Better chat with Copilot led me to an important issue with the 1130 MRAM board - and it appears to be the root cause of my issues!

USING COPILOT TO REPEAT CONVERSATION WITH GOOGLE AI

I repeated my discussion from earlier, describing the observed signals and the sporadic failure of the SLT card AC trigger to recognize my sense pulse. Copilot quickly led me over to the drive of my pulse, which I described including the transistor type and expected sink current of 35-40ma. 

It then asked me details about how I was driving the transistor and pointed out that the 2K base resistor I was using would only provide about 1.35ma to the transistor which would NOT put it fully into saturation. That would mean that even with the minimum beta of 25 from the spec sheet, the transistor may not actually sink the 34ma I was expecting. Too little sink current would fail to pull the SLT input down low enough. 

That does match the signal trace I saw - I initially assumed the low level a bit below 1V was due to impedance in the path and circuit details inside the SLT card, but it very well could be a marginal logic level to trigger SLT. Copilot suggested switching to a 470 ohm resistor.

It then had a cogent discussion of how I was driving the base resistor - a 74LVC08A chip with 3.3V supply voltage. It confirmed that the chip has plenty of drive to deliver 5.7 to 7 ma to the base of the transistor. The chip is designed for up to 24ma per output. At the minimum beta, the transistor could sink up to 142ma of current but the SLT circuit will not deliver more than 1 ma or so. 

I even asked for an analysis of the current into the input pin of the SLT register card with the "AC trigger" - falling edge sensitive input. It gave a very nicely reasoned analysis, calculations and background. It also realized that the input was triggered by a falling edge, thus the capacitor is charged through the +3V rail on the SLT card. Gemini claimed it was charged by the -3V rail. 

Somehow when I looked at the spec sheet for the BVS52LT1G transistor I am using, I thought that my base drive current was sufficient. It doesn't directly give any number for what base current puts the transistor into saturation, which is the necessary condition for the transistor to fully drive the output. 

As you can see in the top line of the chart above, I should have a large signal (DC) gain or beta of 25 with 1ma draw. The saturation voltage for the base to emitter junction is 0.7 to 0.85 V and I was providing 3.3V through the 2K resistor. Naively I assumed the transistor was saturated and able to deliver the 25X gain from a base current of 1.23ma worst case - sinking over 30ma from the connection to the SLT card. 

DOING A TEST CHANGING THE BASE RESISTOR FOR BIT 2

Since the sporadic failure I keep seeing is on B register bit 2, I located that resistor on my PCB and swapped it out. I looked all over the shop for a 470 ohm 0805 size resistor - a very very common value. However, the nearest I could find was a 499 ohm resistor which is what I installed on the board in the bit 2 position.  I then fired up the machine and looped reading memory to see whether the errors disappeared. 

The machine ran for 22 minutes without an error on bit 2. It eventually failed when bit 0 was dropped, understandable since I hadn't changed any of the other 17 base resistors. I ran for a while with no probes attached at all - bit 2 was 'healed'. 

The pulses drive down further toward ground and are sharper to boot. I have a bit of ringing at the low point but the SLT logic is insensitive to such fast signals and was already triggered by the falling edge thus unaffected. 

The falling edge is FAST. The scope is set for 100 ns per division, showing the falling edge as essentially vertical, in the range of hundreds of picoseconds. It appears to fall to about 0.15V, far below the specification for a valid SLT logic low level. 

THE ROOT CAUSE APPEARS TO BE IDENTIFIED AND THE FIX IS EASY

Based on this test, it seems that I was driving a marginal signal into the SLT card due to inadequate base current on the fast transistor I have in the board. It was close but failed sporadically after a few millions of successful operations. Stepping up the base current 4X with the resistor swap gave me a very solid and reliable operation for bit 2. With the machine performing almost 278,000 reads per second, it completed more than 380 million successful reads before a different bit failed. 

I will replace all eighteen of the base resistors on my board. Once that is done, I will run the core memory tests and other CPU diagnostics for a couple of hours just to convince myself the memory replacement is solid. 

I DO AGREE WITH ONE OF MY READERS - COPILOT BETTER THAN GOOGLE GEMINI AI

This is the second time that I did a comparison of the conversations from Gemini (through the Chrome browser) and Copilot. At least for vintage technology subjects, It didn't take that much prompting to move the conversation to productive areas, unlike Gemini which just iterated apologizing and straying in new ways. 

I have a loose lower head cable clamp in the Diablo 31 drive that must be reattached before heads can be installed

BASED ON THE EXPLODED PARTS LIST DIAGRAM, THE HEAD CLAMPS ARE TOO LOOSE

three clamps at upper left

Two of the clamps have a body with a tang that fits into a hole in the metal plate that is central in this diagram excerpt. If the bolts are loosened too far, the tang comes out of the hole and the clamp can spin around thus blocking the head from sliding into place. A third clamp must be oriented with its long axis vertically so that the sides grab on the two heads and lock them in place. When loosened, gravity causes it to turn 90 degrees so the long axis is parallel to the ground. 

The lower head clamp is not only too loose to have its tang in the hole, the bolt has disconnected entirely! You can't see the gap in the photo - very limited access for pictures or for working - but when I put an allen wrench on the bolt I can pull it away from the clamp entirely. 

I will have to do some microsurgery, getting a tool in to hold the clamp while I get the bolt inserted and the threads turning. I then have to tighten it up while maintaining its orientation until the tang enters the hold in the plate. It should only be loose one-half turn from a tight position. 

The upper head clamp is still threaded on the bolt, but its tang is out of the plate. This will be a somewhat easier surgery than the lower head clamp because all I need to do is hold orientation while I tighten it. Once it is in place, one-half turn loose should be enough to let the heads be installed.

When the upper and lower head clamps have their tangs in place and are appropriately loosened, I will try to install the heads while jockeying the third clamp vertical, as you see it in the picture above. I will then tighten it first, then loosen one-half turn. 

When the heads are installed, I can figure out what to use as the adjuster to align the heads. Diablo provides a tool (but I don't have one) that is threaded, to insert through the plate, with a conical head that will push against the side of a diagonal notch in the heads to make them move forward as the tool is screwed inward. 

Prepared the power supplies and cable for the Diablo model 31 drive I am using with the Archiver - powers on

CONNECTED WIRING TO THE WINCHESTER MRA9 SOCKET CONTACTS

I bought three regulated power supplies to support the Diablo drive. Two of them deliver 15V at 7A for use with the drive motor and the rotary arm positioner, the third is a dual voltage +15 and -15V supply at 2.5A each which powers the circuit boards of the drive. The two high current supplies were hooked up so that one delivered +15V and the other delivered -15V. 

These have a common ground wire which will become contact C of the MRAC-14 socket. The high current +15V is wired to contact K. The high current -15V is wired to contact R. The low current +15V for the circuit boards goes to contact H and the low current -15V is wired to contact P. 

Diablo actually uses a single supply for +15V and a single supply for -15V, but routes two separate wires from each. The heavy current on one of the wires drops the voltage on that wire a bit but the other wire stays at the regulated voltage to more reliably power the circuit boards. 

I chose entirely separate power supplies to ensure the cleanest logic power to the Diablo. The power supplies were not expensive and could have other uses once the archiving is complete. I twisted all the wiring together as recommended by Diablo. 

The three power supplies were connected to the AC mains in parallel, so that all three come on or go off as the plug is inserted into the wall socket. If this will become a more permanent drive - perhaps as a second drive for my IBM 1130 - then I will mount the power supplies in something more professional looking (and safer than exposed terminals). 

Test fit of the contacts in the body

+15 rails are good

-15V rails are good

Test fit of power connector

DISCOVERED A GENDER ISSUE WITH THE MRAC 14 CONNECTOR

The part I bought appears to match the part that was installed on the Diablo - other than the Diablo side has the male contacts and mine has female contacts. This locks the tightening screw and body in place. I have to loosen the nuts on the side to let the screw and receptacle rotate to thread onto the Diablo side screw and receptacle, then tighten the nuts when it is fully pulled tight. Since I haven't glued the contacts in the body yet, I only tightened it enough to verify it fits and delivers power properly. 

WILL GLUE THE CONTACTS INTO THE MRAC-14 SOCKET BODY

I plan to JB Weld original epoxy to glue the connectors into the socket body, hoping it would provide enough strength to at least connect the cable one time. I did NOT sand the contacts because the spec sheet mentions they have beryllium in them and I will not risk inhaling any. I did clean the contact and the body with isopropyl alcohol first, then applied the epoxy. It required 24 hours to cure before I could attempt to move it or insert it into the drive. 

CABLE ONNECTED TO THE DRIVE AND DID A POWER ON TEST

The key was to see if the drive would power up and be happy. A bonus if I could get the motor to start spinning. I turned on the power supplies and saw the Power lamp illuminate as well as the Unlock lamp. The drive allowed me to open the cover to insert a disk cartridge. I put in a cartridge (but as i don't have the heads installed I won't spin it up to the point where the drive tries to load the heads) and got the motor spinning the platter for a few seconds. The servo was also locked in place unless I pushed the servo unlock button inside the drive.

The drive appears to be pretty healthy. This was not a checkout, thus we could have failed ICs or other issues yet to resolve, but power is good and the machine didn't do anything strange. 

Fighting to install the heads in the Diablo 31 drive

CHOSE THE REPAIRED HEADS BUT HAD TO CLEAN THEM 

The heads that my friend carefully polished to remove the head crash scratching actually looked good enough that I decided they would be the first set I try to fly. However, when examining them under the microscope I saw that the polishing powder was still on parts of the head. The bits of the powder are large enough that they would certainly cause a head crash as they flaked off during the vibration and airflow of disk operation. 

I could see it in the two round airholes that establish the cushion pressure as the head flies over the surface. I could see a lot more caked up behind the head, around the coil and on the attachment points. As I touched it with tools, it moved and came off in sludgy sections. 

I carefully removed as much as I could and bathed the heads in isopropyl alcohol in a container to try to wash away as much as I could. They seem pretty clean after all that work. 

HEADS STUBBORNLY REFUSING TO SLIDE FULLY INTO THE ARM

The heads should slide into the arm where clamps would be tightened to hold them down. They were not going in sufficiently. Unfortunately, the clamps are hidden inside the rotary motor and arm assembly, barely visible from the side using a light. I thus can't really see what is blocking the head from sliding into position. 

SEEING THE PROBLEM BY STUDYING THE PARTS CATALOG DRAWINGS

The excerpt from the parts drawing shows the three clamps and the slots on the right where the heads slide in (from the right moving inward to the left). It appears from the diagram that two of the clamps have a tang that should sit in a hole in the metal block. I have loosened them too much and they are now turning freely and not held in the proper alignment.

Once I get the two clamps tightened with the tang in the hole, I can loosen they slightly and they should remain in alignment. The third clamp should be oriented as shown but it freely spins round the bolt when there are no heads installed. I will then need to turn that to the proper vertical orientation before I attempt to slide the heads into the arm and get them to settle down between the clamps. 

I will attempt this on my next visit to the workshop. Once the heads are properly inserted and connected up, the drive should be ready for head alignment. The Diablo drive has a pushbutton inside that blocks the servo from moving the heads. I will hold that down and move the heads to a far cylinder manually then wait until the heads load. That way, any damage to the special CE cartridge used for alignment will be past the point I need to use. If the heads fly safely, I can move it back to cylinder 100 where the alignment data is written. 

Getting insight into what is happening during failure of 1130 MRAM doing reads - no more quantum effects

CURRENT SITUATION

When storing words of all 1 values (0xFFFF) into all of memory and then setting the machine to do continual storage reads looping through memory addresses, I will get sporadic parity errors where bit 2 fails to be set. Since the 1130 MRAM board has calculated parity based on that bits stored value of 1, the parity check fails and the machine stops.

I had been unable to see what was occurring because any time I put an oscilloscope probe or the logic analyzer on the incoming pin for setting the bit, the machine never failed. I finally dumped about over $500 into acquiring an active FET probe (used) which has less loading effect. 

MORE EXPERIMENTS WITH ACTIVE FET PROBE

I set the active probe to 10X attenuation and AC coupling, with a direct ground lead to the ground pin on the same SLT card as the incoming sense bit pulse. That worked as I wished - the rate of sporadic dropped bits didn't change thus my loading was not affecting the measured circuit. 

What I saw when a parity check was triggered was that the pulse which attempted to set the B register bit 2 was slightly different shaped than the others. It is there, but somehow it isn't flipping on the bit thus we get the parity check. 

The yellow trace is from the active FET probe recording a negative going pulse from my 1130 MRAM board which is intended to cause the SLT card to flip on the bit. Thousands prior to this successfully turned on the bit but this one didn't. I looked closer at the failed bit attempt as well as the successful one that came just before.

Pulse which fails to set the B register bit 2

Example of a pulse successfully setting the bit

The failed set involved a pulse that dropped about 2V whereas the successful ones show a pattern that comes from the capacitor discharging inside the SLT card and getting down to about .25V. The edge detector in SLT is a capacitor that is charged up through a resistor by an enable signal; then when the falling edge pulse arrives, it discharges the capacitor, resulting in the flipflop being turned on. 

The transistor on my 1130 MRAM board pulls the signal line down to ground. A pullup resistor on the SLT backplane causes the line to sit about +3V until the transistor fires to drop the line. We see the pin barely gets below 1V in the failing case but down to 0.25V when it succeeds. 

The transistor has a minimum beta of 25 and with a 1.6ma drive current it should be sinking over 40ma which should be sufficient for the activation of the IBM edge detector. The successful pulses reach a threshold of about 0.9V and then we see the capacitor delivering energy as it is discharged down to 0.25V. The failed pulse reaches about the same initial level but we don't see the capacitor delivering energy. 

I had a long, long chat with AI about what might be happening. Lots of speculation that didn't make sense, but I did 'listen' carefully and think about the phenomena being described. A good refresher on EE topics. Discussions about varying signals from other processing in the 1130 didn't make sense as this is a pure memory display loop.

As the AI pointed out, the SLT backplanes are designed for slow signals and higher frequencies such as from my board's fast pulse edge can ring across the backplane and cabling. The speculation was that ringing reflections could randomly cancel out my pulse edge if it arrives at just the wrong time However, the active FET probe is not showing any 'long term' ringing. 

When I started the discussion with the sporadic nature and failure rate of about 1 in 200,000 memory accesses, the AI asked me to look for a beat frequency of 1.388Hz but that is assuming it is always deterministically 1 in 200,000. Other suggestions assumed that the IBM flipflops are clocked, but they are really asynchronous circuits. Still, it did push me to think along many lines. Slight timing drift between my board generating the pulses and the clocks in the 1130 could line up bounce and dips on rails. 

I do remember seeing ground bounce in earlier versions of the board and that might still be an issue I need to address. I will add a braided ground strap from my board to the 1130 ground bus and see what effect that has. Another idea was to temporarily add a .1uF capacitor from ground to the -3V rail input of the SLT card and another .1uF capacitor between ground and the +6V rail input on the SLT card. This will absorb some high frequency bounce that might be caused by my fast pulse edges. 

The shape of the failed pulse looks to me as if the capacitor is not discharging. It has to be charged by the SLT circuit prior to my negative going pulse arriving. AI speculated that the resistors on the SLT cards have drifted high and are barely recharging the capacitor in time thus very random timing differences between memory cycles might eventually arrive just too soon to trigger. Bounce of the rail that charges the capacitor might also cause it to fail to charge sufficiently, it mused. 

This problem has bounced around the machine between the SLT cards. The B register is implemented across eight double width SLT cards, two bits per card. Previously the most common failure was bit 14, but recently it is bit 2. Thus it is unlikely that several cards have degraded to have exactly the same vulnerability. The only commonality I remember is that the errors always occurred on even numbered bits - those on row 3 of the SLT backplane. Thus cracked traces for power or ground could be a factor and only impact the cards that used that row. This may be coincidence however and the same issues might be possible on row 2 connections - odd bits. 

Bottom line, I can now see the failures where before they were masked by the loading of the probes. This should allow me to drill down to figure out what is causing my issues. I am making progress but don't yet have the smoking gun that pins down the exact cause. 

PLAN FOR NEXT OBSERVATIONS

When I next get to the workshop, I will add oscilloscope probes to the +6 and -3V pins of the SLT card and watch in AC mode for any activity on the rails that coincides with the failures. 

Started master file list from all 2315 cartridges and stored on Google workspace

ARCHIVED VIRTUAL DISKS FROM AN IBM 1130 SIMULATOR

I have been using the IBM 1130 simulator from Carlos Vincenzi and looked at the virtual disk drives from release 4.4.1.R9 as that is the version I have used the most. I grabbed 9 virtual disk drives from their and ran off the contents from the LET, FLET and SLET of each cartridge.

I entered them in the master spreadsheet I decided to use. This will let me quickly search for a file name and see all the cartridges that contain that file. It also provides an inventory of the cartridge as a whole. 

I will use the Unique field to assign some kind of globally unique identifier since I already have several virtual cartridges that had the same four hex digit ID assigned. For LET and FLET entries, it is the number of sectors the file consumes on disk. For the SLET entries, it shows the size in words of that phase. As well, the type field for the SLET will be the two character phase ID. 

Some files have multiple named entry points, thus the size and type fields are blank for the additional names. The names FADD, FSUB, FADDX and FSUBX are all contained in the one file whose initial entry is FADD. The file is in Disk System Format and takes up 8 sectors of the LET. 

In addition to the compact DSF format, files may be stored in Disk Core Image (DCI) format or in Disk Data Format (DDF). A DSF file must have all its linkages resolved and be combined with all other files it requires to produce the final core image that is what executes. Thus DSF is more compact, but DCI skips the time needed to link together all the files so it is faster to execute. DDF files are not executable. 

Files such as FADD are standard FORTRAN subroutines, loaded when the Fortran compiler was installed on this cartridge. Other files can be applications and utilities, either provided by IBM, from contributors or developed by the user. I will use the Purpose field to note what I discover about the files that are not standard parts of the Disk Monitor System and its compilers. 

The nine cartridges gave me 4, 095 entries in the spreadsheet. I envision the entire archive will be in the range of 100,000 to 200,000 entries. This is why I hosted it on Google. In addition, I can share it easily with other hobbyists and researchers. 

Work progressing on Diablo Archiver project

PYTHON APPLICATIONS DEVELOPED TO READ LISTINGS AND BUILD CSV FILE

I whipped up some Python code to open listing files - printer output of the IBM 1130 simulator running tasks to print the LET (Location Equivalency Table), FLET (Fixed Location Equivalency Table) and SLET (System Location Equivalency Table), extract all the file names along with some salient information, and create a comma separated values (CSV) file to add those to a spreadsheet in the format that I have initially defined. 

They ask for the Cartridge ID, the four hex characters that label the 2315 disk cartridge, a secondary name that I can maintain which is unique across all cartridge images, then read the printer file and spit out the CSV format file that can be used to import those values into a master spreadsheet recording the details of every file stored across all the archived cartridges. 

SOLDERED TOGETHER THE CONNECTOR ADAPTER BOARD

The Diablo came with a cable that has a Winchester MRAC-42 connector on one end and an IDC 40 pin connector on the other. The ribbon cable has a metal ground plane across its breadth, which supports signal integrity for the 37 signals thar are carried across the cable. 

I developed my Archiver PCB with an IDC 50 pin connector on a ribbon cable, supporting 25 signals with a ground wire between each signal wire. I whipped up a design for a board to connect those 25 signals to the corresponding pins of the IDC-40 connector on the Diablo cable. The board mounted one each IDC-40 and IDC-50 socket. 

WORKING ON THE CHALLENGE OF THE DIABLO POWER CONNECTOR

The rear of the Diablo drive has a male Winchester MRAC-14 connector, with five active pins that carry ground, +15V, -15V, another +15V and a second -15V power rail to operate the drive. The power supplies need a MRAC-14 socket. I don't have a cable for that.

MRAC connectors are fairly rare and have strong demand which forces high prices. The used sockets on eBay are all sold as plastic bodies without any metal contacts inside. The female contacts came in several types depending on the wire that would connect to it, whether by crimp or soldering. For my purposes I need the larger solder types.

The female contacts are sold in groups for several hundred dollars. Not a price I want to pay. At worst case I would have removed the male MRAC-14 connector and switched to a more readily available connector type for power. However, I did think I had found a solution that would come in  around $60 total. I found a female 9 pin connector that had the female contacts installed in the body, and I found a female 14 pin socket. Move the contacts, of which I only need 5, and viola.

However, when I received the 9 pin socket yesterday I realized that I had made a mistake. The Winchester connectors come in two flavors - MRA and MRAC - with the difference being that the MRA have the contacts permanently molded into the body, while the MRAC have contacts that can be removed and inserted. The contacts are not compatible between them, as the contacts for MRAC have a metal tab that locks them into the body.

What I had purchased was a MRA 9 pin socket with permanently molded contacts and a MRAC 14 pin body. I decided that I will find a way to re-use the contacts and put them into the 14 pin body. I spent a half hour carefully releasing the female contacts, although it required the destruction of the body. 

I will find a way to anchor the contacts into the 14 pin body. It might require epoxy or some other adhesive, but after I solder wires onto the contacts I will affix them to the body so that I have a workable power connector to hook to the Diablo.

SELECTING HEADS TO MOUNT IN THE DIABLO DRIVE

I pulled the heads from the Diablo drive (the two on the left in this picture) and collected a few heads I was assembling on compatible holders. I will decide which are the best and insert those into the Diablo drive.