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.