1130 SYSTEM NOW SOLID - POWER AND MEMORY ISSUES RESOLVED
When I was last testing the cartridge facility, I began to encounter parity stops due to continued degradation of traces inside the core memory stack of the system. I had previously added wiring to bypass broken traces on several sense lines, but another cropped up. I realized that the memory was going to continue its slide into oblivion due to whatever corrosion was occurring inside the memory stack on a surface to which I had no practical way to gain access.
I designed a replacement for the core memory, based on a magnetic RAM chip (MRAM) which would preserve memory contents when power is off just as the original IBM core memory does. During testing of the core memory substitute board, I encountered a worsening of a second problem that had been plaguing the machine - a circuit breaker tripping on the +6V power regulator.
This would happen mostly when I had powered down the machine for a short time and then turned it back on. Very occasionally it would turn off while the machine was working. The rate of the breaker trip began to increase, so I wanted to resolve that before I could finish the memory board testing.
I changed components on the Standard Modular System (SMS) card that performs the regulation inside the regulator module, until the flaky behavior disappeared. With solid power, I wrapped up the memory board testing. The machine was now ready to support my testing of the disk project.
THE VIRTUAL 2315 CARTRIDGE FACILITY BEING TESTED
The Virtual 2315 Cartridge Facility enhances the 1130 system, allowing the user to use the disk drive more flexibly and reliably while preserving the experience of the original disk drive operation. The disk drive in the IBM 1130 uses cartridges that contain a 14" disk platter inside a plastic housing. These slide into the disk drive. The drive spins the platter inside the cartridge and uses a pair of read/write heads, one per side of the platter, to access the 512K words of storage available on a cartridge.
The disk cartridge is organized into 203 concentric circles on each side of the platter, reached by the drive moving the read/write heads on their arm inward to the hub center or outward to the edge of the platter surface. The movement of the arm to the 203 positions is called a seek.
When the drive operates with a physical cartridge (IBM 2315), the motor spins it up to a speed of 1500 RPM for use. The arm moves in and out making a grunting sound as it moves. The 1130 system vibrates slightly due to the mass of the spinning platter and shakes a bit more due to each arm movement. The sound of the motor is also audible.
The Virtual 2315 Cartridge Facility modifies the disk drive so that the read/write heads do not actually move down onto the surface of the disk platter. They arm still moves, the platter still rotates, and all the sounds and sensations are the same. However, the data that would have been read from the surface is instead coming from a small plug-in virtual cartridge. Any changed data that is written out by the 1130 is captured and put onto the small plug-in virtual cartridge.
This eliminates the risk that the heads will crash on the surface of the platter, damaging both themselves and the cartridge. It also allows the user to have as many of the small cartridges as they wish, rather than being restricted to 2315 cartridges which are no longer sold. Finally, the files on the small virtual cartridge can be used with an IBM 1130 simulator running on a PC or Mac. This interchangeability gives access to a much wider range of disk content that is likely to exist when constrained to using only actual 2315 cartridges.
The virtual cartridge is implemented on a micro SD card that is installed inside a small plastic housing that looks like a small version of a 2315 cartridge. The SD card can be pulled out of the housing and inserted in a reader for use with the simulator, then re-installed to use it with the 1130 system.
The Virtual 2315 Cartridge Facility involves a small control box that sits under the top lid of the IBM 1130, into which the virtual cartridge is plugged. It is connected to an interface board inside the 1130 that sits between the disk drive and the 1130 logic.
A small motorcycle battery and related hardware ensures that the Virtual 2315 Cartridge Facility remains operating when power is abruptly removed from the 1130, long enough to ensure the virtual cartridge has preserved all new data written just prior to the power-down.
BACK TO TESTING THE SEEK FUNCTIONALITY
The disk diagnostic programs from IBM have run against the Virtual 2315 Cartridge Facility, with all the reading and writing of data passing completely successfully. The seek of the drive is the one area that was not fully correct - sometimes the virtual facility would be out of sync with the disk drive arm position.
The facility is simply observing the movement commands sent to the disk drive and should therefore always match the location of the read/write heads. In cases involving longer movements, it was sometimes off by a few positions, leading to the diagnostic issuing error messages.
The 1130 commands the disk drive to move 1 or 2 cylinders (positions), either forward toward higher cylinder numbers (the center of the platter) or backwards towards the outermost position on the platter which is called the home cylinder (0). Long movements consist of sequences of 2 cylinder movement commands, only followed with a 1 cylinder movement command if the total movement is an odd number of positions.
It was possible that noise on the connection to the V2315CF was causing it to miss a move or spuriously record a seek that did not occur. Another vulnerability was that the disk drive documentation claimed that the seek was started with a short pulse on -Access Go but the logic of the disk controller inside the 1130 actually asserts -Access Go for a long time, dropping it once the seeking is over.
I worked on the state machine logic to make it as bulletproof as I could. The main signal that advanced the state machine when I shadowed the actual seek operation between the 1130 and the disk drive was +Access Ready which I debounced. I counted time periods when the signal was high or low, updating a running counter which I allowed to move between 0 and 10.
A sequence of 9 positive states followed by 9 negative states would never advance the counter to 10. Similarly a sequence of up to 9 negative states followed by a long run of positive states would never drop the counter to 0. Thus I had to have a steady signal to consider +Access Ready to be true or false.
When we saw the -Access Go arrive, the state machine would next check that +Access Ready was true. If not, this is considered a spurious condition and we go back to idle. If it was true, we then sit and wait for the +Access Ready signal to become false, which occurs about 5 ms into a seek. A safety timer will count off 15 ms and if we are still sitting here waiting, we consider this a spurious seek and go back to idle. If +Access Ready did go false, we update the cylinder location and then wait for +Access Ready to return to true which is the end of our seek.
The control signal we generate when a seek is requested happens for one cycle when we see a falling edge on -Access Go and the drive is online and ready. This is what moves the state machine out of idle. The previous version was generated continuously as long as -Access Go was asserted low, which is not correct but was driven by the error in the disk drive documentation that shows -Access Go as a single downward pulse.
SIMULATION TO TEST THE SEEK LOGIC PRIOR TO TESTING ON THE 1130
I ran a simulation of the FPGA logic in a testbench to try out the functionality, including as many cases of noise and intermittent signals as I could think of. Once I was satisfied that the logic worked as I intended, it was time to fire up the 1130 system and test with the real hardware.
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