Stumbled upon defect in the As routines for SPI links

VIRTUAL 2315 CARTRIDGE FACILITY USES SPI LINK BETWEEN PICO AND FPGA

The heart of the Virtual 2315 Cartridge Facility (V2315CF) is a unit that houses a Lattice Field Programmable Gate Array (FPGA), enough RAM memory to hold one disk's worth of data, and a Raspberry Pi PICO. The PICO mainly manages the user interface while the FPGA mainly implements the disk drive emulation and controls the RAM. 

The PICO implements a Serial Peripheral Interface (SPI) link to the FPGA, with the PICO acting as the master. The PICO exchanges two bytes with the FPGA as a transaction, the first byte having the command from the PICO and the second byte recieves one byte from the FPGA based on that command. 

For example, a transaction code of x88 is a request for the FPGA to return one byte of data from the RAM. The address for memory access was previously transmitted by the PICO as a set of three x05 commands that set the 24 bit RAM address. When the PICO is writing back the contents of a virtual 2315 disk cartridge to the microSD card file that stores the data, it sets the RAM address for each of the 1,624 sectors, reads 642 bytes sequentially from the FPGA and then writes the sector's 321 words of data to the file. 

PICO LIBRARY PROVIDES ROUTINES TO CONTROL THE SPI LINK

Exchanging data over an SPI link is done with a few library calls provided for the PICO. The caller first starts the transaction by asserting the SPI chip select line, then calls the  spi_write_read_blocking function giving it the address of the buffer holding the outbound data to send to the FPGA, the address of a buffer to receive the data coming back from the FPGA, and a count of how many bytes to send. The caller then deasserts the chip select signal. 

The transfer proceeds with the SCLK signal being alternated for the number of bits being transmitted - in this case, that is 16. The MOSI signal sends the bits serially from the PICO to the FPGA. The MISO signal comes from the FPGA and provides the inbound data that is returned to the caller. Thus, an SPI transaction is an exchange. 

The routines that I inherited from the RK-05 Emulator had defined the outbound and inbound buffers as arrays of uint8_t, an eight bit unsigned value (byte). The length of each was 2, since the intent is to exchanges two bytes between master and slave in a transaction. There was also a 2 byte array of uint8_t declared in the routine but never used. 

EXPERIENCED ERRATIC RESULTS AND LOOKED CLOSELY AT THE FUNCTION CALLS

When my logic tried to write back the contents of RAM to the microSD card in the virtual 2315 cartridge, I would get some correct results and some corrupt data returned. I looked for every possible cause and investigated, but wasn't getting anywhere.

The routines that I inherited from the RK-05 Emulator had defined the outbound (OUTBUF) and inbound (INBUF) buffers as arrays of uint8_t, an eight bit unsigned value (byte). The length of each was 2, since the intent is to exchanges two bytes between master and slave in a transaction. There was also a 2 byte array BUF of uint8_t declared in the routine but never used. 

After adding code to verify that the last bits had been exchanged before deasserting the chip select, yet seeing no improvement, I was considering slowing the SPI clock just in case I was getting errors due to hardware signal issues. However, this didn't make sense because at startup the same link transferred 1,624 sectors of data down from the microSD card file to the RAM without a single failure. 

I happened to remove the unused variable BUF from the routines but when I next tested, the corruption rate increased to almost 100%. That was unexpected. 

MY ASSUMPTION WAS THAT THE FUNCTION WAS RETURNING TOO MANY BYTES

Perhaps the issue was the function, which might be overrunning the buffers I passed it. The count was a hard coded integer 2, which should be what controls this completely, but perhaps it was instead returning integers, a longer data type than the uint8_t bytes in the arrays. 

I increased the size of OUTBUF and INBUF to four from two and tried again. The code worked flawlessly. Somehow it was stepping on the stack and causing issues until I added extra room in teh buffers. The code is below. 

uint8_t read_write_spi_register(uint8_t reg, uint8_t data)

{

    uint8_t out_buf [BUF_LEN+2], in_buf [BUF_LEN+2];

    uint8_t buf[2];

    out_buf[0] = reg;

    out_buf[1] = data;

    cs_select();

    spi_write_read_blocking (spi_default, out_buf, in_buf, 2);

    cs_deselect();

    return(in_buf[1]);

}

The cs_select and cs_deselect calls are inline routines:

static inline void cs_select()

{

    asm volatile("nop \n nop \n nop");

    gpio_put(PICO_DEFAULT_SPI_CSN_PIN, 0);  // Active low

    asm volatile("nop \n nop \n nop");

}

I still don't understand what is going wrong but increasing the buffer fixed the problem. This must be why the original programmer left the spurious BUF variable in the routines, something they found solved a problem even if the root cause wasn't understood. 

Testing a write after conditioning the Sector Marker signal for the Virtual 2315 Cartridge Facility

FIXES MADE BEFORE DIAGNOSTIC TESTING RESUMED

The new Signal Conditioner board is in place and ensuring that glitches on the -Sector Marker signal receive by the IBM 1130 logic won't lead to incorrect results. Since that signals looked really good now, I expected everything to work properly now. 

LOGIC ANALYZER SHOWS THAT THE SECTOR MARKER ISSUES ARE GONE!

I recorded the same signals that showed me that doubled edges were caused by the ringing of the signal and led to a spurious termination of the write after only half a sector - 5 milliseconds instead of 10. This time, everything was perfect and the write continued for the full 10 ms. 

The flipflop logic that blocks every other 5ms raw sector marker pulse (+Alt pulse and -Alt pulse) are alternating just as they should, thus the -Sector Marker signal at 5ms is blocked and only the second one is passed through as -4 Sector Pulses and that terminates the write. The probe lead had fallen off the raw sector marker signal, which would have been signal 5 (green) in the trace, but we know the pulse comes at the 5ms point in the write operation. 

ALMOST PERFECT TEST OF WRITE IN VIRTUAL MODE ACHIEVED

This time I did achieve a good capture of the data I wrote to the virtual 2315 cartridge, including the entire sector. I set up a known pattern in a memory buffer in the 1130 and issued an XIO instruction to write to a given sector of the disk (cylinder 0, head 0 and sector 1). I issued an XIO to read in the same sector to a different buffer, confirming that the contents matched what I had written. An offline view of the mini 2315 cartridge using the Showsector.exe program I provided confirmed the content of the sector as well. 

It was not flawless, however, as there was a single bit shift in the data for the last 48 words of the 321 in the sector. If you look at the data from the sector as it was extracted with Showsector.exe, you will see starting at word 0x111 the data was recorded as 0x0224 when it should be 0x0112. Every word from that point forward is the same as the data I tried to write out except for the one bit shift, ending with 0x0282 which was intended to be 0x0141. 

The data that I wrote out was almost the same as in my previous tests, however, words 2 and 3 were changed to 0x2600 in the buffer where they had been 0x0002 and 0x0003 in my original test. Thus I wrote what we see above, at least up to word x111 where things went slightly astray.

I had previously identified that the signal -Write Clock Phase B which comes from the V2315CF into the 1130 might cause this kind of issue. The signal will advance the shift register after each clock. A spurious double edge would cause the register to shift the data by one bit. I had finessed the signal termination to really ameliorate signal issues but they were not ideal. It does appear that we had this one error in about 6,500 clock intervals during the write. That is quite good but not good enough.

The solution is the sledgehammer! One board has five channels of signals that it can process. The -Write Clock Phase B signal is definitely worth slamming into shape, so it is going to be processed. On my next visit to the shop I will be wiring the other four signals I identified so that they pass through the Signal Conditioner board. I don't anticipate any errors after I sledgehammer the signal into shape. 

Testing signal sledgehammer for the Virtual 2315 Cartridge Facility

FILTERING OUT RINGING AND UNDERSHOOT OF SIGNALS

The Virtual 2315 Cartridge Facility (V2315CF) sits in between the internal 2310 disk drive and the IBM 1130 computer's logic gates, either relaying signals between the two end points or generating a signal in place of the one that the disk drive might produce. I found that in virtual mode (disk drive not turned on, the V2315CF simulating it entirely), certain signals exhibit an undershoot to under -3V and quite a bit of ringing, when the nominal voltage levels are +5V and 0. 

The ringing caused the 1130 logic circuits to see the falling edge of the signal as multiple falling edges, thus confusing the logic and creating incorrect results. The path from the V2315CF to the logic gate of the 1130 is complex, with transitions between PCB traces, ribbon cables, more PCB traces, twisted wire cables, additional PCB traces, a pin on a backplane and then wiring in the logic card. There are many impedance transitions and places where signals can be distorted - made worse by a very fast falling edge because of the modern FPGA and logic gates in the V2315CF which switch so quickly. 

SIGNAL SLEDGEHAMMER IMPLEMENTS SCHMITT TRIGGER FOR HYSTERESIS

I designed a circuit that will employ hysteresis, so that the output of the circuit changes from logic high (+3V) to logic low (0V) or back at different thresholds depending on the direction of the change. That is, the output won't switch to logic low until the incoming signal from the V2315CF drops below 0.85V and then it won't switch back to logic high until the incoming signal rises above 4V. Any ringing in between these levels is blocked from the output. 

TESTING ONE CHANNEL WITH THE SECTOR MARKER SIGNAL

I initially only located and cut the wire that carries the signal causing me the most problems, -Sector Marker which produces a falling edge for 160 microseconds once every 5 milliseconds. This marks one eighth of a rotation of the disk platter, used by the logic circuit in the 1130 to determine where the four sectors begin on one rotation. 

A peculiarity that leads to the incorrect operation of the 1130 logic is that the disk drive reports every one-eighth of a rotation but the 1130 implements only four sectors per rotation. Therefore, it has to ignore every other sector marker pulse. The ringing in the signal from the V2315CF was causing the logic to see two sector marker pulses rather than one, so it assumed it had already moved a full one-quarter rotation. The signal conditioner should ensure it counts correctly and sees only one falling edge for every -Sector Marker pulse. 

WATCHING THE RESULT OF THE SIGNAL CONDITIONER

I put a scope on the input pin of the 1130 logic that receives the -Sector Marker pulses, to observe how the signal was shaped by my conditioner. I performed a write to a target sector, which previously had been aborting halfway through the sector due to the false edge count. The goal was to see it complete the sector write and the signal to look as close to perfect as possible on the scope.

Rising edge

Falling edge

The yellow trace at the topp is the output from the V2315CF main unit before it runs through all the cabling to get to the 1130 circuits. The purple trace is the signal received at the 1130 after the Signal Conditioner finishes with it. The blue trace is the output of the first inverter inside the 1130, showing that it also produces a single edge. 

By comparison, here are the same three signals without the Signal Conditioner aka sledgehammer - 

WILL ADD ADDITIONAL SIGNALS TO THE SLEDGEHAMMER

I picked four other signals that depend on falling edge detection in their operation in the 1130 logic, since they had the potential to trigger incorrect operation of the logic. Some won't malfunction even if they see multiple falling edges close together, such as the -Index Marker pulse that arrives once per rotation to reset the sector number counter to 0. Two edges on that signal wire still generate the same sector number in the counter. 

The incoming clock generated by the V2315CF, -Write Clock Phase B, is used by the 1130 logic to output the pulses (-Write Clock and Data) that get written on the disk platter to record both clock pulses and data bits that have a value of 1. The design of the disk produces no pulse for a data bit if its value is 0, so the only way to determine whether we have a 0 data bit value is to depend on the timing signals coming from the disk drive (or V2315CF in this case).

When reading a sector from disk, the V2315CF generates the clock and data pulses that would be extracted by the disk drive, however the data actually comes from our mini 2315 cartridges and sits in RAM on the V2315CF. Therefore, the V2315CF generates the -Read Clock and -Read Data pulses that would be seen if the disk drive heads were actually reading data from a full sized 2315 cartridge inserted in the 2310 disk drive. Those two signals are given a channel each on the conditioner board.

A feedback signal from the disk drive mechanism, -Access Ready, is used by the 1130 logic to recognize when a step of the disk arm has been accepted and when it completes. The 1130 converts a request from the program to seek a number of cylinders into a chain of small steps, a single 1 cylinder step for an odd count preceeded by 2 cylinder steps to achieve the full count. False edges might cause the 1130 to believe a step has completed when it has not, throwing off the total movement. 

I protect against false edges in the V2315CF where I watch over the arm movement, but the disk drive and the 1130 are fully in control of arm movement when we are in real mode (disk drive is operating with data coming from mini 2315 cartridge). The V2315CF has to shadow the arm movement so that it can present the data from the correct cylinder when a read or write is done - we change the RAM address based on the current arm location between cylinders 0 and 202. 

The terminator resistors for all the signals passing through the V2315CF must be pulled up to 5V in order for the 4V threshold of the signal conditioner to recognize a logic high, even though the disk drive circuits generally use 3V as logic high. This is one on the terminator board of the V2315CF. 

Another complexity of the V2315CF is my desire to support two modes - real and virtual. The intended use is real mode, where the 2310 disk drive inside the 1130 is spun up. The disk drive is passing along the real -Sector Marker and -Index Marker pulses, with the V2315CF just observing or shadowing them. The disk drive arm moves when the 1130 commands a seek to a different cylinder, with the V2315CF shadowing the end location. 

Virtual mode allows operation with the disk drive left turned off. The -Sector Marker and other signals are all produced by the V2315CF itself. It responds to the seek requests and generates -Access Ready as well. Thus, -Sector Marker, -Index Marker, and -Access Ready can come from either the V2315CF or the 2310 disk drive depending on mode.

The signals like -Sector Marker and -Access Ready are passed along through an electronic analog switch on the 2310 Interface Board which merges the wires from the disk drive, 1130 and V2315CF. In real mode, the switch connects the disk drive output to the 1130 input, while the V2315CF leaves its output line undisturbed. In virtual mode, the switch is off and the V2315CF is pulling the line down to logic low to implement those signals. 

I need the signal coming into the 1130 to work properly in both modes - real and virtual - with different electronics producing the signal in each mode. This made it impossible to select terminator resistors for an impedance that worked correctly in both modes - some choices worked for one mode but not the other. 

When these signals are conditioned by the new board, it will be time to rerun all the function tests for the V2315CF, once in real mode and another time in virtual mode. That plus the correct completion of the IBM 2310 disk diagnostic program constitute an adequate test of the V2315CF. 

Preparing wiring of the signal sledgehammer into the IBM 1130

SCHMITT TRIGGER TO CLEAN UP MESSY SIGNALS ENTERING THE 1130

I have some signals that are generated in the Virtual 2315 Cartridge Facility (V2315CF) that I built and they are fed into the disk drive controller logic in the IBM 1130. I had limited ability to change the 1130 circuitry and the connections into it. The main hardware in the V2315CF generated the signals in a FPGA and drove them out with an open collector driver and a terminator board. 

Typically the terminator board had a pullup and pulldown resistor pair to create the logic high voltage and an appropriate impedance to match to the destination. The details of the Solid Logic Technology (SLT) circuitry in the 1130 is odd by today's standards and often would not work properly when fed with the standard resistor pairs in the terminator - it might not detect a logic low reliably as one example. Thus in some cases I couldn't create a good impedance match and also get good behavior from the 1130 circuitry. I would experiment until the results were good.

When the impedance is not well matched - a difficult condition to achieve since the path from the V2315CF to the SLT input pins involved multiple mechanisms with differing individual impedances - the signal would have poor characteristics where it entered the SLT circuits. Signals may ring with substantial swings, thus swinging up and down between levels that would be a logic high and a logic low to the 1130 circuit. Sometimes the signal had an undershoot to a very negative level. 

Parts of the 1130 logic was designed around edge detection - a falling edge would trigger an action. If that action changed a counter or flipped between states, then the ringing could act like multiple edges when the intent was to have just one edge. What I needed was a way to ignore all the junk such as the ringing that occurs as part of a falling edge sent from the V2315CF, so that the 1130 sees a single clean edge. 

A Schmitt Trigger is a form of hysteresis that controls whether a signal becomes a logic low or logic high, using different thresholds for the transitions in each direction. If we don't consider a signal to be logic low until the voltage drops below 1V, but won't change it back to a logic high until the voltage rises to 4V, then all the oscillations between those voltage thresholds are ignored. 

I designed a signal conditioner that I nicknamed the signal sledgehammer because it forces a very ugly incoming signal to be a clean nice output signal. It uses an operation amplifier to force a very clean dramatic snap between logic low and logic high. Normal logic circuits can enter a grey area when the voltage is above the maximum logic low voltage but below the minimum logic high volage. This forbidden zone may produce an output that is not clearly high or low in normal circuits. Using an op amp meant that when it switched, it swung fully to the extreme. 

I used a voltage reference chip and appropriate resistors with the op amp to set it so that it would flip to logic low when the input falls down past 1V and won't flip back until the input rises up past 4V. Much more of a hysteresis spread than a typical Schmitt Trigger logic gate. I then used other circuitry to produce an SLT type output that would swing fully to its logic high (3V) or logic low (nearly 0V) following the op amp.

INTERCEPTING THE WIRING OF THE SIGNALS TO BE CONDITIONED

IBM connected the 1130 disk driver logic circuits to the internal 2310 disk drive mounted in the computer using a cable with about 20 twisted wire pairs, with SLT style connectors on each side. The SLT connector, used on the circuit cards, is a 2 column by 12 row grid of pins. The backplanes into which SLT cards plug are composed of many SLT connector slots. One of the slots in the backplane where the disk driver logic is implemented is where the cable is connected. The disk drive electronics, also built with SLT, has a smaller backplane and one of its slots is used to connect the other end of the cable. 

The V2315CF sits between the 1130 and the internal disk drive, so that it can make a mini 2315 cartridge appear to be spinning on the internal disk drive on a real 2315 disk cartridge. A 2315 cartridge has a single 14" disk platter inside the plastic enclosure. The cartridge is inserted into the 2310 disk drive and read/write heads of the disk drive will read or write data on the platter and provide it to the 1130 computer memory. 

A printed circuit board named the 2310 Interface Board has the cable from the 1130 plugged into it, rather than to the disk drive. Another cable comes from that PCB and is plugged into the disk drive. A pair of ribbon cables come from the 2310 Interface Board and connect to the main unit of the V2315CF. That unit has an FPGA, a Raspberry Pi Pico processor, RAM and a lot of interface circuitry. 

Some signals come from the disk drive and are passed directly on to the 1130. Some signals come from the 1130 and are passed on to the disk drive. The V2315CF will snoop on those signals as necessary to stay in sync with the disk drive. However, signals that would have come from or gone to the read/write heads of the disk drive are instead processed fully in the V2315CF, which creates signals that appear to come from the disk drive but are actually based on data in the mini cartridge. 

A complication is that the V2315CF can operate in one of two ways. It can completely simulate the internal disk drive, while the disk itself remains powered down. More usually, it works cooperatively with the internal drive. The disk drive spins the platter and produces timing signals (e.g. Sector Marker) that indicate where the platter is in respect to the read/write heads at any moment in time. In the other mode, the Sector Marker is generated within the V2315CF. 

Requests from the 1130 to move the read/write heads to one of the 203 tracks (cylinders) on the platter will have the disk drive move the arm with the heads. The V2315CF has to snoop on this so that it knows which track the heads are at. When the 1130 reads a sector of data, the V2315CF has to grab the data from its RAM at the location corresponding to that cylinder, so the two must stay in sync. In the other mode of operation, the disk drive does not move its arm and only the V2315CF is involved in the movement request. 

The cable from the 2310 Interface Board to the 1130 runs through the inside of the 1130, enters compartment C1 of logic gate A and is plugged into a slot of the backplane in the compartment. Its twisted wire pairs were held together with waxed string lacing then covered by a sheath of electrical tape. I can't easily intercept a signal once it enters a slot in the SLT backplane, so the cable is my point of interception.

MOST IMPORTANT CONDITIONING WAS THE SECTOR MARKER SIGNAL

My biggest challenge is when the V2315CF is in the mode where it fully simulates the disk drive. The signal -Sector Marker is produced once every 5 milliseconds during the 40 milliseconds it takes for a single rotation of the 2315 disk platter. For the 1130, IBM decided that it needed a full 10 milliseconds for a reasonable sized sector of information. Therefore the 1130 logic will ignore every other sector marker pulse. 

When doing a write of a sector of data to the V2315CF, the 1130 logic will stop the writing process when the second sector marker pulse arrives marking the end of the logic sector. The falling edge of the sector marker pulse (160 microseconds long) alternates some logic, to alternate whether the sector marker pulse is passed along or blocked. 

Because of the ugly signal arriving at the 1130 logic, the logic might see a sector marker pulse falling edge as two edges instead of one. That would mean that the write ends after only 5 ms since the blocking logic malfunctioned. 

FINDING THE CORRECT WIRE PAIR FOR THE SECTOR MARKER SIGNAL

I opened up some of the electrical tape sheathing, cut the lacing and had access to the individual twisted wire pairs near the end of the cable near where it will enter the compartment of the 1130. Using a signal tone tracer I attempted to locate the wire pair corresponding to the -Sector Marker signal. It wasn't definitive, but I also used a continuity tester hooked across the two connectors at the ends of the cable. I did locate the wire pair and cut it.

PREPARING TO SOLDER IN THE CONNECTORS TO THE SLEDGEHAMMER

I created two short twisted wire pairs that would connect to the two cut ends of the signal wire, as well as continuing the ground in the other wire of each pair. They were hooked to two connectors which will plug into the signal conditioner PCB that I had mounted on the side of logic gate A near where the cable enters the compartment. One of the short pairs goes to the input connector of the PCB, bringing the signal coming from the V2315CF. The other short pair goes to the output connector, bringing the cleaned up signal into the 1130 logic. 

Next session, I will solder these short pairs to the cut ends and insulate everything, then plug the input and output connectors to the PCB. I can then fire up the 1130 and watch the signal on an oscillocope to confirm that it looks clean. As well, I will watch the write process to verify that the full sector is written, not cut short by the defective sector marker blocking. 

Installation of signal sledgehammer into IBM 1130

FILTERING OUT RINGING AND UNDERSHOOT OF SIGNALS

The Virtual 2315 Cartridge Facility (V2315CF) sits in between the internal 2310 disk drive and the IBM 1130 computer's logic gates, either relaying signals between the two end points or generating a signal in place of the one that the disk drive might produce. I found that in virtual mode (disk drive not turned on, the V2315CF simulating it entirely), certain signals exhibit an undershoot to under -3V and quite a bit of ringing, when the nominal voltage levels are +5V and 0. 

The ringing caused the 1130 logic circuits to see the falling edge of the signal as multiple falling edges, thus confusing the logic and creating incorrect results. The path from the V2315CF to the logic gate of the 1130 is complex, with transitions between PCB traces, ribbon cables, more PCB traces, twisted wire cables, additional PCB traces, a pin on a backplane and then wiring in the logic card. There are many impedance transitions and places where signals can be distorted - made worse by a very fast falling edge because of the modern FPGA and logic gates in the V2315CF which switch so quickly. 

SIGNAL SLEDGEHAMMER IMPLEMENTS SCHMITT TRIGGER FOR HYSTERESIS

I designed a circuit that will employ hysteresis, so that the output of the circuit changes from logic high (+3V) to logic low (0V) or back at different thresholds depending on the direction of the change. That is, the output won't switch to logic low until the incoming signal from the V2315CF drops below 0.85V and then it won't switch back to logic high until the incomign signal rises above 4V. Any ringing in between these levels is blocked from the output. 

FINISHED THE BOARD WITH THE CORRECT PART I SHOULD HAVE ORDERED ORIGINALLY

Part of the design of the Schmitt Trigger uses an operational amplifier with a reference voltage of 2.5V generated by a special chip (LM285D) on my board, with the components of the operational amplifier chosen to set the low and high thresholds I mentioned above. However, when I ordered all the components to build my board, which provides five copies of the signal conditioner to handle five signals, I inadvertently ordered the LM285D chip that sets a 1.2V reference voltage, not 2.5V.

This caused the circuit to switch on at a voltage below 3V and it would not switch off even at 0V on the input pin, because the circuit was offset by the difference from my design spec. I had to order the correct part and wait for it to arrive from Digikey. I used a bench power supply to deliver 2.5V in order to validate the operation of the circuit once the correct part is installed. Once it arrived, I soldered the correct voltage reference chip on the board and went back to my testing setup. It passed with flying colors on the workbench, thus it is time to hook it into the signal path between the IBM 1130 and the V2315CF. 

CHOSE MOUNTING LOCATION AND INSTALLED THE BOARD

The main circuitry of the IBM 1130 is built into two structures (called gates) that each hold three compartments of Solid Logic Technology (SLT) cards. A backplane in each compartment hosts the slots into which the cards are plugged; some of the slots instead have a cable connector plugged in to route signals out of the comparment. The two gates are hinged at one side, so that they can be swung out of the 1130 cabinet for servicing, as well as swung apart. Normally the two sit parallel to each other and parallel to the front of the machine. 

Logic gate A is the rearmost of the two, holding compartments labeled A1, B1 and C1. Compartment C1 is closed to the hinge. The circuitry that implements the disk drive controller is installed in gate A, compartment C1. A cable runs from this location out to where the internal 2310 disk drive is mounted, normally plugged into the back of the drive. 

I built a mounting plate from acrylic and fasted that to the side of logic gate A at the hinge point. The printed circuit board with the signal sledgehammer is bolted onto that plate. Wires run from the PCB to the terminal strip in the 1130 where power is distributed, providing +12V and ground. The PCB also has two five pin connectors on it, one for inputs and the other for outputs of the five inbound signals to the 1130 that can be conditioned by this PCB.

The cable from the 1130 that is now connected to the 2310 Interface Board of the V2315CF runs from gate A compartment C1 out by the hinge, wrapped in electrical tape holding the twisted wire pairs of the signals together. I need to intercept the signals from that cable, cutting the wire and routing it through the signal sledgehammer. 

I prepared for the task by unwrapping a three inch segment of the cable to gain access to the twisted wire pairs inside. Once I identify a signal that needs to be conditioned, I will cut the white wire and expose some conductor of the black wire of the pair. The black wire is a ground signal and white carries the actual signal. I will add a short bit of twisted pair to the V2315CF side, another short bit of twisted pair to the 1130 side, and put pins on the two white wires of the new pairs. The black wires of the new pairs are joined together and joined to the exposed black wire of the original cable. 

FINDING THE WIRE PAIRS FOR THE SIGNALS TO BE CONDITIONED

I have to find which of the 20 or so wire pairs in the cable are associated with the inbound signals from the V2315CF that need to be conditioned. I purchased a signal tracer that injects a tone into a wire pair which is detected by a probe at the other end. I should be able to disconnect the cable from both the 1130 and the V2315CF, then inject a tone into the pin on the connector for each signal I need to trace. The wire can be identified, then cut so that I can insert the signal sledgehammer into the signal path. 

Once the tracer arrives, I will find the wire for the -Sector Marker signal that caused the most problem, wire it to the input and output connectors and then test the behavior after conditioning. 

Assembled and tested the Teletype ASR-33 paper tape reader power supplies

MISSING PAPER TAPE READER POWER SUPPLIES CAN IMPACT A TELETYPE RESTORATION

The paper tape reader on the left side of the model 33 ASR teletype has a power supply that feeds it 160VDC for the magnet that advances the tape. In addition, if the paper tape reader is the automatic type rather than the manual version, it has a 48V DC supply and a control relay included alongside the 160VDC circuits. 

While the power supply for the paper tape reader can be mounted up in the body of the teletype, often it was mounted down inside the pedestal stand. That freed up room for the various communications options such as a telephone dialer. It is not uncommon for someone to pick up an ASR-33 for restoration but not receive the pedestal stand. The teletype will work just fine sitting on a tabletop, except that the reader power supply will be missing, disabling the reader capability.

DESIGNED A SUBSTITUTE POWER SUPPLY FOR WHEN THE ORIGINAL ONE IS MISSING

I designed a printed circuit board based on the schematics from Teletype so that it could be connected into an ASR-33 and allow the reader to operate. The circuit is the same, although some of the components are compatible but not identical. I didn't try to match the shape and mounting exactly, since I didn't have good measurements available. 

The ASR-33 models come with a cable that has a 15 pin Molex socket on the end, meant to plug into a 15 pin Molex receptacle on the power supply card. I build my card with a 15 pin Molex connector that is hanging from the wiring coming from my card, allowing the owner of the teletype to just plug this in and have it work just like the original. 

SOLDERED TOGETHER BOTH TYPES - AUTOMATIC AND MANUAL READER VERSIONS

The manual paper tape reader version of the model 33 is operated by moving a switch on the paper tape reader to the Start position, which begins the process of reading the tape until it runs out, jams or the operator turns the switch to the Stop position. The automatic version supports a version of the switch on the tape reader that has a momentary Start position, a 'Run' position, and a momentary Stop position. In addition, if the teletype receives an XON (DC1) character, it will start reading the tape. If it receives an XOFF (DC3) character or a WHOIS (ENQ) character, it stops the paper tape reading process. 

Making a manual reader version of the power supply is done by omiting components necessary for the automatic function, such as a control relay, and wiring up fewer pins on the 15 pin Molex connector. Otherwise, the same board is used. I built two supplies for an automatic reader model and two supplies for a manual reader model, in support of someone restoring teletypes for a museum. 

BENCH TESTED THE SUPPLIES

My first test was of the section that implements the relay for the automatic reader version. This is powered by 48VAC, rectifies it with a single diode and that closes the relay. Two contacts on the relay are closed when the coil is activated, which I could check with a continuity tester. The relay began chattering, not fully closing. I finally noticed that I hadn't connected a third wire from the board to one side of the 48VAC. 

One side of the incoming AC goes through the rectifier and on to the relay coil. The other side of the relay coil is only connected to the other side of the AC input when the relay is activated. The filter capacitor across the output of the diode, however, is always connected to the other side of the incoming AC, thus there are three connections to 48VAC not two from the board. 

That caused the chattering because nothing was smoothing the 30 pulses per second of 48VDC through the coil. I added the third connection, so that one side of 48VAC went to the diode and the other side went to the other two connections; this would activate the relay with a smoother DC voltage. I tried this wiring and the relay reliably activated and released.

I picked up a 115VAC relay to act in place of the distributor trip magnet that fires off the transmission of a character that has been sensed by the paper tape reader. I also used it to act in place of the feed magnet that pulls the tape forward one position since I don't have a teletype on hand for testing. 

The process of reading begins with the circuit energizing the distributor trip magnet. The distributor in the teletype converts a 6 bit character from parallel into a serial stream with a start bit, the six data bits and the stop bits. This is done with a rotary mechanism. At the end of the rotation, the distributor activates a switch that closes the circuit to power the feed magnet in the tape reader. This causes the tape to advance one position, after which the distributor trip magnet is again energized unless we are stopping the reader. 

I used the inexpensive relay I bought to observe the operation of the circuitry, verifying that closing a switch (simulating the end of a rotation of the distributor) will energize the relay that is simulating the feed magnet. This section is driven by the circuit that rectifies the 115VAC into 115VDC to activate the feed magnet. 

Testing the 115VDC circuit, I saw the current limiting resistor emit a burst of light as a thin slice vaporized. This is a 22 ohm resistor in the input line to the bridge rectifier, whose peak load is when the AC mains are first turned on and the filter capacitor of the bridge rectifier is charging up. The surge is very brief but will definitely blow out a normal resistor type which is what I used. Wirewound resistors are typically used as they can absorb the very brief surge without failure. 

I will install 5W wirewound resistors on the boards instead of the surface mount part I originally used. There is a good surface mount part that I could have put on there, but it would require a new version of the PCB since it is larger than the footprint of the original part I used. The original was a 16W component with a very large SMD size already, but less than the wirewound SMD part requires.   

For the automatic version of the power supply I operated some switches that stand in for the momentary Start, the momentary Stop, as well as the XON, XOFF and WHOIS detection switches. I checked that the relay on the power supply board activates and turns off appropriately based on those simulated switches. I also verified that the distributor trip magnet (the relay simulating it) activates when the control relay is turned on and does not activate when it is turned off. 

Once the functions were validated and the voltages and currents verified, and the new resistors installed, it will be time to pack these up and ship them off to the restorer who is waiting for them to make use of the paper tape reader. 

Built my signal sledgehammer for the Virtual 2315 Cartridge Facility and gave it a test

SIGNAL ISSUES TO BE ADDRESSED BY THE CONDITIONER

Certain signals produced by the Virtual 2315 Cartridge Facility (V2315CF) that drive logic in the IBM 1130 have significant ringing, bounce and undershoot problems due to the nature of the wiring that ordinarily connects those 1130 circuits directly to the internal 2310 disk drive in the computer. The V2315CF sits in between the two and generates some signals in lieu of the disk drive. 

Logic in the IBM 1130 is often designed around edge detectors, so that a ringing signal can pr,esent itself as more than one edge which causes the 1130 disk controller logic to malfunction. I have particularly observed this when the V2315CF is operating in virtual mode; the -Sector Marker signal exhibits the signal quality issues because it is generated by the FPGA and fast gates in the V2315CF, unlike real mode where the 2310 disk drive creates those signals. 

DESIGN OF THE CONDITIONER

The conditioner is designed to employ hysteresis, holding the state of the output steady during the perturbations of the bad input signals. It will not respond to an input level above 0.85V when the input was logic high and begins dropping. Once it hits that low threshold, the output switches to logic low state and won't exit that condition until the input signal climbs to at least 4V. All the undershoot and ringing that happens with the input signal is masked, giving a single clear falling edge to logic low. 

I used an operational amplifier circuit as the Schmitt Trigger, with the components chosen to cause it to switch to logic low when the input goes below 0.85V and to switch to logic high only when the input goes above 4V. The op amp is then buffered by a MOSFET and the output of the MOSFET is inverted by a bipolar transistor to create an output signal at the appropriate voltages for the 1130 circuits receiving the signal.

I created a board with five channels - five identical signal conditioning circuits as described above - plus an onboard voltage regulator and a reference source at 2.5V for the Schmitt Trigger. This is intended to be mounted on the end of logic gate A in the 1130, close to where the signal cable enters compartment C1 housing the disk controller logic. I will cut the wires of the signals that need conditioning, pass them through a channel on the board and then into the 1130. 

SOLDERED TOGETHER THE BOARD AND COMPONENTS TODAY

I had received the parts from Digikey and the printed circuit boards from JLCPCB thus I did the assembly today. After soldering all the parts onto the board, it was ready for testing. First I would test its functioning on the bench and then install it to condition the -Sector Marker signal and observe the behavior of the 1130 logic. 

TESTED THE CONDITIONER - DISCOVERED I ORDERED THE WRONG PART

I set up a bench test to verify the hysteresis behavior, varying the input voltage and observing the output line. I saw it switch too early, well under 4V, and then refuse to switch back to off even when the input voltage was zero. 

I checked the voltages on the board and discovered that my voltage reference, which should establish a 2.5V level for the op amp schmitt trigger behavior, was sitting at 1.2V. A careful read of the package made clear that this was in fact a 1.2V reference part, and that was what I had ordered somehow. 

I put in an order for the correct part but this means I have to wait about four work days before I can finish the validation test and then put the conditioner into the circuit between the IBM 1130 and the V2315CF. 

USED TIME IN THE SHOP TO FINISH THE CONNECTORS FOR MY ASR-33 POWER SUPPLIES

I wired up two connectors for the automatic reader version of the Teletype model 33 paper tape reader and two connectors for the manual reader version of the model 33. These are ready to solder onto the boards when I build them tomorrow. Digikey delivered my parts today and the DHL truck is wandering around in my area with the PCB shipment.

FURTHER GLUE WORK ON THE V2315CF CABINET

I added braces and set up the holes for screws to allow the top panel to be removed for servicing. I have to drill the matching holes in the top plate and drill several mounting holes in the bottom plate to complete the construction so that the cabinet is ready to attach to the 1130 system permanently.