Constructing the new 1130 MRAM board, part 2

SOLDERING THE INTEGRATED CIRCUITS ONTO THE BOARD

The four remaining chips, two tristate buffers, the MRAM memory device and a quad NAND gate, were soldered down first.  These were also installed using solder paste, a metal stencil and my hot air gun. 

Based on a suggestion by one of my regular blog readers, I have ordered a hot plate to work atop while using the hot air gun and solder paste (for future soldering work). I tend to put on paste one chip at a time, then flow the solder paste, which I thought would be slowed by having the entire board heated. I will need to adjust my workflow for soldering to add in the hot plate. However, for chip removal it will always be better than starting with a cold board and directing the hot air gun to loosen each part. 

OTHER PARTS ADDED TO THE BOARD

Next up were 18 surface mount transistors plus 18 pullup resistors. These were all hand soldered with a traditional soldering tool. Once all the active parts were in place, I moved over all the gold pins that form the three connectors for the IBM Solid Logic Technology (SLT) cables. Each connector has 24 pins in two rows of 12. 

I had to extract all 72 pins from the prior board and clean them up, before I could install them in the new PCB. I used a connector I stripped out of an independent vendor's add-on product that would connect to 1130 or 360 systems. This served as a guide for all the pins, to keep them aligned until I soldered them in place, so that the IBM cable connector will slide smoothly onto the pins. 

The last item to add will be the FPGA board which plugs in with the USB connector at the outside left edge of the finished assembly. I had previously programmed the Digilent CMOD S7 board with the control logic to drive my design. 

I called it a day before finishing, not having added the third connector, but everything else is assembled on the board and ready for inspection and then testing. 

All chips, transistors, resistors and capacitors are installed

24 pins to be removed - one SLT connector - from old board

Pins from three connectors removed and ready for new board

Similar connector to the SLT 2 x 12 cables

The spare connector positioned in place as guide

Pins will be inserted through board into guide connector

Completed installation of 24 pins into one connector

FPGA board sitting in place on board

Constructing the new 1130 MRAM board today, part 1

FIRST STEP WAS THE POWER SUBSYSTEM

I installed the two linear voltage regulators which drop the 12V input power to deliver the two power rails for the board, 3.3V and 5V. A quick test with a bench power supply verified that they worked properly. I also installed the decoupling capacitors.

In addition to the power supply parts, I installed the power on reset logic since it sat beneath where the Digilent CMOD S7 FPGA board would be mounted. I soldered down sockets into which the FPGA board will plug. Below you can see the decoupling capacitors on the back side of the printed circuit board, plus a pair of resistors that drop the incoming +12V to an acceptable level to feed into the FPGA as a 'power available' signal. 

SOLDERING THE INTEGRATED CIRCUITS ONTO THE BOARD

I took advantage of the stencils I had from the prior invocation of the board to solder each chip using solder paste and a hot air gun. There were 13 chips to add to the board, but in today's session I only finished installing nine of them (excluding the power on reset and the linear voltage regulator chips). 

Five74LVC08 quad 2 input AND gates produce the 16 data and 2 parity output pulses. These will drive BVS52 transistors to pull down the 1130 sense output lines which in turn flip on a bit in the Storage Buffer (B) Register. Four 74HC86 quad XOR gates generate the correct parity for each 8 bit half of the 16 bit data word. These were installed today. 

In my next workshop session, I will solder down a 74HC00 quad NAND gate which implements a bit of glue logic for the design, two 74LV240 octal inverting tri state buffer gates, and the MR0A16A 1Mbit Magnetic Random Access Memory device. Once these are in place, the 18 BVS52 transistors with 18 pullup resistors will be soldered down. The final step is to transfer the 72 gold pins that form the three connectors into which the 1130 memory cables are plugged. 

Redesigned 1627 Plotter controller card while waiting for my MRAM board and parts

WAITING ON ALL PARTS TO ASSEMBLE THE NEW 1130 MRAM BOARD

I already have the printed circuit boards from JLCPCB and most of the components from Digikey, but the final shipment won't arrive until Monday sometime. In the interim I focused on an older project which also had some challenges using timer chips like my earlier MRAM design.

REDESIGNED BOARD FOR PLOTTER USING FPGA

I chose to use another Digilent CMOD S7 board mounted on a PCB to implement a revised design. I whipped up the design in KiCad yesterday. Today, I wrote the Verilog for the controller function in the FPGA and simulated it to verify proper operation. 

This greatly simplifies the board, as it only has a linear voltage regulator and two open collector hex inverter chips. One of the open collector chips drives the output signals to the 1627 plotter, basically pulling a line down from 12V to ground to command various plotter motions. The other chip pulls 1130 signal lines down from 3V to request an interrupt and to output the status bits when an XIO Sense Device command is issued. 

I am waiting to hear from PCBWay, who had offered to do a sponsorship with me but due to timing urgency, it wasn't a good fit while I was working on the MRAM project. However, I don't have time urgency on the plotter effort, since I will be focused on the 1130 MRAM project next week and then the 2310 Virtual Disk Cartridge project after that. Once the boards arrive and I have a suitable lull in the workshop, I will put together and test the card that acts as a driver for an IBM 1627 Drum Plotter. 

1130 MRAM testing observations while I wait for the new board for the updated design

PCB AND PARTS ARRIVING ON MONDAY

I had redesigned the board to utilize an FPGA to drive all the timing, dropping the use of timer chips and simple combinatorial logic. The FPGA code was developed, simulated, and I loaded the bitstream into the new Digilent CMOD S7 board that will be installed of my new PCB.  The PCB blank and some final components are due to be delivered on Monday, so that on Tuesday I can build the new board to begin testing. 

OBSERVATIONS DOING MANUAL DISPLAY AND LOAD MODE CYCLES

When I set the rotary mode switch to Load or Display and push the Prog Start button, the 1130 takes one memory cycle and either loads memory with the value set in the Console Entry Switch (CES) toggles switches or reads the contents of the memory location and shows it on the SBR lights. 

I was able to load patterns and get reliable readback until I got over about five 1 bits in a word. At that point, some of the bits did not display correctly when read back. A pattern with four 1 bits would be reliably written and read back - this may be more than four bits because there may be parity bits in addition to the data bits that have a 1 value. 

The new design will set only three bits at a time, skewing the setting across multiple 85 ns cycle times of the FPGA. Thus whatever is causing the issue observed should not recur. 

When words with a high number of 1 bits are loaded, I would observe that when displaying, some words would come back as all zeroes while others either had the pattern or the pattern with some of the 1 bits missing. 

The final anomaly that I observed was that when resetting the 1130 using the Reset button, from time to time the value of the word at location 0 was erased instead of having its previously written value. This may be a fault caused my prior design where it reacts poorly to the instantaneous changes in signals during the reset and as the processor comes out of reset. 

Since my new design changes how the control signals are generated, now using the FPGA onboard, this issue is probably not going to recur. An advantage of having shifted to an FPGA for timing and control signals, is that I can change the behavior with changes to Verilog code and update the FPGA board without requiring hardware changes. 

Looking at 6V regulator SMS card to resolve 1130 power issues - part 3

CIRCUIT OPERATION TO MAINTAIN 6V OUTPUT VOLTAGE

The comparator that controls the voltage of the 6V supply is formed with a pair of 026 transistors sharing an emitter resistor. One is fed a 3V reference voltage from a Zener diode, while the other side is fed a fraction of the current output voltage. 

The reference voltage (3V) acts on the base based on the emitter voltage, which is produced by the voltage drop on the emitter resistor from the summed currents flowing through the two 026 transistors. As the current goes up in the emitter resistor, the current draw of the reference voltage side decreases meaning that the other 026 has its current increased due to the output voltage being above its 6V target. If the output voltage drops below 6V, the current in the reference voltage side has to increase since the emitter resistor current lessens. 

It is the output current of the reference voltage side 026 that drives the power transistors through an amplifier to pass more or less current to the output. Voltage drop in the load is based on the current flowing through it, thus more current increases the voltage on the output terminals and less current causes the output voltage to drop. 

OBSERVED BEHAVIOR DURING FAILURE - WHEN THE CIRCUIT BREAKER WILL TRIP

The output voltage is oscillating (with small voltage swings) around an average of 3V when I see the regulator then trip its circuit breaker. This happens when I power up the system when it has been previously powered and turned off for only a short period. 

This implies that the output current is lower, since the voltage is caused by voltage drop in the load (1130 circuitry fed by this regulator). This is paradoxical however because if the current from the regulator is low, then the draw through the circuit breaker should also be low. Instead, it trips in about one second. Thus, a defect in the regulator that causes it to feed insufficient current to the load should not produce the breaker trip. 

AREAS TO LOOK AT THAT MIGHT CAUSE THE SYMPTOMS

If the load were to demand far too much current (partial short), that would result in the observed symptoms if the regulator were unable to push enough current to meet the demand. The regulator has a capacity of 24A at 6V, based on using six IBM 108 transistors in parallel. We would have to see a demand well above that, perhaps 50A, to sag to 3V and trip the circuit breaker. This would be a failure out in the load, not in the regulator.

However, using my LTspice model, imperfect as it is, I was able to reproduce the failure mode if one of the 026 transistors were to form a short across the emitter and collector. It is the transistor that samples the output voltage. I turned my attention to the 026 transistor. 

STUDYING 026 TRANSISTORS ON THE CURVE TRACER

I grabbed a couple of 026 transistors from a donor board and watched them on the curve tracer to see what a good transistor looks like. I then removed the 026 transistor in question from the regulator and watched it. I also examined the remaining 026 from the regulator since I want some balance between the two transistors if I have to replace one. 

There was no problem at all with the 026 transistors when I tested them. I checked all the transistors on the board and they all worked properly. Same with the Zener diode and the regular diode. Based on this I instrumented the board so that I could observe how various points behave when the card is both working correctly and misbehaving. See my prior post where I was unable to reproduce the failure mode once the instrumented card was reinserted in the regulator. 

Watched pot syndrome - can't get the 1130 6V power regulator to trip now that it is instrumented

REMINDER OF THE 6V REGULATOR ISSUE

The 1130 logic rails are +3V, +6V and -3V in addition to some special voltages for other purposes. DC is produced in a power supply unit and fed to voltage regulators that produce the three logic voltages. The regulator for +6V is where the issues arise. A circuit breaker on the regulator will trip if the current demand is too high; in addition it will trip if the output exceeds a voltage threshold and triggers an overvoltage clamp. 

If the 1130 has been previously powered up but is turned off for a relatively short period, when it is turned on again, the circuit breaker would trip. I monitored the current being demanded through the regulator and found that it did NOT go to high levels when the problem occurs. I verified that the overvoltage clamp card was not firing. 

When I watched the output voltage, I would see 6V during normal operation but when the circuit breaker trips, the output was around 3V and I could see it oscillating a relatively small amount around that average. 

I replaced all the resistors on the regulator control card and tested some of the transistors on my curve tracer to see if I could find a semiconductor that would fail if it was hot from prior operation. I didn't find anything suspicious. 

INSTRUMENTED FOUR SPOTS ON THE REGULATOR CARD

Circles around four wire taps

I soldered wires to four spots on the regulator card, in order to have four traces recorded on my oscilloscope. I wanted to see how the circuit behavior changed when it went to the 3V oscillation before tripping the circuit breaker. 

MANY POWER CYCLES ATTEMPTED WITH NO ISSUES ENCOUNTERED

I hooked up the scope and powered up the 1130. I then tried for an hour to run it for various lengths of time, power down for a short while and bring it up again. It NEVER tripped the CB nor dropped the output voltage to 3V as it had been. Everything I tried failed. 

This is an example of the watched pot that never boils. At this point I have to assume that something I did while tacking on the four wires to the regulator board is the reason that it no longer failing. That might be a cold solder joint or a cracked trace. I will have to examine the board very very closely at the four points where I added the wires. 

New rotate and tilt tapes for the 1053 typewriter of the System Source Museum's 1130

TAPES SNAPPED ON THE CONSOLE PRINTER OF THE SSM 1130

The console printer (1053) of the 1130 computer is based on the IBM Selectric typewriter. It has a rotating and tilting typeball that moves across the print line while the paper stays fixed in position rolled over the platen (roller). The typeball on the moving carrier will spin or tilt to type one of the 88 characters on the ball. 

A pair of metal tapes connect to the moving carrier and are threaded over pulleys on the two sides of the typewriter - when the pulleys pivot they turn or tilt the typeball regardless of where the moving carrier is sitting along the print line. There are reasons why the metal tapes may break, thus requiring replacement. A misadjusted typewriter can stress the tapes, the tape might have had a crease that leads to metal fatigue breaking, or people turning the typeball by hand might put too much tension on the connectors at the ends of the tapes. 

I suspect that visitors to the museum have played with the typeball one time too many and caused the tape to snap during operation, as the broken tape and its partner had signs of such abuse. It is very tempting to touch the typeball, a natural reaction to curiosity for someone seeing the Selectric mechanism operating for the first time. I think it will be important to add a plexiglass box over the typewriter to protect the ball from someone twisting it. 

BOUGHT NEW TAPES AND INSTALLED THE ROTATE TAPE FIRST

The museum dropped off the 1053 during a recent trip to Florida and I removed the old tapes. I ordered new ones, as fortunately there are still plenty of spare parts available on places like eBay. Today I installed the tape that spins the ball. 

The typeball is mounted on a coil spring to provide rotary tension. One end of the tape is connected to the disc that holds the coil spring. First the type ball is turned to wind up the coil spring, then the typewriter is triggered and manually cycled to the halfway point of a typing stroke, where a lever locks the ball from turning; this keeps the tension on the ball while the tape is installed.

One end of the tape has a T shaped hook that fits into a slot on the coil spring holder disk. It is threaded out of the movable carrier and routed to the left where a pulley is mounted on a lever. A selection mechanism in the typewriter will pivot the lever and pulley to one of eleven positions, which are intended to rotate the typeball left or right up to five steps of about 16 degrees.  

Slot for T shaped connector

The tape goes round the pulley and then is routed from left to right side of the typewriter passing underneath the carrier. As the pulley pivots out or in, it pulls on or releases the tape to cause the typeball to turn.

Left pulley on lever

On the right side of the typewriter frame is a lever with a pulley on the end. The lever pivots to move right or left, which would pull on or release the tape as it pivoted. This pulley pivots between two positions, intended to place the typeball in the middle of one or the other hemisphere, so that it can access one of 44 characters on that hemisphere; traditionally the hemispheres were for upper case and lower case characters. The tape goes around this pulley and then routed back to the left. This end of the tape has an eyelet attached which will hook over a screw on the right side of the carrier. 

Right pulley and shift lever

Once the tape is correctly routed and attached to both the disc and the carrier screw, the typewriter manual cycle can be finished to release the lever so the coil spring can wind up the tape as the ball turns. With the tape in place, the lever on the left frame selects a rotational position for the ball and the right side lever pulls enough to spin the ball 180 degrees to pick which hemisphere to use. 

Tape path

NEXT STEPS - TILT TAPE AND ADJUSTMENTS

Another tape with pulleys is used to tilt the typeball so that it selects one of four rows around the ball. A typeball has eleven rotary positions and four rows on a hemisphere, thus containing 44 characters on each side and 88 for the full typeball. I have to attach the tilt tape on my next visit to the workshop.

The metal tapes can stretch very slightly over their lifetime, plus manufacturing variances mean that the position of the typeball with replaced tapes may not be at the exact same position as it was with the prior tapes just before replacement. As a result, a sequence of adjustments must be made to ensure that the typewriter will tilt and rotate the ball so that the character to be typed is centered vertically and horizontally as the ball strikes the ribbon and paper. This will be done after the other tape is installed.