ramac oscilloscope observations joseph feng magnetic disk heritage center computer history museum...

RAMAC Oscilloscope Observations

Joseph Feng

Magnetic Disk Heritage Center

Computer History Museum

June 22, 2007

Acknowledgements

• Thanks to Al Hoagland, Pat Connolly, and the students at Santa Clara University for bringing up the RAMAC

• Thanks to the IBM Corporation for loaning the RAMAC drive

• Thanks to Hitachi Global Storage Technologies, Steven Lambert, and Terry Whittier for providing computers and oscilloscopes

• Thanks to the Computer History Museum for providing the laboratory facilities

OD track (-01)complex waveform

RAMAC readback waveform - “bottom” head at OD CE track (Tektronix)

• Top trace: expanded view showing details

• Bottom trace: one full record (20% of track) showing starting sync burst, start gap, 100 characters of data, and end gap

ID track (100) complex waveform

RAMAC readback waveform - “bottom” head at ID CE track (Tektronix)

• Same text data as on the OD waveform - vertical scale changed for top trace, same for bottom trace

• Note the wider pulse widths, lower signal amplitude, and poorer signal-to-noise ratio

Customer data track - blank data

RAMAC readback waveform - “bottom” head at a customer data track (Tek)

• This is representative of the most commonly observed data pattern

• Repeating blocks of 10000001, which decodes into blank data (unpunched column in IBM card)

Decoding the RAMAC data1

• The RAMAC readback waveforms for two consecutive revolutions were captured with a LeCroy 9345AL digital oscilloscope– 10 ms per waveform, no averaging

– 100ns/sample or about 100X oversampling

– 1,000,000 samples per waveform

• The apparently high noise levels in the waveforms is attributed to the increased bandwidths in the modern electronics and the oscilloscope– scope bandwidth ~500 MHz

– a 733-based preamplifier had an estimated bandwidth of about 10MHz and a net gain of about 5X; its two key functions were to provide the proper load impedance for the heads and to act as a differential-to-single-ended adapter from the heads to the oscilloscope

Decoding the RAMAC data2

• A program was written to decode the digitized waveforms

• After mathematically reducing the noise, 100% of the 10,054 sectors (out of 10,200 possible) were decoded without any obvious errors

– one sector had real errors, apparently due to bad overwrite

– two characters were decoded with proper parity, but “not allowed”

– 28 “unformatted” tracks were observed, most had low-amplitude transitions, variable envelopes, maybe due to low write current

– one track had been used for experiments by SCU students

Contents of the RAMAC data1

• Most of the CE tracks (-01 and +100) had test track identifying text

• Most tracks had blank data or tables with references to Canadian companies, including one to “EXPO 67 CANADA”

• Six sectors had “non-boring” readable text, with typographical errors

• One reference on three sectors to the final results of the 1965 World Series, with October 14, 1965 date, the correct result, and a humorous (and obviously deliberate) misspelling of Sandy Koufax

• The 40+-year old data appears to be well-preserved. There were no errors that could be attributed to data decay. In most cases, the tracks with low amplitude/bad envelopes were adjacent to good tracks with no signs of distress or readback errors

Waveform envelopes - 1well-aligned sectors

• Head moved across one data track from OD (rim) to ID (hub) (LeCroy)

• Top trace: 100 ms or two revolutions, showing data envelope

• Lower trace: index pulses

• The spikes at the start of each record, seen on the ID side of the scan, is due to old data, with the new data written slightly OD and down-track compared to old data

Waveform envelopes - 2showing cross track misalignment

• Head moved across one data track from OD (rim) to ID (hub) (LeCroy)

• Envelope modulation in 10ms blocks due to cross track offsets between sectors

• The modulation pattern on the ID (hub) side is the complement of the pattern seen on the OD (rim) side

• Decoded waveforms taken at the point of minimum amplitude variation

Waveform envelopes - 3showing a “bad” sector

• Head moved across one track data from OD (rim) to ID (hub) (LeCroy)

• The third sector after index shows a loss of amplitude compared to the other sectors

• Symmetric signal loss on both hub and rim sides means it was not due to cross track misalignment

Waveform envelopes - 4showing localization of “bad” sector

• Head moved across five consecutive tracks from OD (rim) to ID (hub) (LeCroy)

• This shows the signal envelopes for the tracks near the one in the previous slide

• The adjacent tracks do not show a similar loss of amplitude, so the signal reduction may not have been due to a scratch or other defect