Running IBM disk diagnostic 309 against the Virtual 2315 Cartridge Facility - part 2

RESULT OF DEBUGGING SESSION ON SEEK ERRORS

My first test was performed by setting up manual XIO commands to seek forward, seek in reverse, read 1 word of a sector and to sense the device status word. I used them to manually command the Virtual 2315 Cartridge Facility (V2315CF) and the attached internal 13SD (2310) disk drive in the IBM 1130.

I had set up the two seeks to move x00C8 cylinders, one forward and the other in reverse. That is a move of 200 cylinders, thus stopping on cylinder 200. The home bit (cylinder 0) was on in the device status before the seek and off afterwards. 

I then did a seek in reverse of 200 cylinders which should have returned us to cylinder 0. The home bit should be on in the DSW and when I read a sector, the first word should be 0x0000 to 0x0007. 

When it finished, the home bit was NOT on. The read displayed a relative sector number of 0x0020 (32) which corresponds to cylinder 4, not cylinder 0. I did another reverse seek of x00C8 but the value read from the sector showed we had only moved to cylinder 1, not to zero. It took a third reverse seek to light up the home bit and to read 0000 as the relative sector. 

This proved that the diagnostic seek to 0 should have been successful, but somehow it read sectors 112 to 119 instead. Similarly to the experiment with the manual XIO commands I set up, the reverse movement is not moving far enough. 

This rules out the diagnostic as the source of the errors. I may be the IBM 1130 disk controller logic, the V2315CF, the 2310 disk drive or some kind of signal integrity problem on the cabling between 1130, V2315CF and 2310. 

The home cylinder status is passed directly from the 2310 to the 1130, thus we are really not backing up as far as the command requests. The V2315CF decision on what cylinder we reached begins with 0 at startup of the machine and then tracks purely by the seek commands from the 1130 controller logic to the 2310 disk. 

I did update the logic so that when the home condition turns on from the 2310 disk drive, we immediately force the V2315CF to set cylinder to zero. This will keep them synchronized whenever the drive goes to cylinder 0. I don't believe this is the cause of the error, but it was something that made sense as I thought about it. 

The data returned by a Read command is based on the cylinder that the V2315CF believes we are at. Thus if the seek logic in V2315CF is not working correctly, it might get out of sync with the cylinder where the arm of the 2310 is flying over. 

I am busy tomorrow but should be back at the workshop on Thursday where I can load the new bitstream to the FPGA and test again.