Working hard on making seek shadowing state machine impervious to glitches for Virtual 2315 Cartridge Facility

LOGIC ANALYZER SHOWS SHORT GLITCHES THAT CAN AFFECT STATE MACHINE

When looking at the logic analyzer traces I collected while doing seeks with the Virtual 2315 Cartridge Facility (V2315CF), where the internal 2310 disk drive of the IBM 1130 was used in concert with the virtual disk cartridge contents, I saw brief glitches. These were 10 nanoseconds or less in duration, thus completely ignored by the Solid Logic Technology (SLT) of the IBM 1130 and the disk drive. However, they are enough to cause problems in the FPGA logic due to the high speed electronics.

The 1130 requests a movement of the disk arm to move one or two cylinders at a time either towards the home cylinder 0 or out towards cylinder 203 which is the highest location where data is stored on a 2315 disk cartridge. A seek command issued by a program specifies a relative number of cylinders to move; the disk controller converts this into a set of 1 or 2 cylinder steps until the desired total movement is achieved. 

The -Access Go signal drops low to request a step, with the drive responding with a feedback signal +Access Ready that goes low 5 milliseconds after the seek request and remains low for another 10 milliseconds before returning to high to indicate that the movement is complete. The originating -Access Go line is returned to high when the 1130 sees the +Access Ready line drop low 5 ms into the seek. 

I captured a seek command where the program requested a movement of 40 cylinders, which the 1130 drive controller electronics converts into 20 movements of two cylinders each. The seek shadowing function in the FPGA only counted 38 cylinders while the 2310 disk itself moved the full 40 cylinders. There are two glitches on the -Access Go line that were the cause of the state machine in the FPGA going awry. 

top is -Access Go, next is +Access Ready

When I zoom in on the glitches, they are all shown as 10 nanoseconds long, which is the sampling interval so the actual glitch is likely even shorter. None of the cabling in the 1130 is designed to eliminate very short glitches like this because the logic family, called 30 ns SLT, is impervious to short duration perturbations. If I were to try to eliminate all of that, it would involve considerable work to change parts of the 1130 such as altering the type of cable used in the machine. Instead, making the FPGA ignore these is the better direction to take.

WORKING ON FPGA LOGIC THAT IS IMPERVIOUS TO SHORT DURATION GLITCHES

I worked on the state machine that shadows the seek movement by watching the signals between the 1130 and the 2310 disk drive, working to ensure that it produces the correct results with brief glitches in the signals. In order to test that, I had to develop a good testbench for simulation that would introduce glitches of every type so that I could watch the behavior of my logic. Building the testbench was more work than the state machine itself, but it did help me achieve the performance I need from the seek shadowing. 

I introduced glitches of several types and in different places during a sequence of simulated movements. Glitches of 100 nanoseconds, far longer than I was observing with the V2315CF, were introduced. First was a brief drop of -Access Go when the 1130 was not actually trying to move the arm. Next was a brief return of -Access Go to high before the drive has responded with a low level on +Access Ready. Third was a brief drop of -Access Go during the final 10 ms of a step, when +Access Ready is low. I also sprinkled a short glitch in between successsful step movements. 

This had to work in both modes of the V2315CF - real and virtual. Virtual mode is when the 2310 disk drive is not switched on. The FPGA simulates the sector and index marker pulses that indicate the rotation of the drive and it simulates the feedback signals such as +Access Ready. In real mode, the drive itself generates the marker pulses and responds to the step requests with feedback signals. 

OUT OF ROOM ON FPGA CHIP WAS NEXT SNAG

With the functionality validated using simulation, I moved to the Lattice IceCube2 tool to generate the bitstream to load onto the V2315CF. Unfortunately, the changes I made expanded the required number of elements like Look Up Tables (LUTs) and D Flip Flops (DFFs) beyond the capacity of the chip installed in the V2315CF. 

I have to find ways of trimming this down without impacting the functionality I just tightened up. I suspect that the big issue was the need to add a second 19 bit register to hold the value of a timer so that I could continue a countdown across a glitch - that register and all the comparisons to certain values that will be contained in it require a lot of DFFs and combinatorial LUT logic. 

I looked over the logic to see if I could find clever optimizations that might reduce the count of DFFs and LUTs while maintaining the working logic. I had to go back to simulation to be certain that these changes had no adverse impact. I also had to hope that I could trim it enough to fit. I have had issues trying to fit in the past, requiring me to strip out what I could from the FPGA code. 

SUCCESSFULLY REMOVED NEED FOR SECOND REGISTER AND ALL THE COMPARISONS

The code was tightened further and works just as well. The result fits in the existing FPGA chip! The final counts of parts are:

• 1,209 Look Up Tables
• 622 D Flip Flops
• 181 Carrys
• 1 Global Buffer
• 3 Global Buffer I/O blocks
• 86 Input Output buffers

The chip is an iCE40 HX1K in a quad flat pack TQ144 package. The FPGA runs under a 40MHz clock. It shares the main board of the V2315CF with a Raspberry Pi PICO processor. The two communicate over an SPI link.  A 32Mbit SPI flash ram holds the FPGA bitstream and initializes it on powerup. The FPGA also controls a 256Mbit DRAM that is used to hold the contents of the virtual 2315 disk cartridge when it is being used by the 1130. A 2315 cartridge holds 512K 16 bit words,  far less than the size of the RAM chip. 

IMPLEMENTED THE CARTRIDGE ICON TO DISPLAY ON V2315CF MAIN PANEL

I converted the image of the 2315 cartridge into a pixel array that is written to the LED panel of the V2315CF main box when a virtual cartridge has been loaded into the RAM and is ready to use with the 1130 system. 

The refresh rate of the LED conflicted with the iPhone camera a bit, so the image doesn't look as good as it does in person. 

Switching over to electronic switch version of 2310 Interface Board for Virtual 2315 Cartridge Facility

ELECTRONIC SWITCH ADDED FOR IMPORTANT SIGNALS RELATED TO SEEK

The Virtual 2315 Cartridge Facility (V2315CF) makes use of a PCB that sits between the signals that normally run between the IBM 1130 disk controller logic and the internal disk drive (2310). It routes those through ribbon cables to the main box of the V2315CF, plus it supports functions such as managing the Unlock lamp on the 1130 console and setting real or virtual mode. This is the 2310 Interface Board.

In past incarnations, the 2310 Interface Board would route a signal from the 1130 to the V2315CF main box, the FPGA would pass the signal through a four stage chain of flipflops to avoid metastability issues, then simply drive the same value out on the signal that would run to the internal disk drive. This introduced some delays in signals and added signal quality issues due to the very fast edges driven by the FPGA. 

This caused problems for commands to seek the disk arm to one of the 203 cylinders on the disk platter inside the 2315 disk cartridge. The signals between 1130 and 2310 involved feedback and responses that could fail due to the delay introduced by signals passing through the FPGA. I decided to directly connect the 1130 and 2310 for those signals, eliminating the FPGA's role and cutting out the delays. 

This posed two small challenges. First, when the V2315CF is switched to virtual mode, the 2310 is not powered on and does not generate the feedback signals that go to the 1130. Instead the FPGA produces thos in a pure emulation of the 2310. Secondly, the V2315CF creates the data stream that would have flowed from 2310 to the 1130, but instead comes from the virtual 2315 image inside the main box; it captures writes from the 1130 to update the virtual image as well. To send the data associated with the current cylinder location that the program has moved the disk arm, the main unit must know which cylinder is current. 

To keep track of the cylinder that the disk arm is current flying over (or emulate it in virtual mode), the main unit must see the signals between the 1130 and the 2310 that constitute a seek - moving the arm. Thus, even when I have directly connected those signals between the 1130 and the 2310, they must also be routed into the FPGA so that it can shadow the movement of the 2310 disk arm (or emulate it in virtual mode). 

To accomplish this, I installed electronic switch chips on the 2310 Interface Board which connect the pin for certain signals between the 1130 and the 2310 cable connections, but only when the V2315CF is in real mode. In virtual mode, the switch is turned off so that the signals to the 1130 originate from within the FPGA and not the 2310. These TMUX1112 chips support four signals each. I applied these to the signals related to disk arm movement (seek commands). I also direct connected the sector and index marker pulses that reflect the current position of the disk platter rotating under the heads. 

TERMINATOR CHANGED TO OPTIMIZE SIGNAL QUALITY

The IBM 1130 technology - Solid Logic Technology (SLT) - is a diode-transistor logic type of circuitry with voltages that (mostly) swing between 0 and 3V for logic 0 and 1. The cables in the 1130 are designed around a nominal impedance of 92 ohms. The main box of the V2315CF does the voltage shifting to accomodate the logic levels.

Generally in SLT, a pullup resistor keeps a gate input high unless it is actively pulled down to ground to drive a logic 0. Without a current flow down to ground, the gate input is considered logic high regardless of the voltage or absence of voltage on that pin. The pull-up resistors are in the 1130 or 2310 circuitry depending on the direction of the signal. Thus for the signals that are switched directly together, nothing is needed in the main V2315CF box. 

However, when in virtual mode, there is no 2310 providing the pullup and the source of the signal is the V2315CF. The FPGA drives 0 or 3V on those lines, rather than using a pullup resistor. Thus, for most signals, since the ribbon cables and the V2315CF main box are relatively close to the 1130 nominal 92 ohm impedance, I decided that terminator resistor pairs are not needed for most signals. However, for a few signals I still needed terminator pairs, especially for the ones that will be generated by the FPGA - when in virtual mode or for disk data streams that always flow between FPGA and 1130. 

I made these decisions based on signal quality observed while I tested the V2315CF. I can easily tweak these over time as necessary. I made the changes to both terminator boards since I am building two V2315CF systems, one for the Vintage Computer Federation's InfoAge museum in New Jersey and the System Source Museum in Maryland. 

FPGA LOGIC CHANGES NEEDED TO AVOID DOUBLE DRIVING OF CIRCUITS

Since the FPGA must drive a logic 1 or 0 for signals when in virtual mode, but we want to use the electronic switch to short 2310 to 1130 when in real mode, I needed to change the FPGA so that it emitted a logic 1 on those signals when in real mode. The interface chips in the main unit of the V2315CF are essentially an open collector gate, thus only active when driving a logic zero. 

By setting them to 1, they are not conducting and thus not acting on the line other than through the pullup resistor in the main box. The pullup resistor function is provided by the terminator board resistors - with 179 ohms connected to +5V and 249 ohms connected to ground. This should be overpowered by the open collector interface chip pulling the line to ground, producing a voltage low enough to register as a logic 0 by the SLT circuits to which it connects. 

FIRST TEST WITH NEW BOARD BUT FORGOT TO UPDATE THE FPGA

I fired up the system with the new 2310 Interface Board and the newly modified terminator board installed. The V2315CF reacted to the seek commands but the disk drive did not move. When I looked at the signal levels seek at the 2310 (and at the 1130 for the blue signal below), the logic swing was insufficient to get down below about 1V. That is too high to register as a logic 0 for the SLT gate input, thus the disk controller commanded the first step of the seek (yellow signal dropped to 0) but the feedback signal (green and blue) did not drop far enough to register. 

I realized that I had not updated the FPGA with the version of the logic that would emit a logic 1 to the interface chips for those signals that are switched together on the 2310 Interface Board when in real mode. In addition, I didn't have a good signal ground on the scope thus the noise showing up on the traces. 

I updated the FGPA logic and will install it onto the V2315CF main box when I next get to the shop. If the signal still doesn't get down to 0 adequately, I will remove the resistor pairs for those signals. That will mean that in virtual mode I won't see a logic high on the scope due to lack of a pullup resistor, but the SLT gate should still respond properly. I will add a high resistance pullup to 3V somewhere to make diagnosis easier in the future. 

PRODUCED 2315 CARTRIDGE IMAGE FOR LED PANEL OF MAIN V2315CF BOX

The main box of the V2315CF has an LED panel that displays the four hex character ID of the virtual 2315 disk cartridge along the bottom of the screen and had been showing a larger drive number on the remainder of the area. The RK-05 disk drive that the box was originally designed for supports multiple drives on a cable string, thus it displayed the address of which disk the box was emulating. This has no meaning for the 2310 which does not share an interface cable with other drives. 

I took an image of the top view of a 2315 disk cartridge and converted it to the 80 x 40 pixel footprint of the original drive numbers being displayed on the screen. I modifed the code in the Raspberry Pi PICO that is in the main V2315CF box so that it displays this image when a virtual 2315 disk image is loaded into the system. 

Testing my new seek logic for the Virtual 2315 Cartridge Facility - part 14

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. 

Work on the internal disk drive of IBM 1130 - part 3

CHECKING 257 TRANSISTOR FROM DONOR SLT CARD

I put the transistor that I pulled off a spare card and tested it with my curve tracer. I compared it to the two transistors on the M2/M3 card which made the weak/failing transistor obvious. Apparently Germanium power transistors can fail gradually with the gain increasing until there is a complete failure. The base junction is thinned out by tin whiskers or holes punched by voltage spikes, causing the current to increase through the base which drives up the beta. 

SWAPPING THE TRANSISTOR AND TESTING THE DRIVE

I removed the failing transistor and installed the donor transistor. The M2/M3 card is now working properly, with no current through the voice coil during idle times. I used the opportunity to adjust the dead band level up until the voice coil began oscillating and then backed if off which is the the procedure for the adjustment. 

I started up the drive and ran a program to seek back and forth 200 cylinders at a time. However, while the drive grunted with the detent cycling 100 times, it didn't move forward from cylinder 0. The second seek in the program, moving backwards 200 cylinders, ends immediately because the drive knows it is already at cylinder 0, the home cylinder. 

DRIVE MOVES FORWARD AND BACK WITH CE SWITCHES BUT NOT BY PROGRAMMING

I had previously moved the arm back and forth using the CE switches - In/Out, 10/20 Step and Single/Continuous movement. When I hooked up the cable from the Virtual 2315 Cartridge Facility (V2315CF) it should disable the CE switches and obey only the signals from the IBM 1130. The arm was not moving forward, yet the signals from the 1130 were correct for forward movement. 

The logic that responds to the -Access Go signal as well as the -Reverse and the -10 Mil control signals does merge the CE switch signals with the signals from the 1130 before activating the disk drive movement. I probed around and realized that the Out/In switch was set to the Out position, which means reversing towards cylinder 0. I noticed that this overrides the signals from the 1130 when the +CE Interlock signal is low. 

MY ERROR - MY HANDLING OF CE INTERLOCK IS BACKWARDS

The intent of the +CE Interlock signal is that when the cable from the 1130 is plugged into the disk drive, the CE switches should not have any effect. Pulling the cable out enables the switches. As I looked closer at the logic diagrams, I realized that I had things backwards. The 1130 delivers +3V on the +CE Interlock wire and the disk drive has a pull down resistor to -3V on the same pin. Thus, when the cable is pulled, the line is logically low and when the cable is attached it is logic high.

I designed for the inverse. I set up the cable to pull the +CE Interlock signal to ground (logic low) thus the signal is always low as far as the disk drive is concerned. Since the Out/In switch was set to Out, it forced the drive into reverse. 

Somehow, every time I tested the V2315CF before this, the Out/In switch was set to In and had no impact. It only overrides the direction when it is set to Out due to the way that Solid Logic Technology gates work. 

REWORKING THE 2310 INTERFACE BOARD TO FIX CE INTERLOCK

The cable from the V2315CF to the disk drive has the wire for +CE Interlock on pin D07 that is hooked to the ground of the 2310 Interface Board. The wire coming from the IBM 1130 to the 2310 Interface Board does does nothing with the incoming signal on pin D07. 

My fix was simple. I removed the disk bound cable wire from D07 and added a bodge wire to tie it to D07 of the cable bound for the IBM 1130. That ensures that the 1130 is driving +3V through the cables which will block the CE switches from working when the cable is attached to the disk drive. Pulling the cable lets the internal pulldown resistor to -3V drive the signal to logic low. 

Grounding signal at board

incoming +CE Interlock

PROGRAM SEEKING BACK AND FORTH 200 CYLINDERS CORRECTLY

With the fix, the program worked properly. The arm moved smoothly back and forth. I will continue with the fine tuning adjustments but everything is very promising. 

FINALIZING THE ENCLOSURE FOR THE V2315CF 

I designed an enclosure for the V2315CF that is bolted to the top plate of the internal disk drive, so that it is hidden under the top cover of the IBM 1130 but accessible by opening that lid. The main unit and most of the power supply and power fail ridethrough equipment is installed inside this enclosure. There is a smart battery trickle charger, a 12V motorcycle battery, and the 2310 Interface Board that are mounted externally, the rest is inside this enclosure.

A friend is building a much more realistic looking mini cartridge mechanism that will also be in this enclosure. This mechanism looks like a miniature version of the internal disk drive as viewed from the front of the IBM 1130. The blue handle will operate to allow a mini cartridge to be inserted, then closing the handle inserts the mini cartridge into the V2315CF circuit. 

design visualization

The mini cartridge is now 2.25" in diameter, a slight increase over my original version but one that permits the cartridge to look much more realistic. I will provide a lot more detail and pictures of this as he completes the work. 

I finalized the laser cutter files to trim the acrylic sheets to make the enclosure. It will be 4 1/4" tall, 13 1/16" wide and 11 5/16" deep, with the main V2315CF unit and the disk drive mounted on the front face. Once I have the acrylic cut, I will glue it up and begin mounting the equipment in it pending only the mini disk drive completion. 

Work on the internal disk drive of IBM 1130 - part 2

INVESTIGATING THE REASON THE VOICE COIL IS ENERGIZED AT ALL TIMES

The internal disk drive of the 1130 has a disk arm that is moved by a linear voice coil, much like an audio speaker. It should only be active when the drive is attempting to move .01 or .02" at a time. The circuit will energize it in the movement direction, then switch the current direction to decelerate it after a tachometer shows it having reached a target velocity. 

This should be turned on with the falling edge of -Access Go or when the drive is first powered up. At power-up, it energizes the reverse direction movement causing the arm to move back towards the home cylinder (0) and that is turned off when the Home microswitch is activated by the arm reaching cylinder 0. Activation during regular operation is by -Access Go which is returned to high when the disk controller sees that the feedback from the disk drive, signal +Access Ready, drops to low. 

Inside the drive, the drop of -Access Go turns on the Go Latch which remains active until the tachometer driven trigger determines that the target velocity was reached. That turns off the Go Latch and allows the tachometer to slow the arm down to zero speed which will complete its movement of .01 or .02 inches (10 or 20 mil). 

I studied the circuit diagrams until I understood how they work and what levels I should see at all the observation points offered by the disk drive circuitry. I annotated the Solid Logic Technology (SLT) card pins that correspond to each observation point, which I used to investigate the reason that the drive was pulling hard in the reverse direction at all times. 

Example - diagram of reverse acceleration

Capacitor C1 in the diagram above is initially charged to +6V and should remain there while the arm is not moving. Once we are in the acceleration phase of a movement, the current from the tachometer amplifier lowers the voltage on the capacitor. When that voltage decreases to a trigger level that is set by circuits below, it turns off the Go Latch and the drive is in the deceleration phase.

During deceleration, the tachometer output drives the power amplifier to energize the voice coil in the opposite direction of which it was powered during acceleration, thus slowing the arm. When the tachometer voltage gets low enough, the voice coil is shut off and the arm comes to rest. The trigger levels were set to move the arm 10 or 20 mils between the acceleration and then slowdown actions. 

These circuits are almost all on card F2/F3, pictured below. The final four transistors are on card M2/M3. Most adjustments for the disk drive are made with potentiometers on F2/F3 and any circuit failure is likely to be on that card.

card in F2/F3

OBSERVATIONS

The tachometer amplifier was driving the voice coil to the rear because it hadn't been properly balanced to zero out when idle. I suspect that the dead band adjustment was also not correct. That sets the minimum differential from the tachometer amplifier before the voice coil is driven in either direction. 

CAUSE AND RESOLUTION

The potentiometers on the card are fine pitched - they have many turns to adjust from one end of the setting to the other - but they no longer have stops to indicate when you have reached an end. I had to take the card out and adjust the pots to get them at their halfway point in resistance. Since one of them had fixed resistors across it as well, I unsoldered it first then adjusted it. 

Once I had the circuit behaving well, producing the same low voltage on both the forward and reverse outputs of the card, the arm shouldn't be trying to move either way. These outputs are routed to card M2/M3 which have the four bridge transistors that switch current to the voice coil based on the outputs coming from card F2/F3. 

However, when I tested, card M2/M3 was driving the voice coil strongly to the reverse direction. With the inputs at the same level, it shouldn't be. That was the next candidate for debugging. Meanwhile, I don't have a good setting for the dead band yet because I need to arm to not try to move while I adjust it so that I can see if/when it begins to oscillate and back off the setting. That will come later. 

INVESTIGATING THE DRIVER CARD M2/M3

I pulled the card, which only has five large germanium transistors on it. One of them is used to select which read/write head is active, so I can ignore that. Each side of the circuit (forward or reverse) has an NPN transistor and a PNP transistor that switch +48V and ground to the voice coil, but the pairs are wired to inverse to each other so that a pair either drives the coil one way, 48 to ground, or the other way. 

I hooked the transistors up to my curve tracer to see if one of them was bad. The two PNP transistors performed identically and had good looking traces. However, one of the NPN was quite different from the other. I suspect that one, although the curve shape wasn't definitive. This is an IBM type 257 transistor

FOUND DONOR CARD IN MY STOCKPILE WITH THE TRANSISTOR ON IT

I did find an SLT card in my pile of spare and ebay auction cards that has the 257 transistor on it. I will pull it off, test to make sure it matches the presumed good one on the M2/M3 board, then replace the bad NPN transistor to repair the card. When that is done, during my next shop visit, I should have a drive whose arm is not activated when not attempting a seek. I can make the dead band adjustment and then resume testing of the drive.

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.