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.