunction. 

For example, here I zoomed in on +Access Ready, the second signal in the trace, dropping low for a short (invalid) period which triggers the disk controller to step the count register and in this case to switch from 10 mil to 20 mil steps. It also confused the controller into turning off its =Access Go request but it turned it back on because +Access Ready was back on. 

An even more flakey case is captured on an attempted seek of 3 cylinders, which actually moved the arm just one step. The count stuttered very rapidly and reached a full count condition that stopped the seek. The drive saw only the single -Access Go with -10 Mil Step asserted, thus it moved only one cylinder. 

The arm physically moved to cylinder 175 when we requested the seek of 193 cylinders, because the count was being updated by +Access Ready glitches. 

The root cause is incorrect +Access Ready signals as seen by the 1130 disk controller. These are also being seen by the FPGA logic in the V2315CF which causes it to miscount in its own way. The state machine behind the seek shadowing function expects +Access Ready to drop 5 milliseconds after -Access Go is set low, for +Access Ready to stay low for another 10 ms and then return solidly to high. The glitches cause the state machine to advance when it shouldn't or fail to advance. 

NEW VERSION OF 2310 INTERFACE BOARD PCB ORDERED TO HELP WITH THIS

The new version has the electronic switch circuits that will directly route the +Access Ready signal coming from the disk drive out to the disk controller logic, so that it does not have to pass through the FPGA at all. The FPGA is able to see that signal from the disk drive, so that it can shadow the arm movements properly. 

I also switched the terminator resistors on the V2315CF to better match the impedance of the 1130 cables and ribbon cables between V2315CF and the 2310 Interface Board. Hopefully these will help with reflections and other signal issues for +Access Ready. 

CHANGE TO FPGA LOGIC TO AVOID THE ACCESS READY GLITCH ISSUES

In virtual mode, the V2315CF generates the +Access Ready signal using timers so that it conforms exactly to how the disk drive should work. It drops 5 ms after a seek begins and returns to high a further 10 ms beyond that. This worked perfectly in testing.

I will send that same generated +Access Ready to the disk controller logic, not the signal coming from the disk drive, so that real mode will not have the improper feedback to trip up the operation of the disk controller logic. 

My shadowing logic must be made bulletproof even with odd behavior of the detected +Access Ready signal, so that it too counts properly. I will drive it off the same timers that generate the signal for virtual mode. If we receive an -Access Go, we will step through the timing without regard to the feedback signal coming from the disk drive. I may test the state of +Access Ready in order to detect failures of the disk drive, but it must be a soft enough check that the glitches won't bother it. 

Testing my new seek logic for the Virtual 2315 Cartridge Facility - part 4

UPDATES MADE TO LOGIC OF THE V2315CF

I made a few updates to the C code for the PICO and the Verilog for the FPGA of the Virtual 2315 Cartridge Facility (V2315CF) based on the prior day's testing. I had noticed that a completely valid sequence of steps did not perform in a way that makes sense. The state machine was changed to handle this in a more reasonable way.

The user inserts a mini cartridge containing an image of a 2315 disk cartridge on a microSD card inside the holder, plugging it into the V2315CF main box. The switch is set to Load which causes the V2315CF to read the contents into memory so that the 1130 can read and write to that just as if a physical 2315 disk cartridge had been inserted into the 1130's internal disk drive in an unmodified system.

A placeholder 2315 cartridge is inserted into the disk drive in the 1130, one which is only used to generate timing signals and provide realism. The run switch is turned on for the disk drive and it spins up. After about 90 seconds the disk drive believes it has loaded the read/write heads down to fly atop the surface of the disk platter inside the cartridge. The File Ready lamp lights on the 1130 console and on the V2315CF main box.

After accessing the disk by software instructions run on the 1130, the run switch of the drive is turned off. The File Ready lamp goes out and the drive slows to a stop. The Unlock lamp on the 1130 console lights, indicating that the 2315 cartridge could be removed and another inserted. If, however, we just turn the run switch back on, we should be able to continue using the virtual cartridge we loaded into the V2315CF just as we would continue to use a physical 2315 cartridge in an unmodified drive.

After the drive spins up and waits 90 seconds, the drive thinks it loads the heads and wants to turn on the File Ready lamp. However, it does not light, nor can we access the virtual disk from software. This does not make correspond to what happens with an unmodified 1130 and disk drive, where we could turn it on again and it would be ready for access. I had to unload the mini cartridge and reload it to get the V2315CF to start up the drive and make it ready.

When we load a mini cartridge into the V2315CF, the state machine sits (lets call this state W) waiting for the File Ready signal from the disk drive before it advances to turn on the File Ready lamp (call this state R) and allow access. My change was that when we had been in this second (R) state, but turned off the drive, the state machine returns to that first state (W) where it is only waiting for the drive to turn on File Ready.  

I also removed the Schmitt trigger filter I used with the +Access Ready incoming signal, because that adds additional delay which may have been causing the 1130 disk controller logic to malfunction. That was one possible cause of the seek errors I was seeing - a seek of N cylinders sometimes moved only L cylinders instead, plus the V2315CF believed it had moved to cylinder L-1 instead of L where the arm actually stopped. 

INSTALLED THE LOGIC ANALYZER ON THE DISK CONTROLLER LOGIC

I hooked up the 16 channel logic analyzer to key signals in the disk controller logic that might point me toward the cause of the seek anomalies. I watched the nine bits of the count register, as well as the signal to move the arm (-Access Go) and the feedback signals from the drive (+Access Ready and +Home). I also hooked the analyzer to the signal +Access Control generated during an XIO Seek instruction and the internal control signal -full_word_count that indicates the count register contains all ones. 

When an XIO instruction requests a seek of N tracks from the current location, the value N is inverted and loaded into the count register. That is, every bit is inverted to form the ones-complement of the count from the XIO. The disk controller logic then sends -Access Go signals and bumps up the count register, taking steps of 1 or 2 tracks, until the count is complete because all nine bits of the count register are 1. 

When the count N from the XIO instruction is an odd number, the low order bit of the count register has a 0 in it. This causes the disk drive to move only 1 track in the step (-10 Mil Step signal). As soon as the drive responds to the -Access Go about 5 milliseconds later with a drop of +Access Ready, the low bit is forced to 1. When +Access Ready returns to high, the count register has 2 added to it and -Access Go is again asserted to take the next step. This will always be a 2 track step. Only the first is 1 track and only when the count N is odd. 

This restart of -Access Go is blocked when the count register is all 1s, causing the seek operation to complete. I will look at the interaction between -Access Go, +Access Ready, the 9 bits of the count register and the -full_word_count condition, which should proceed according to the rough timing of 5 ms for +Access Ready dropping and 10 ms after this, +Access Ready returning to high. -Access Go should go off when +Access Ready drops. The seek operation should not end until we have moved the full count N tracks. 

A challenge with the logic analyzer is having enough recorded information to find the defect. The logic analyzer runs fast, but the timescale for a seek is glacial by comparison. Each step takes around 15 milliseconds so a seek of 200 tracks would span 1.5 seconds. The practical limit for recording in the 256MB buffer depends on the sampling rate and number of channels being recorded. For 16 channels being captured, at a sampling frequency F we will have 16/F seconds recorded. 

To watch the signals based on nominal behavior, we have to catch signals that change in the neighborhood of once each 5 to 10 milliseconds. If I set the sampling frequency at 5MHz, then we record about 80 Mbits in a second; with a memory of 256Mbit to hold them I can capture about 3.2 seconds worth. PPPp

Using the rule of thumb that we should sample 10X the rate we expect signals to change, I should be able to see signals changing on the scale of 500KHz with a 5MHz rate. That corresponds to about 2 microseconds. I can't be certain to capture glitches that occur on a shorter timeframe with this recording rate, but I can certainly count the steps and validate the change of the count register. 

If I see that weird things sometimes happen, with counts advancing too rapidly or the hint that the dance between -Access Go and +Access Ready is sometimes happening faster than the 15 ms we expect, I can increase the sampling rate and try some short seeks that still fit in memory until I have visibility of whatever is triggering the malfunction. 

FIRST OBSERVATION FOR A SEEK OF 193 CYLINDERS

I set up the XIO with a seek count of 0xC1 which is 193 in decimal. It is an odd number as well as very long, where previously we had very repeatable failures to seek the entire distance. The logic analyzer set to 5MHz sampling can easily record the entire operation, triggered by the +Access Control signal that is generated as the XIO Seek instruction executes. I can store the contents of the data and use the DSView software to look at it. 

The V2315CF agrees with the disk arm position, which is a significant improvement from last time. Unfortunately, the saved file from the logic analyzer didn't save enough to capture the end, so I couldn't count the seek steps to verify that it matched the seek count. 

ZOOMING IN ON A SEEK OF 3 CYLINDERS

If I set the analyzer to the max rate for 16 channel recording of 100 MHz I can capture about 167 milliseconds of signals, good enough for a seek that should last a bit of 30 ms. If I want to just zoom in on the -Access Go and +Access Ready signal waveforms, plus the +Access Control trigger and the -full_word_count end signal, the analyzer can capture at 400MHz for the same 167 ms. That lets me see signals with good accuracy down to about 20 nanoseconds or two ticks of the FPGA main clock. 

Again, due to my low familiarity with the DSView software that captured the trace, I didn't save enough duration to see the end of any of the small seeks. The only one that I was able to see in its entirety was a seek of 40 tracks, which the trace confirms that it issued -Access Go 20 times and I did verify that the disk arm was moved exactly 40 tracks. 

I also don't have good traces for two signals, which I suspect means that I misconnected the two leads from the logic analyzer since the two missing ones are both on the same SLT card slot. 

However, I did do some individual 1, 2 and 3 track seeks which did move both the disk arm and the V2315CF cylinder variable the correct amount. 

ANALYSIS OF RESULTS

It appears that most of the issue I was experiencing was due to the Schmitt trigger filter which I removed from the FPGA logic. However, it was my impression that the disk arm stopped at around cylinder 161 when it should have ended at 193, but I am not certain. 

I plan to go back to the workshop and more carefully record the signals including ensuring that the traces fully capture the entire duration of the seek command. I was triggering on the wrong signal to start as well. Stay tuned for the update, which will either confirm that the seek is fully corrected or help identify a remaining issue with long seeks. 

DISK DRIVE ISSUE WHEN ATTEMPTING TO SEEK PAST CYLINDER 202

When I do a seek that attempts to move the arm past cylinder 202, the drive should keep the arm at the proper location and report completion. What I am experiencing is a steady buzz with the drive continually attempting to move the arm past 202. Even doing a system reset of the 1130 system does not stop that error. Since the reset will ensure that -Access Go is not asserted, it is an issue entirely inside the disk drive. 

The drive has 'crash stops' fixed to block the arm from moving outward past cylinder 0 (Home) and from moving inward past cylinder 202. These are mechanical stops that are adjustable.

The electronics that moves the arm makes use of an integrator to add up the speed pulses from a tachometer on the arm. When the integrator gets to the preset trigger level, it shuts off the access go latch, turns off +Access Ready, which in turn causes the 1130 disk controller logic to return -Access Go to high. 

In one document, it makes mention of a resistor that will charge the integrator if the arm is not able to move, due to the mechanical stops. It claims this takes about 40 milliseconds to turn off, longer than the 2.4 to 3.8 ms during movement that reaches the integrator trigger level. I don't see that 'resistor' in any of the other documents for the disk drive, but I will have to study the circuitry in detail to see if this is actually implemented. If it is implemented, then that charging path is defective, probably due to an open resistor. 

I should do a test where I attempt to seek 2 tracks in reverse, starting at track 1, to see if the drive malfunctions in the same way. It may not, because the access go latch is turned off my reaching the home cylinder which turns on a microswitch. There is no such microswitch nor turnoff path for reaching cylinder 202. 

The issue may be due to the crash stop being set incorrectly. There is a minimum gap between the stop and the arm when it is at cylinder 202, which might be far enough to let the integrator level reach the trigger point. If the space is too small or non-existent, the tachometer won't record enough pulses. 

I can quickly check the spacing of the crash stop, but if that value is acceptable then I have to dive deeper. I suspect that the paths that charge the integrator are represented in the schematic below, as I have colored the paths to show my speculation. 

When the forward direction is active, transistors Q19 and Q22 apply +48V (green) and ground (blue) across the voice coil in the proper direction. The resistor at the start of the red path has a voltage drop proportional to the current flowing in the voice coil. If the coil is stalled it should increase the current and therefore increase the voltage of the red line relative to ground. This voltage flows through a diode D1 thus it only acts to effect things if the voltage is above the voltage drop (about .3V for germanium) of the diode. 

The red line is injected into the yellow path that runs to both transistors Q5 and Q6 of the integrator. Normally, the pulses from the tachometer will cause Q3 or Q4 to pulse the either Q5 or Q6 to drain energy from the capacitor C1 which is initially charged to +6V and pulled down by Q5 or Q6 towards -3V until the comparator triggers the reset of the accelerator go latch. 

However, with no movement to pulse the tachometer, the voltage from the red line flows through the balanced resistors connected to transistors Q5 and Q6. If the balance of the yellow line is offset by current from transistor Q9 (pink) which is active when the -Access Forward line is low (asserted). This causes some current flow to discharge C1 albeit more slowly than if the tachometer pulses were driving Q5/Q6 actively. 

I struggled to find a component that is not working properly for the injection from the red path, but works properly for normal forward and backward movements of the voice coil, other than diode D1. It would have to be a short circuit to stop the red voltage entering the yellow path. If the gap at the mechanical stop for cylinder 202 is too small to allow the tachometers to drain C1 far enough, then I will try to find diode D1 on the circuit board and test it. 

Changing the terminator to improve impedance matching for the Virtual 2315 Cartridge Facility

IMPEDANCE FOR SIGNALS IS COMPLICATED

IBM engineers the 1130 for 92 ohm impedance on cables, although in practice there are ribbon cables, traces on printed circuits, wire wrap on the backplane, and twisted pair wiring involved in the path between the 1130 and its internal disk drive. However, since the engineering goal was matching 92 ohm impedance, I will use that as a working start.

The 2310 Interface Board is where the twisted wire cable from the 1130 and a new twisted wire cable to the disk drive are connected, allowing the Virtual 2315 Cartridge Facility (V2315CF) to sit in between the signals of the 1130 disk controller and the internal disk drive (called 13SD or 2310). It has two IDC 40 pin ribbon cables that run from the 2310 Interface Board to the V2315CF main unit. That unit is a significantly modified version of the RK-05 Emulator from George Wiley, since the RK-05 is a derivative cousin of the 2310. 

CURRENT TERMINATOR VALUES ARE SUBOPTIMUM

The RK-05 Emulator has a terminator board for the signals arriving there over the ribbon cables which uses 178 and 383 ohm resistors pulling up to 5V and down to ground respectively. That provides an impedance of about 122 ohms, which is a mismatch to both the IDC ribbon cable and to the 92 ohm IBM cabling on the 2310 Interface Board. We will use an impedance of 102 ohms for the IDC cable. 

On the 2310 Interface Board, the mismatch between 92 ohms and 102 ohms is not too bad, particularly with the relatively slow rising edges from the 1130 and 2310 electronics over the size of the interface board. However, the mismatch between the 102 ohm ribbon cable and the 122 ohm terminator board is going to lead to some ringing, reflections and power loss. 

CHANGING THE TERMINATOR BOARD TO IMPROVE SIGNAL INTEGRITY

If I switch the terminator resistors to 169 ohms and 249 ohms, still pulled up to 5 and down to ground, it will give me a 100 ohm impedance. I ordered the resistors and will remake the terminator board when the parts arrive. 

Adjusting my Virtual 2315 Cartridge Facility design to increase signal integrity between 1130 and disk drive

LOGICALLY THE V2315CF SITS BETWEEN THE DISK AND THE 1130 COMPUTER

The cable that ran from the disk controller logic cards in gate A compartment C1 of the 1130 originally plugged into the internal 13SD disk drive, but we inserted the 2310 Interface Card between the two. The original cable plugs into that card and a new cable from the card plugs into the disk drive. Two ribbon cables connect the 2310 Interface Card with the Virtual 2315 Cartridge Facility (V2315CF) main box where the signals are processed.

The V2315CF operates in one of two modes - real or virtual. A switch on the 2310 Interface Card selects the mode of operation. 

In real mode, the 13SD disk drive is operating and performing all of its functions except for the data streams to and from the read/write heads. The V2315CF uses a virtual cartridge file in a mini cartridge to generate the bit stream that the drive should have produced, if that virtual cartridge were a real 2315 disk cartridge inserted in the drive. It also captures data that the 1130 intends to write on the disk drive, instead updating the virtual image file in the V2315CF. 

The disk drive spins a 2315 cartridge, although what is on the cartridge doesn't matter since the read/write heads are never lowered onto the disk surface. The cartridge generates sector and index pulses that identify which sector the disk head is flying over at any instant. The disk drive indicates when the cartridge is spinning at the proper speed and it has completed the load sequence, so that a File Ready light illuminates and the 1130 is allowed to command the disk drive. 

The 1130 commands the disk drive to move the arm with the read/write heads inward toward the hub or outwards to one of 203 circular paths on the surface of the disk, called tracks or cylinders. The control signals and feedback signals flow between the 1130 and the disk drive by entering the V2315CF and being passed out to the other side. 

The V2315CF must snoop on the movement of the arm, in order to understand which track the 1130 believes it will be reading from or writing to. The data being read or written is to that track's position with the virtual cartridge file. 

In virtual mode, the V2315CF produces the sector and index pulses and indicates when to turn on the File Ready lamp and allow access. The disk drive is not powered up, does not spin and the arm does not move in and out. Everything that the disk drive did is now generated in the V2315CF and all interactions are between the 1130 and the V2315CF. 

REAL MODE SIGNAL INTEGRITY AND DELAY ISSUES ARE POSSIBLE

Signals from the disk drive must flow into the V2315CF so that the FPGA inside can direct a copy out to the 1130, and conversely signals from the 1130 must first be detected in the FPGA so that the FPGA can generate the same signal out to the disk drive. This adds delays of 80 nanoseconds or more to the passage of the signal. It may also result in the signal developing glitches that cause incorrect behavior. We are troubleshooting issues with seek - moving the arm to a different track - which might be caused in part by these signal integrity issues.

REDESIGN TO CONNECT THE 1130 AND DISK WIRES TOGETHER

I came up with a redesign of the 2310 Interface Card that will connect together the corresponding wires when we are in real mode, so that the signal produced by the 1130 is directly received by the disk drive, and vice versa for signals in the other direction. We can still snoop on those signals, in order to keep track of the arm position on the real drive, but we don't detect them and the generate a copy. 

This is possible because the V2315CF main box, derived from George Wiley's RK-05 Emulator, uses specific interface chips for incoming and outgoing signals. Most importantly, it uses open collector output drivers for signals generated by the FPGA. Thus, if we set that driver signal so that the transistor is not conducting, then it is electrically isolated. 

I added electronic switch chips - TMUX1112 - that connect the input and output sides just like a physical on-off switch. The switch that determines whether we are in real or virtual mode is used to activate those electronic switch chips when the switch is set to real mode. Thus, the -Access Go signal from the 1130 disk controller is connected to the -Access Go wire to the disk drive, allowing the signal to flow without any reshaping or delays.

In real mode, we control the output drivers for those signals so the transistors are off. We connect the two sides using the electronic switches. The input buffers that allow the FPGA to read the signals coming from the 1130 or the disk drive are still operational, thus the FPGA can see what is happening on the 'wire' we connected between the two sides. This supports the shadowing of arm movements and other needs of the logic inside the V2315CF. 

In virtual mode, the switches are turned off and the signals going to the 1130 are produced by the open collector output drivers in the V2315CF. Signals from the 1130 are detected by the buffer chips. Nothing is generated on the output drivers going to the disk drive, since it is turned off. We don't pay any attention to signals that would come from the disk drive since it is not operating. 

NEW VERSION OF 2310 INTERFACE CARD PCB DESIGNED AND PARTS ORDERED

I added a couple of the TMUX1112 electronic switch chips into the schematic, wired them up to bridge pairs of signals that should flow directly between 1130 and disk drive while in real mode, and controlled those switches by the real/virtual mode switch on the board. I placed the parts, routed the connections and prepared the files to send to a PCB fab.

I ordered the two electronic switch chips and changed the logic in the V2315CF to keep the transistors of the output drivers turned off in real mode. Once the new PCB arrives and is assembled, I will place the new version of the logic into the FPGA and test the V2315CF. 

I will finish debugging the seek behavior of the V2315CF and if it turns out that signal integrity or delay plays any role in the issues, I will send the board out for manufacture and buy the electronic switch chips. If everything works solidly after any defect resolution of my logic or disk/computer hardware, I won't spend the money nor invest the time to implement this change. 

Testing my new seek logic for the Virtual 2315 Cartridge Facility - part 3

MORE OBSERVATIONS OF SIGNAL FOR A SEEK WITH AN ODD COUNT

I used my hand programmed code to make several seeks of different lengths in order to watch what the disk drive itself as well as the V2315CF logic saw. For example, when I requested a seek of 161 tracks, it should have stopped the disk arm at 161 and reported 161 on the V2315CF log. What I saw instead was the arm stopped at 141 and the log was also not reporting 161.

I then set up a longer seek to 193 from the home cylinder, with the drive appearing to be at 167. Requesting 81 from the home cylinder left the arm at 77 but the V2315CF believed it was at 76. A seek from home to 41 put the arm at 37 and the V2315CF reporting 36. A seek of 21 put the arm at 17 physically and the V2315CF believed it was 16. 

ANALYSIS OF THE OBSERVATIONS

There are three entities that could be defective - the 1130 disk controller logic, the V2315CF logic and the disk drive. I am still not certain which is wrong, but the discrepancy between the drive position and the V2315CF position tells me that I have a problem in my logic in addition to whatever is happening elsewhere.

I could have problems with the interface signal integrity since the V23215CF sits between the disk controller logic and the disk. While the design intends to pass the signals through between the two, if there are signal issues that might throw off the way the two sides interact.

The interface between the two sides is very simple. The controller logic sends three signals to the drive and the drive responds with two signals. Only those five signal wires are involved in seek operations. The outbound signals are:

• -10/+20 Mil Step is a signal that requests moving 1 or two tracks
• -Rev/+Fwd is the direction of the movement relative to track zero
• -Access Go is the request for a seek movement

The return signals from the disk drive are:

• +Access Ready is the feedback, goes low 5 ms after a seek request and returns high by 15ms
• -Home reports low when the arm is at track 0 (home)

The details matter in terms of timing and behavior both inside the disk drive electronics and in the 1130 disk controller logic. 

Timing inside the disk drive

When -Access Go drops to low, the falling edge sets a go latch inside the disk drive which begins some timed operations. The state of the -10 Mil and -Rev signals determine the movement direction and size and are held steady during this part of the seek. 

A timer of 2.6 milliseconds expires before it begins actual movement. The detents are removed from the rack teeth during this time, which would allow the arm to move freely. If the -Access Go edge is at T-0, this runs to T-2.6 ms.

Depending on whether it is a 10 Mil or a 20 Mil step, a timer then runs for 2.4 ms or 3.8 ms. It runs twice, first accelerating the arm and then decelerating it to a stop. That is, the acceleration is from T-2.6 to T-5 ms or T-6.4 ms depending on step size.

At the end of the first 2.4 or 3.8 ms interval (acceleration) the signal +Access Ready drops to low. This resets the go latch. The signal drops at T-5 ms or T-6.4 ms. It also flips the active detent at this time, toggling between even and odd if the step size was 10 mil or not changing if the step was 20 mil. 

During the second 2.4 or 3.8 ms interval the detent begins to engage and the arm stabilizes its position, coming to a stop by the time the tooth engages the rack to hold the position. Thus by T-7.4 ms or T-10.2 ms, the arm is stopped. 

At the time that +Access Ready dropped, at the end of the acceleration, the drive also begins an approximately 10 us timer at the end of which it restores +Access Ready to high. The entire seek timeline runs from T-0 to T-15 or T-16.4 depending on the step size. 

Timing in the disk controller

The disk controller begins a seek during an XIO instruction, setting the counter of how many tracks to move and the direction of movement, asserting -Access Busy for the duration of the entire seek. It then repetitively issues seeks of 10 or 20 mils until it has counted out the total movement. At the same time as -Access Busy goes low, the access drive latch turns on. 

At the completion of the full number of tracks from the XIO instruction, it pauses for an additional stabilizing interval of 22.5 ms then turns off -Access Busy and turns on Operation Complete to request an interrupt on level 2. 

Once the -Access Busy in the controller goes low and turns on the access drive latch, the logic asserts -Access Go to the disk drive. The direction is held steady for the entire duration that -Access Busy is low.  However, the step size -10 Mil signal is determined by the value in the count register in the controller and thus may change between the first step and the remainder.

The low bit of the count, which is inverted in ones-complement from the seek quantity of the XIO instruction, determines the step size of the first step. If the count is odd, then the inverted low bit will be a 0 which makes the -10 Mil Step signal request a 10 mil step. After the first step, the low bit is forced to a 1 thus every other step is going to be 20 mils. Each time the access drive latch is turned on, it adds to the count register in the controller, adding two except for the first step of an odd count which effectively adds one. 

When the signal +Access Ready from the disk drive goes low, it turns off the access drive latch and returns -Access Go to high. We wait until the disk drive completes the seek step, which it signals by returning +Access Ready to high. 

The rising edge of +Access Ready will turn on the access drive latch again unless the value in the count register has reached all 1s, meaning we completed the full count requested by the XIO. Turning on the access drive latch means we assert -Access Go again for the next step. Thus, this logic iterates steps of the drive.

Glitches may confuse the interlocked logic of the drive and controller

If the +Access Ready bounces back high right after it goes low when a seek step is active, the controller might turn on the access drive latch incorrectly, also incrementing the count register incorrectly. The drive won't move because it ignores a falling edge on -Access Go if +Access Ready is not high. Since +Access Ready is generated internally to the disk drive it has it low during the seek when the signal bounced as seen by the controller. Thus the drive ignores one of the movement requests because it is sent at the wrong time, while the drive is actually still busy moving. 

The rate of glitches on that line, if that is what is going wrong, has to be relatively low since most of the movement is completed successfully as far as the V2315CF and the disk drive is concerned. Short total counts are close, the discrepancies grown with long counts. 

HARDENING V2315CF TO BE ABSOLUTELY CONSISTENT WITH THE DISK DRIVE

I am concerned that the V2315CF is off by one when the controller is moving an odd number of tracks. I suspected that it is the timing of when I capture the step size, since the -10 Mil Step signal comes from the count register and that is incremented just as the -Access Go signal is asserted low. Since I wait to grab it until +Access Ready drops, it has already changed and will always be seen as a 20 mil step size. That means I am also missing one step entirely since the count is low not high by one. 

I had previously added a kind of Schmitt trigger as a debouncer for the signal that drives my seek shadowing. My logic tried to minimize the risk of bounces/glitches on the +Access Ready signal by requiring 10 cycles of a steady signal level before I acted on it. However, that gives the counter time to be incremented in the disk controller and for the -10 Mil Step signal to rise so that I no longer captured the signal state when the seek step started. I removed that from the logic so that I grab the step size immediately on the falling edge of -Access Go. 

TIMING RACE CONDITION POSSIBILITY TO CONSIDER

Because the V2315CF must operate in two modes - real and virtual - I was passing the seek signals through the V2315CF. This means that I had to eliminate metastability risks on the input to the FPGA for each of those signals. That adds four FPGA clock cycles before a signal is passed along, 40 nanoseconds of delay. 

The interlocked behaviors from the two sides would typically occur much closer together, but a sequence where one side sees a signal change (e.g. +Access Ready drops) and then changes an outgoing signal (e.g -Access Go rises) will take 80 nanoseconds longer or more than the direct wiring without V2315CF. 

At this time, I can't imagine how that would cause a problem. Therefore I am left with two broad categories where the cause may lie. First is some hardware failure in the 1130 controller logic, the other is a signal integrity issue over the wiring between the 1130, V2315CF and disk drive. 

NEXT DIAGNOSTIC STEPS

I want to identify which of the categories is our problem. I will start by wiring up the logic analyzer to the signal pins on the IBM 1130 gate A compartment C1 where the controller logic lives. I am practically limited to capturing 16 signals at a time.

I  will try to record a long seek and capture all the -Access Go and +Access Ready signal interchanges. I should see if there are the correct number of seek steps commanded and perhaps spot anomalies that might be triggering errors. Another set of signals I will record are the nine bits of the counter, so that I can watch it count off the seek. 

If I see the count change at the wrong time, or see it miscount, it can steer me closer to the root cause.