M2488 USER’S GUIDE INTRODUCTION
April 1997 CG00000-011403 REV. A 1-3
1-3.8 Seismic Data Function Feature Option
The Seismic Data Function (SDF) feature is a standard M2488 with modifications to provide access
to digital data from the read channel after the 9:8 decode and the error correction is performed. The
digital “Seismic Data” is provided for what is usually termed RAW, Read-After-Write verification
during the actual write transfer to tape. In addition, the same Seismic Data Function is provided dur-
ing a read playback of the recorded tape. For more information, see the M2488 Cartridge Tape Drive
Supplemental Manual, SDF Feature For Seismic Data Gathering Applications (CG00000-0128xx).
1-4 DATA INTEGRITY
The M2488 has been designed to detect data errors when they occur so that data integrity can be main-
tained. Data integrity through the M2488 data path is ensured by extensive use of CRC (Cyclic Redun-
dancy Check) and ECC (Error Correction Code) circuitry. CRC detection was chosen for superior
detection capability over parity schemes. Parity detection exists only on the SCSI interface. The remain-
der of the M2488 data path uses at least one level of CRC to ensure data integrity.
The M2488 data path integrity can best be described by listing the elements of the data path and then
describing error detection schemes present. The elements of the data path are listed below;
SCSI Interface - Data protected by parity as defined by ANSI SCSI-2 specification.
Host Interface Data Path - Multiple CRCs are used to protect the data as it is reformatted to 3490 tape
standards (EDRC Compacted, EDRC Non-compacted). The CRC used depends on the data transfer
type selected. If the operation is Write EDRC Compacted, the EDRC circuitry also performs a Decom-
paction Readback check on the Compacted data.
Data Buffer - Data in the 2MB buffer is protected by CRC.
Formatter Data Path - CRC is used to protect the data while it is still in “parallel” format. Once the data
is broken in tracks for movement to tape, “media CRC” and ECC information is added. The ECC is a
Reed-Solomon algorithm as defined by the 3490 Media Interchange Specification. The Formatter logic
also performs a Readback check of the data written to the tape media. The read heads are used to pick
up the just written data, and this read data is verified for integrity by using the Read ECC and CRC
check circuitry.
On read operations the Read ECC circuitry is used to correct for media induced errors on up to four
tracks at once. The Read “media CRC” ensures that any corrections performed by the ECC circuitry are
valid, since the media CRC was generated when the data was written.
1-5 DATA COMPATIBILITY
The M2488 Cartridge Tape Drive records data in a format that is compatible with the binary arithmetic
coding algorithm, ANSI X3.225-1992, and allows interchange of tapes with other manufacturers’ tapes
that comply with this standard. The M2488 reads 18-track and 36-track format tapes and writes in 36-
track tape format.
1-6 RECORDING CAPABILITIES
The M2488 tape drive uses the DD-NRZI 36 Track (36-Track Serpentine) method of recording. It
records 18 tracks in the forward direction (wrap 1) and 18 tracks in the reverse direction (wrap 2).
Table 1-1 presents the performance capabilities of the drive.
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