Verified configuration of Diablo drive for use with the archiver

CHECKED OUT THE CIRCUIT BOARDS IN THE DRIVE

The drive has six logic boards inside (plus a power PCB and a backplane). By examining them I could look to see what options are installed on the drive - and whether it will work properly to read 2315 disk cartridges that had been written on an IBM 1130. None of the board numbers match the manuals available online; these appear to be older versions of the Diablo logic. 

I found one board (J1)with some wires added, which might have modified the behavior. I would need to back it out if so, bringing the drive back in conformance with the specifications. You can see a few violet wires, one end tacked to the connector on the left and a transistor that appears to have been added to the board by drilling holes. 

When I compared to the more modern versions of the board, I found the same transistor wired into the circuit in exactly the same way. I am assuming that this is a rework to add that transistor into the circuit from the previous design that was built into the board. As such I am leaving it in place. 

I looked over boards J8 and J10 to verify the options that might be installed. I did find that the drive was set up for daisy chaining. I see from the schematics that some drives were set up so that they could not, but this option also includes the support for the four signal lines Select 1 to Select 4 that determine which drive on a daisy chain will work. I also saw the jumper plug that configured this drive to respond to Select 1. 

I also saw that the drive does generate the Logical Address Interlock signal, asserted if a seek is requested to a track address that is not between 0 and 203. That might be option 064 which is for Varian computers, implementing four attention lines and the Logical Address Interlock signal. I didn't look at the attention lines since I don't need it nor care about that functionality.t around the circumference of its hub. There is one modification to shorten the sector marker pulse w

The drive does have the circuitry to report sector numbers for up to 32 sector versions of the drive. There is no way to change the machine to choose the number of sectors it supports. Instead, the Index Marker resets the counter thus it automatically supports the number of sectors that the 2315 cartridge has cuhen the drive is configured to support 32 sector disks, but it is not installed on this drive. 

I then checked board J10 for two important options. First, I verified that a 10 ohm resistor tied the Write Gate and Erase Gate signals together. If they did not, then option 008 would be installed to implement an independent Erase Gate. Second, I had to be sure that this was option 001 - 720KHz data rate and not option 002 - 781 KHz. That was determined by the value of a a specific resistor pair. If the main resistor was 9.8K then it is the wrong option, but I found the 10K value that matches option 001. 

DETERMINED CONFIGURATION IS GOOD FOR USE WITH THE ARCHIVER

It appears to have option 063 - off-white paint for Varian, based on the paint on the drive. It also has option 019 - extended mount - so that it can sit in a standard rack but have the drive extend forward about three or four inches. 

The disk heads are the metallic standard density type, not the ceramic heads you find with the high density drives. Exactly what should be installed. 

MODIFIED TERMINATOR TO MATCH THE SIGNALS I WILL USE

I started with the terminator that was closest to the version I need - it had pairs of resistors to terminate signals for all but two of the signals I will use. Using the VOM as a continuity checker, I located the open pads on the terminator where a 120 ohm resistor would pull up to +5V and a 330 ohm resistor would pull down to ground. I soldered new resistors in place at those locations and now have a fully functional terminator for the drive when connected to my archiver. 

CONNECTOR WITH RIBBON CABLE TO IDC-40 CONNECTOR

Here is the MRAC-42S connector that fits onto the rear of the Diablo drive. It is connected with a 40 wire flat ribbon cable that has a metal plate bonded along the length as a ground shield. The other end of the cable ends in a female IDC 40 pin connector (often used with internal hard disk cables in a PC). 

Building the cable for the 2315 cartridge archiver using the Diablo disk drive

SIGNAL CABLE NEEDED FROM MY PCB TO THE DISK DRIVE

The archiver I build connects to certain signal lines on the Diablo model 31 disk drive using a flat ribbon cable which has an IDC 50 pin connector on one end (commonly used for SCSI internal connections on PCs) and a Winchester MRAC-42S or MRAC-42P connector on the other end. 

The drive has both male and female 42 pin connectors on the rear. A terminator is connected to one side and the signal cable attaches to the other. Depending on the polarity of the connector you use,  you need the opposite gender terminator. There is also an MRAC-14S socket that brings power to the drive, connecting to the plug at the top. 

Every other wire in the ribbon cable is connected to ground, as a means of minimizing crosstalk between active signals. I started with a standard 50cm cable, two IDC 50 sockets on the ends, and removed one of the end connectors. I then split out the wires and soldered the appropriate signal wire to the associated pin of the MRAC-42 connector. 

AVAILABLE CABLES AND TERMINATORS

I have four terminators (three plug and one socket type) and several connectors and cables that I saved from the time when I built an archiver to use the Diablo model 31 high density drive to read and save all the 2315 cartridges from the Xerox Palo Alto Research Center (PARC) library before they disposed of them. These were all used on their Alto systems decades before. 

One challenge is that the Alto used the Diablo in non-standard ways. For one thing, they hot wired the Read Gate signal so that it was always on, emitting clock and data pulses back to the Alto. That unfortunately means that the cable from the Alto days does NOT have a wire to drive Read Gate. No wire in the cable and no socket for the signal on E of the connector. The designer sometimes cut off the terminator resistors based on the exact components they used in the Alto, ignoring impedance matching.

Another challenge that is endemic to the Diablo model 31 and 33F is all the options that exist with that drive. This means lots of variations in signals used and even in what signals had terminator resistors installed on the terminator boards. It requires me to carefully check the terminators I might use against the resistors that should be installed; I might need to modify one of them if none of the terminators match what I need. 

I do have two MRAC-42S connectors with the flat ribbon cable and an IDC 40 pin connector on the other side, but I needed to first determine which pin on the IDC-40 connector was hooked to each MRAC-42S socket, then determine how to shift the signals with an adapter of some type. 

QUICK ADAPTER BOARD DEVELOPED

After finding the signals on the IDC-40 connector of the existing connector I had on hand, I worked out the mapping between the IDC-40 connector pins and my IDC-50 connector pins. I used KiCAD to build a quick and dirty adapter PCB that housed two sockets - an IDC-40 and an IDC-50 - then routed the signals between them according to the map.

I shipped that off to be built and made certain that I had the IDC sockets on hand to solder this together when the parts arrived. I am waiting for quite a few shipments before I can put the archiver together completely - components from Digikey, the main circuit board from PCBWay.com, the adapter PCB, the sockets, the three power supplies, braided ground wire and parts to build the MRAC-14S power connector.

POWER CABLE CONSTRUCTION

The Diablo drive asks for four power rails to be delivered via the MRAC-14P connector on the rear. It uses +15VDC and -15VDC for everything on the drive. However, to minimize noise and the voltage drop on the wiring when heavy currents are drawn by the motor and the arm movement servo, the designers ask for a second set of +15 and -15 rails on their own wires to power the other electronics. 

I have bought two high current 15V supplies and a lower current dual supply that outputs both +15 and -15V, thus can feed the four power rails over wiring to an MRAC-14S socket. I used 16ga wire, as recommended, plus a braided ground wire. 

I found that the MRAC-14S sockets were quite rare and commanding a steep premium on eBay. The Winchester socket pins themselves go for up to $4 each and are almost always separated from the housings to extract maximum value for the sellers. I did find an MRAC-9S socket that had the socket pins included, where the price was cheaper than just buying the socket pins. Now I have to find a female MRAC 14 body or 3D print one if the prices remain too high on eBay.

NEXT STEPS WHILE I WAIT FOR ALL THE SHIPMENTS

I can install the best set of standard density heads into the Diablo drive and set it up for the alignment process. Aligning involves using a precious CE cartridge that has a special pattern recorded on cylinder 100 that allows me to move the arm to be centered on that track, by watching signals on an oscilloscope while turning setscrews on the arm; all this while the drive is spinning and the heads are on the almost irreplaceable CE cartridge. 

Therefore I need to find my least valuable 2315 cartridge, one that I can afford to lose if anything goes wrong when I first load the heads on the Diablo drive. I will clean that carefully and then verify that the heads fly without scraping, crashing or otherwise misbehaving. I will clean that cartridge's platter (as well as the CE cartridge platter) using 91% Isopropyl Alcohol and laboratory quality low lint wipes. The heads get the same treatment.

Of course, I need to cross check the Diablo drive against the options and configuration I need for the archiver. I have to plug it to respond to Select Unit 1. It must be standard density with the 720KHz clock rate - option 001 - or be convertible to that. I have to check other options to determine whether I have an independent erase gate (option 008) as that requires me to bridge some wiring in the cable assembly or modify the drive. 

Since I disregard the sector counter outputs, I don't care if this was configured for 12, 16, 24 or 32 sectors other than to validate the timing of the sector marker and index marker pulses. Other options like write protect, interrupts, attention lines, and logical address interlock are irrelevant and can be ignored. Other options cover the paint scheme and mounting brackets, also not important. 

This drive was originally installed in a third party manufacturer's cabinet that provided this as a second disk drive for an IBM 1130, along with providing an adapter to use a third party line printer as an IBM 1403 printer as far as the 1130 was concerned. As such, the drive should have option 001, not have option 008, and have option 026 for standard density 8 sector configuration.

The options list are not all listed on the physical drive. Many of them were just on the sales order, which I don't have, but I can look at the circuit boards and other aspects of the drive to determine which are included or omitted. 

I could even complete the alignment of the heads before I have all the shipments to build the archiver, as this process is done without any signal connector. All I would need is the power supplies and a temporary way to connect them to the drive.

Orders placed for interface PCB for the Diablo drive archiver for 1130 based 2315 disk cartridges

ARCHIVER PROJECT UNDERWAY

The Archiver makes use of certain models of the Diablo Model 31 disk drive to read the contents of 2315 disk cartridges that were used on an IBM 1130 system, then export the archived contents to files on a PC, Mac or other system. 

The main logic for the archiver is implemented in Verilog on a Digilent Arty S7 FPGA board but that requires a custom printed circuit board to manage the electrical requirements of the Diablo disk drive and the voltage maximums of the FPGA board. I designed a board to accomplish this.

PCBWAY.COM SPONSORING THE MANUFACTURE OF THE CIRCUIT BOARD

I uploaded the design to PCBWay.com who have offered to sponsor the production of the board. It is a four layer board, less than 4 by 5 inches, that installs on top of the Arty S7 FPGA board. It provides power to the FPGA board and its own circuitry, driven by a 9V wall wart DC supply. A 50 wire ribbon cable with an IDC connector - commonly used for SCSI internal drives - connects the PCB to the connectors on the rear of the Diablo drive. 

Above is the visualization from the PCBWay.com web site of the front of my board. The order was uploaded and I expect to have the completed boards in a couple of weeks. I chose non-standard solder mask and silkscreen colors which adds about half a week to the production versus standard colors.

COMPONENTS ORDERED FROM DIGIKEY

I also ordered all the parts for the PCB construction from Digikey. They will arrive before the board so that I can move directly into construction once I have the circuit board in my hands. 

GITHUB PROJECT STARTED

I opened a Github repository to share all the files related to this project - board designs, Verilog code, documentation and pictures - so that others can build this if they have IBM 2315 cartridges whose content they want to archive.  

Given some pricy advice to make progress debugging the 1130 MRAM memory infrequent bit failures

1130 MRAM BOARD HAS BITS DROP ONCE IN MANY THOUSAND READS

The failure is rare. When I loop through memory, there are almost 278,000 reads done every second. With just 8K of memory implemented, this means the system has looped through every location almost 34 times in a single second. 

The symptom is a drop of bit 14, when it should have been set, at varying addresses in memory. It only happens after thousands of successful reads of the same word. When I put my Rigol oscilloscope probe on the pin where the pulse arrives from my board into the 1130 memory register, the error goes away. Instead, bit 10 will now fail at a even less frequent rate. Putting a second channel's probe on that bit leads to almost flawless operation. 

THE QUANTUM MECHANICAL NATURE OF THIS FAILURE

Therefore, when I put on the scope probe or a logic analyzer probe, the failure does not happen. It only happens if I am not observing. That means I can't see how it is going wrong, because I can't get the failure to occur while I am measuring.

ATTEMPTED MANY TIMES TO CREATE AN EQUIVALENT LOAD TO THE PROBE

I can't run the machine with my Rigol scope permanently attached. Further, it may be that after 20 minutes or five hours, another bit will fail to be set. Since the presence of the probe seems to cure the situation that causes the failure, I hoped I could develop an equivalent set of components that I could attach to the backplane pin in lieu of my scope probe. 

However, this is a very messy and complicated situation. Many inductances, capacitances and resistances are involved in the scope probe through to the input amplifier. I looked up the probe and the Rigol scope schematics, but those only show the discrete components. They don't show the wire and connection capacitance, inductance and resistance, but those values are also involved if you want a correct model to analyze the resultant equivalent circuit. 

ADVICE ON HOW TO BYPASS THE QUANTUM MEASUREMENT EFFECT

My friend CuriousMarc suggested that if I used an active FET probe, which has much lower loading than the passive probes, I can probably observe the failure. If I can see what is going wrong, I have a better chance of fixing it. 

I immediately went online to look for active FET probes for my Rigol DS1054Z scope. What I saw set me aback. Rigol is a very cost efficient brand of scopes, but the active FET probe choices ran from about $1,500 to well over $4,000 depending on bandwidth. 

Since I don't regularly have a need for that kind of probe, I would be spending that money just to measure this situation - a one time process. I can't even be sure that what I observe would definitively lead me to a solution, thus it is a gamble on top of a serious investment. 

LOOKING INTO DO IT YOURSELF ACTIVE FET PROBES

At this point, I am willing to consider building my own active FET probe. I did find a few projects done by competent engineers. This gets into serious RF magic territory, well beyond my usual engineering experiences. I picked one and am seriously considering the effort.

To control the characteristics of the PCB for this probe, I would have to shift from ordinary PCB material (FR4) to Rogers material, which would push the price for the bare board up to around $150. The design I found is centered on a MOSFET amplifier used in UHF analog television circuits - the BF998 dual gate enhanced mode N channel FET. This has dual gates and characteristics like extraordinarily low gate capacitance.

Of course, with the advent of digital television, the market for those devices has dried up. The chip is obsolete and not stocked by any of the reputable distributors. Of course there are many offers for the device on Amazon and eBay, all from operators in China, but the risks that I would be sold a lesser quality FET that was relabeled are just too high. If the gate capacitance and other specs aren't what the design expects, it isn't going to work correctly. 

I did find one location that has stock of the FET, is not in China and seems to have a good reputation. Assuming they didn't just by the stock from the scammers, but instead had a legitimate supply that came from NXP through a distributor, then this solves the availability problem.

I found an Instructable from someone who built the probe successfully.  They characterized it as less than $20 but that assumes free PCB construction and a substantial electronics lab to measure and adjust the probe. The values they determined for filters to make this work linearly are only for the board they built. Because - RF magic. 

Minute variations in the PCB, the components and even in my soldering will mean that when I build it, I may need to do the measurement and tuning process myself. Even the presence of a bit of solder on the traces beyond what is strictly needed to mount a component adds capacitance and inductance. Also, too little solder turns the joint into an unwanted capacitor. 

It isn't the most practical Instructable. There are only simplified schematic fragments. The gerber files show many components, about half of whose values are NOT specified. Instead of showing a full schematic, the person uploaded an equivalent analytic model in NI AWR Microwave software format - thus having elements that stand in for straight and right angle traces, as well as actual components. 

As one caveat, they mention that "you may need a membership in a laboratory, or acquire some people's affection who do (i.e. via cold beverages). Also, the 20$ price tag suggests you only need to buy the special components, not the standard things which lie around in an RF-lab anyway." They also assume I will be milling my own PCB from 512µm Rogers RO4003 w. 17µm copper dual-sided board material. 

I was able to take their gerber files and get them into KiCad so that I could produce an acceptable set of gerbers and drill files to send to a professional PCB fab house. I am working hard to identify the components they installed at all the open gaps on the PCB, so that I can purchase them. If I am successful in developing a complete bill of materials and can find the parts, I will then order the PCB and commit to the project. 

The component values which are totally custom are the ones needed to form a filter to compensate for a resonance around 800 MHz. He started with components such as 1.8 pf capacitors and 22 nH inductors, but suggests that I have a well stocked supply of different values to try. This might involve some iteration ordering parts from a supplier and stretch out the completion. 

The current snag in finding parts is the SMA connector he uses - it is a straight SMA female with a solder-on pin, but has a mounting flange with two holes. The flange is at right angles to the connector, but the inner conductor comes out straight. All I have found so far that have the flange at a right angle bring the inner conductor out at the right angle, which isn't going to work. If anyone recognizes this and knows where I can find it or at least how to describe it when searching, let me know. 

This probe produces a 50 ohm output impedance. My Rigol DS1054Z oscilloscope has a 1M input impedance. The solution is to add a 50 ohm terminator in a Tee connector at the input of the scope. I will have an SMA to BNC cable hooked to the tee where it is terminated, the other side of the tee connected to the scope input. 

Invested time simulating and correcting code to archive IBM 2315 cartridges on a Diablo model 31 disk drive

ARCHIVAL NEED

I own several dozen 2315 cartridges that contain software for IBM 1130 systems. Rather than risk them directly on the internal disk drive of the IBM 1130 (and rather than risking damage to the very rare disk heads inside the drive), I want to extract the contents and put them on virtual cartridges for use with my Virtual 2315 Cartridge Facility (V2315CF) for the 1130. 

The V2315CF holds the contents of a cartridge on an SD card inside a holder designed to look like a miniature cartridge. When inserted into the V2315CF, the contents are made available for the IBM 1130 to read and update while it believes it is accessing the physical internal disk drive of the 1130. 

A dummy 2315 cartridge spins in the physical disk drive, providing the timing information and offering the user the sounds and vibrations of a running disk and its seek movements. The data flow to the heads is instead fed from the V2315CF, giving the 1130 the stream of pulses that would have come from the heads. Any sector that is written to will have the pulses from the 1130 captured and update the contents of the virtual cartridge. 

The archived contents of the physical 2315 cartridges can also be used with the IBM 1130 simulators running on personal computers, expanding the pool of software available for hobbyists and researchers of the 1130 system.

IBM developed the 13SD drive and used it with the IBM 1130 and 1800 computing systems. It took the 14" disk platters and head technology from the larger 2311 and 2314 disk drives, creating a single platter version that was housed in a cartridge (a 2315 cartridge). 

The 13SD drive was installed in IBM 1130 computer systems, inside the main 1131 processor cabinet. Drives were also put into the separate IBM 2310 cabinet, hosting one or two drives in each cabinet to give expansion capacity for 1800 and 1130 systems. The cartridge was pushed into a front loading slot of the 13SD drive and spun up to use it with the drive, then removed when a different cartridge's contents were desired. 

DIABLO MODEL 31 STANDARD DENSITY DRIVE

Diablo licensed the technology from IBM to design their Model 31 standard density disk drives. These used the same 2315 cartridges and were compatible, able to share data between the model 31 and an IBM 13SD drive. A 2315 cartridge in standard density read and wrote bits at 720Kbps with the cartridge platter spinning at 1500 rpm. 

The data is recorded in a self clocking using a 'double frequency' method where each bit is recorded in a cell with space to record two magnetic flux reversals. The first reversal was always recorded - that was the clock bit. The second time interval would either have a flux reversal, to indicate that the data bit value was 1, or skipped a reversal to indicate that the data bit was a 0. 

The platter was organized into 203 concentric rings of information as the disk head moved radially inward and outward towards the center. 2315 cartridges from IBM had the circumference of the platter divided into eight equal pie slices, with a notch in the hub ring generating a Sector Marker as the drive reached each of the slices. A special double notch at one point generated the Index Marker to define which was the first sector of the eight. 

IBM chose to combine pairs of slices to form the four logic sectors that were used in the IBM 1130 and 1800 systems. Diablo offered cartridges with 8, 12, 16, 20 and 24 physical sectors per rotation. 

Diablo then enhanced the drive, doubling its capacity, as the high density version. Many other minicomputer manufacturers licensed the IBM disk technology and made use of the high density approach. Over time, these vendors also narrowed the size of a ring on the platter to implement 406 cylinders rather than 203. None of these were interchangeable with the IBM 1130. 

The disk controller for the 13SD drive handled disk arm movement differently from Diablo. IBM used a toothed comb with pins that held the arm in the 203 positions. An arm movement involved a command to the disk drive to move 1 or 2 positions in either the forward or reverse direction. The drive made a grunting sound as the pins detented into and out of the comb during seeks. 

A microswitch in the drive indicated that the arm was at the home (cylinder 0) position. Modeling the seek meant tracking the arm position based on the 1 or 2 track movements received as well as the home cylinder indication. A feedback signal turned off when a seek began and switched back on when the arm was settled in position on the new track. The command signals to move, the step size and direction were all defined length pulses. 

Diablo dropped the use of the comb and pins, instead implementing a servo that precisely positioned the arm at the 203 possible cylinder; it was much quieter as a result. The drive accepted different commands. It was given a cylinder number to attain and simply asked to seek. The request was maintained until feedback indicated it was accepted. 

The drive would first acknowledge acceptance of the target cylinder number with one feedback signal, requiring the controller logic to drop the movement request once the feedback arrive. Another feedback signal would turn off when the seek began and turn back on once the arm was correctly in position. The drive did not indicate when it was at cylinder zero.

The drive would emit the Sector Marker and Index Marker pulses, just as did the 13SD, but in addition would output a binary coded decimal Sector Number directly. The controller logic no longer had to interpolate the sector by counting SM and IM pulses. 

Other differences included a Restore command that would move the arm to cylinder 0. Error signals were generated if the requested new cylinder number was out of range or if the seek mechanism somehow failed to work properly. 

FPGA LOGIC WRITTEN A WHILE BACK BUT HAD KEY DESIGN FLAW

My version of the archiving logic used the 13SD seek commands and feedback signals. The Diablo model 31 did not conform to that, thus it required redesign to use the Diablo interface signals. 

ADAPTING TO THE DIABLO INTERFACE

Fortunately, I am quite familiar with the Diablo drives and how to drive them with an FPGA, as I did this to archive all the cartridges from the Xerox PARC library a few years back. Those contained code used on the Xerox Alto computers, the pioneering systems that developed the graphical user interface, ethernet, WYSIWYG and other concepts that made their way into the Apple Lisa, Macintosh and Windows. 

SIMULATING TO VERIFY THE UPDATED DESIGN

I changed the logic and then had to carefully simulate the behavior to be certain that my updated design conformed to the Diablo specifications and would work correctly. This was a long slow process as each rotation of the platter took 40 milliseconds to complete, with more than one turn required to be able to sync up with the SM and IM pulses to correctly mirror the sector number under the heads. This took a long time in simulation before I could observe the movement behavior. 

To fully simulate the archiving of an entire cartridge would require 120 ms per cylinder plus 15 ms for the one track seek. This would approach 30 seconds of simulated time, many, many hours of simulation time on the laptop. 

Using new hysteresis technique to protect against noisy signals in some of my FPGA designs

EXTERNAL SIGNALS MAY NOT BE CLEAN ENOUGH FOR CERTAIN LOGIC

Every external signal coming into the FPGA has to be passed through a series of flipflops to reduce the risk of metastable states - the risk that if the pulse arrives at just the wrong time compared to the clock edge in the FPGA, it may assume an incorrect state or even hang up. The conventional solution is to pass through three clocked flipflops to clean up the signal for use inside the FPGA. 

The risk of a signal sitting in a metastable state goes down dramatically by the third flipflop. It does introduce a delay of three clock cycles before you are seeing what has occurred externally, but with a 100MHz FPGA clock and the relatively slow equipment to which I am connecting, this is not an issue.

These signals may have a bit of bounce in them, similar to how contacts behave when a switch is flipped. That is, even though digital logic is driving the signal rather than electrical contacts, I might see a sequence of bits coming out of the three flipflops that don't definitively transition between 1 and 0 states at only one cycle. The output might have a stream of 0 0 0 1 0 1 1 1 for example, where a long sequence of 0 change to a long sequence of 1 but there is a sort of stutter for a cycle or two. 

Often we want to react when a signal changes state, but we take an action only once. In essence we only care about the signal edge, the rising transition. If there is a stutter, than we might detect more than one rising edge when the external situation only changed once. If we are counting based on the edges, we might incorrect count higher than we should. 

In many cases, the external signal is used to advance a state machine, so that it doesn't matter that there is a pair of edges close together, but I have to think carefully about whether the state machine could malfunction with stutters. For example, the state machine may use the falling edge to move onward, which the stutter will appear to deliver. 

HYSTERESIS - SCHMITT TRIGGERING

Schmitt triggers are gates that switch at different thresholds in one direction than the other, designed to eliminate the risk of small variations inadvertently switching the output state. Usually this is an analog approach - perhaps the rising edge must get to 75% of the full on voltage before the gate registers it as a 1, but it won't switch back to 0 unless the falling edge drops below 25%. The signal can bounce or oscillate around the 50% level quite a bit and still not be reflected in the output. 

This is hysteresis - where the change in state depends on the history of the signal not just the immediate value. It adds a delay before the change in the signal is recognized and acted upon, but it can eliminate bouncing or stuttering signals. It involves different thresholds for when it turns to 1 and when it turns to 0. 

MY THRESHOLD FILTERS

I created a module that adds this hysteresis for critical external signals. It first address metastability with the usual series of three flipflops. It then adds or subtracts to a counter based on the instantaneous signal state. The thresholds for when the output changes are specific high and low counts. We might require five successive 1 values before the output is switched to one, when the counter reaches a count of 5. Future 0 values decrease the counter from its maximum, but we don't change the output state until we get to a count of 2. That spread between the rising and falling thresholds is the protection against bounce or stutter. 

The time delay can be an issue - in the example above a signal that rises to 1 won't be recognized for a minimum of 8 FPGA cycles, more if there were stutters. At 100MHz clock frequency, that is 80 nanoseconds or more delay. The case for most of my projects is that the signals are much, much slower than that. Even the stream of clock and data bits from the disk drive are changing more like 1400 nanoseconds, more than 17.5x slower. More importantly, all the signals are arriving wih a similar delay. 

It will add delay for when my output signals to an external device don't react for 80 nanoseconds, but that isn't significant. A sector on the disk takes 10 milliseconds to rotate past the head, thus the signal delay is only 0.0008% of the sector duration. 

I am first applying my threshold filters to the tool I built to use a Diablo disk drive to read and archive all my 1130 disk cartridges. The protocol for the disk is to record magnetic flux reversals in 'bit cells' of two bit times. A reversal always occurs first, which is the clock, then the second bit time skips the reversal if the bit value is 0 otherwise a reversal signals a 1 bit. 

The disk drive itself tries to separate the flux reversals and route them out separate clock and data signal lines, but I have to see the clock pulses to know which bit of a word that a data pulse belongs to. Thus, the edges of clock and data really matter in this process. I expect my filter to help me determine when to watch for data pulses and when to shift the prior result into a word. 

My state machine is driven by the clock pulse edges - the falling edge sensitizes me to watch for any 1 bit coming on the data line. The falling edge of the clock resets the detector, then any data pulse turns it on. The rising edge of the clock is when I check the detector to see if a data pulse had arrived any time during the roughly 1.4 microseconds between those edges. 

Another application where the threshold filter might be very helpful is in the Virtual 2315 Cartridge Facility, where I have had some discrepancies between the seek distances requested by the disk diagnostic program and the attained cylinder position. Since the commands from the 1130 to the disk pass through the V2315CF, if a stutter caused a failure in my FPGA logic I could have dropped a movement or added one as I drove the disk drive from my code. 

Found substitute spring for the Calcomp 565 plotter and verified it works properly

RESTORING CALCOMP 565 PLOTTER

I was given a Calcomp 565 Plotter, one of the first drum plotters, which is the same mechanism as IBM resold under the model number 1627 for use on their 1620 and 1130 computer systems in the early 1960s. Since I own an IBM 1130 I was given this machine as an eventual restoration project.

The machine had suffered a serious blow on the drum, denting it significantly. It had been disassembled and was in a bin when I received it. In addition to the drum damage, it was missing the pen assembly, a solenoid that pulled a pen up away from the paper or dropped it down to draw on paper fed around the drum. 

Recently I found a Calcomp cutter which was the same solenoid and a slightly different top. used to cut films on the same plotter instead of drawing. I bought it and plan to modify it to accept a pen so that this plotter could once again draw graphs under computer control. All that assuming I could repair or find a substitute for the drum. 

I recently repaired the circuitry, which had a few bad semiconductors, then began to reassemble it temporarily, with the bad drum, to confirm whether it could be restored once I solved the drum and pen issues. I discovered it was missing a spring as well as some nuts and bolts. The fasteners are no issue at all, but I needed to replace the spring.

CABLING TO MOVE PEN CARRIER LEFT AND RIGHT

A pair of metal rods across the front of the plotter support a moving fixture into which the pen solenoid is mounted. Cables stretch from both sides of the fixture, around pulleys inside the plotter, connect to a stepping motor, and then are joined by the missing spring at the back. 

These cables are insulated with a plastic sleeve at the front, since they also carry the current to activate the solenoid when the pen is to be held up off the paper. The rear portions of the cable are not insulated and ride over metal pulleys that are isolated from the machine chassis. These two pulleys make the connection of the 24V solenoid power to the cables. 

The routing and winding of the cables around one pulley and around the stepper motor is a bit complex, but the result is that when the stepper moves, the fixture slides left or right. The spring at the back maintains the proper tension on the cables. The two cables have nylon eyelets so that the spring does not make electrical contact, merely providing tension. 

What I didn't have was any idea of how long the spring is and how resistant it is to extension. Too, I didn't know the target tension it should be applying to the cabling. 

KIND ASSISTANCE FROM COMPUTER MUSEUM OF BASEL

I have had help over the years from the Computer Museum of Basel, in Switzerland, as they have a restored IBM 1130 system. We often cooperate on 1130 issues. Alex and Christoph agreed to measure the spring in their Calcomp 565 plotter so that I could then look for a suitable replacement. 

They provide very precise measurements of the rest and extended length of the spring as well as the tension it provided to the cables. That was just what I needed. 

FOUND SUITABLE SPRING AND INSTALLED IT ON CABLING

I did locate a spring that had the correct rest and extended length, as well as the force it should exert when stretched to the target length. I wound the cable through the machine and inserted the spring in the rear to hold the cable ends together. It wasn't a final assembly but was sufficient to verify that this will allow the plotter to work properly once I have the pen and drum issues solved. 

Rough fit test of spring