Result of debugging the initial issues found testing the Virtual 2315 Cartridge Facility in virtual mode on the IBM 1130

INVESTIGATING ANOMALIES OBSERVED DURING INITIAL TESTING

My first testing of the Virtual 2315 Cartridge Facility (V2315CF) hooked to the IBM 1130 resulted in observing a few incorrect results. The File Ready lamp on the V2315CF lit all the time, when it should only light when the simulated disk becomes ready for access. The Fault lamp on the V2315CF lights as soon as the simulated disk becomes ready, but no error condition should have occurred. A phantom bit (bit 10) is injected into the IBM 1130 memory bus when I sense the device status of the disk controller. 

FILE READY LAMP INVESTIGATION

Looking at the debug console output made it clear that the PICO did not believe it had turned on the ready lamp, but it was illuminated. I was preparing to troubleshoot, tracing signals, as well as instrumenting a version of the PICO software with more outputs, when I decided to swap the PICO from the other unit I built since it was a quick test. The Ready  lamp was not lit! 

I hooked up the original PICO and loaded the code base I had on my laptop, but it continued to produce the erroneous Ready lamp. Therefore there is either a damage on the PICO, which was a new one I substituted for the ruined one, or my load file is out of sync with the working code on the final PICO. 

DISK FAULT LAMP INVESTIGATION

I found that the terminator board had a couple of lines I needed that were not fully populated with the paired resistors used for all the others. I moved a couple from lines I don't utilize and soldered them onto the terminator. The disk fault issue disappeared. 

The terminator uses a pair of resistors one pulling up to +5V and one pulling down to ground. Together they set the idle voltage at 3.3V and yield an effective resistance of 120 ohms. These were selected for use with the RK-05 drive and DEC controllers. IBM tends to terminate their long transmission lines with 95 ohm impedance, thus I may need to tweak the resistor values if I see issues caused by ringing or other noise on the lines. 

The most sensitive lines will be those that carry the written data to the disk drive as they are sampled by the drive at specific edges of the clock coming from the drive - thus I might get data corruption if the signals are too marginal. I will scope the lines at a later date to check the suitability of the current terminator and make changes if necessary. 

PHANTOM BIT INVESTIGATION

The phantom bit 10 appears to be in the accumulator register, where it affects computing effective addresses as well as storing the device status from a Sense DSW. I will shoot this later, once the V2315CF is debugged. 

DROP OFF OF 1132 AND 2501 FOR RESTORATION

Today the line printer and card reader for the VCF 1130 arrived. I opened them up for a rapid first assessment. They don't appear too bad, thus I have reasonable hopes that I can restore them both to operation. 

The card reader has a large gear which had plastic teeth attached to an aluminum disk. The plastic had become so brittle that it was already cracking just from movement and breaks further when touched. I will have to make a replacement gear. This operates the joggle plate on the card output which moves the stacked cards back and forth; not a very timing critical part.