Testing my new seek logic for the Virtual 2315 Cartridge Facility - 16 and now works correctly in real mode

TESTED LOGIC REFINEMENT FOR SEEK SHADOWING IN REAL MODE

The Virtual 2315 Cartridge Facility (V2315CF) monitors the seek commands sent from the IBM 1130 system to the 2310 internal disk drive. It has to shadow the position of the disk arm on the 2310 so that it always knows where the heads are positioned. This allows the V2315CF to emit the proper data stream when the 1130 reads from the disk. 

The 2310 records four sectors on the top read/write head at each cylinder, with another four sectors on the bottom head. With 203 cylinders available on the 2315 disk cartridge that is inserted in the drive, it holds 1,624 sectors in total. Each sector holds up to 321 words of data - a word being 16 bits in the 1130. Thus the capacity is about 512K words (1 Mbyte) for each 2315 cartridge. 

The V2315CF has to know which cylinder is under the 2310 disk heads, so that when the 1130 selects the top or bottom head and turns on a read as it reaches one of the four sectors on a cylinder, the appropriate data from the virtual 2315 cartridge image is delivered to the 1130. The V2315CF sees which head by receiving the -Head 1/+Head 2 signal plus keeps track of the -Sector Marker and -Index Marker pulses to know when it is beginning a given sector. Combining this with the shadowed cylinder number allows the V2315CF to retrieve the appropriate sector data from the RAM inside the V2315CF and deliver it to the 1130. 

I made a number of changes to the logic in the V2315CF which monitors the seek commands that flow between the 1130 system and the 2310 disk. Because we see what is sent to the disk drive, as long as the drive correctly moves the arm we should be able to keep our shadow cylinder number in sync. Various glitches were causing the shadow and real cylinder locations to diverge. My changes were intended to eliminate these situations so that the shadow cylinder always matches the actual cylinder of the disk arm. 

SHORT PROGRAM CYCLES THE ARM BETWEEN TWO CYLINDERS

A program will move the disk arm forward by some number of cylinders, then when the drive reports successful completion, it will move the disk arm in reverse by the same number. It then repeats, so the arm should be zooming back and forth between two set cylinder locations. At any time, stopping the program should leave the arm at one of the two positions. 

The IBM 1130 allows the program to request a forward or reverse seek of any number of cylinders up to the maximum  possible movement of 202. The 2310 disk drive only supports moving one or two cylinders in a step. Thus, the 1130 iterates stepping the arm until the full count requested by the program has been accomplished. Moving one cylinder is only done as the first step of a seek and only when the count from the program is an odd number; all other steps are two cylinders at a time. 

The direction and size of each step is controlled by -Rev/+Fwd Direction and -10 Mil/+20 Mil Step signals. The actual request to take a step is sent with -Access Go from the 1130 with the 2310 providing feedback using the +Access Ready signal. Our shadowing of the arm movements will observe each falling edge of -Access Go to begin, change the shadow cylinder number based on direction and step size, then end the step after a complete down and up cycle of +Access Ready. 

Short glitches on these signals are ignored by the 1130 and the 2310 logic circuits, but have tricked the FPGA logic into falsely triggering a step or ending one prematurely. I improved the debouncing functions and added safeguards that should eliminate the impact of short term glitches. 

OBSERVING THE PROGRAM CYCLING BETWEEN CYLINDERS

The V2315CF and the disk arm agreed perfectly. I ran fast and slow patterns, always getting agreement between V2315CF and the disk arm. I seem to have developed a solid approach now. I will do a check for virtual mode when I next get to the shop, followed by running the IBM disk diagnostic program (309) which will validate every aspect of the disk drive. When that runs clean in both real and virtual mode, I move on to wrapping up the project.