This section highlights the differences between EXB-8500 format and EXB-8200 format. For detailed information about EXB-8500 recording format and recording parameters, refer to the EXB-8500 8mm Cartridge Tape
Subsystem Product Specification. For detailed information about EXB-8200
recording format and recording parameters, refer to the EXB-8200 8mm
Cartridge Tape Subsystem Product Specification.
Track Structure—EXB-8500 Format
When writing data in EXB-8500 format, the EXB-8500 uses its two write heads (W1 and W2) to write two partially overlapping physical tracks to the tape during the first 180-degree rotation of the drum. The two read heads (R1 and R2) perform the read-after-write operation during the second 180-degree rotation of the drum.
When reading data written in EXB-8500 format, the EXB-8500 uses its two read heads (R1 and R2) to read the two partially overlapping physical tracks. The single servo head (SVO) reads the servo data that was written on the tape by the second write head (W2). The servo data is used to ensure accurate positioning of the read heads over the tracks.
Of the two tracks in the pair, track 1 has a +20 degree azimuth. This track is written by head W1 and read by head R1. Track 2 has a –10 degree azimuth. This track is written by head W2 and read by heads R2 and SVO.
Track Structure—EXB-8200 Format
When writing and reading data in EXB-8200 format, the EXB-8500 uses the W2, R2, and SVO heads to write and read a single track with a –10 degree azimuth.
Physical Blocks
In both EXB-8500 format and EXB-8200 format, each physical track contains eight physical blocks. A physical block can contain user data and other
information. In both formats, a physical block containing user data includes the following information:
■ 14 bytes of header information ■ 1,024 bytes of user data
■ 2 bytes of cyclic redundancy check (CRC) data ■ 400 bytes of error correction code (ECC) data.
Note: This information is arranged somewhat differently in EXB-8500
format than in EXB-8200 format.
Since each physical track contains eight physical blocks, each track can contain a maximum of 8,192 bytes of user data. The header, ECC data, and CRC data do not affect the data capacity of the tape.
Logical Blocks
A logical block is a block of data transferred from the initiator to the
EXB-8500. In both EXB-8500 format and EXB-8200 format, logical blocks can have fixed or variable lengths. In addition, both formats support logical block sizes from 0 to 240 KBytes.
In EXB-8500 format, each 1,024-byte physical block can contain multiple logical blocks (for example, two 512-byte logical blocks can be written in one physical block). In addition, a logical block can start in one physical block and end in a different physical block. This logical block packing prevents the loss of data capacity for tapes with small logical blocks written in EXB-8500 format.
Note: In EXB-8200 format, only one logical block can be written in each physical block.
For detailed information about logical block packing in EXB-8500 format, refer to the EXB-8500 8mm Cartridge Tape Subsystem Product Specification.
Search Fields
A track written in EXB-8500 format also contains search fields that enable the EXB-8500 to perform high-speed searches at up to 75 times the nominal tape speed. A high-speed search occurs when the initiator issues a LOCATE (2Bh) or SPACE (11h) command. The search fields are the only areas of the tape that are read during a high-speed search. They consist of small data areas interspersed with clock sync areas. The search field data contains information for locating files and blocks and detecting the end-of-data (EOD) mark during high-speed searches.
Note: Tapes written in EXB-8200 format do not contain search fields and do not support EXB-8500 high-speed search. In addition, the EXB-8500 does not support the EXB-8200SX high-speed search feature.
Filemarks
Like the EXB-8200 and the EXB-8200SX, the EXB-8500 supports both short and long filemarks. However, the sizes of these filemarks are different in EXB-8500 format than they are in EXB-8200 format.
The long filemark in EXB-8500 format occupies 48 KBytes of space and allows data to be appended to it.
The short filemark in EXB-8500 format occupies 1 KByte of space but does not allow data to be appended to it.
As described in Table 3-2, the size of the filemarks that can be written depend on the format of the tape and on the setting of the Short bit in the WRITE FILEMARKS (10h) command (see Chapter 30).
Tape Written By...
Type and Size of Filemarks Long
(Short bit = 0)
Short (Short bit = 1)
KBytes Hex KBytes Hex
EXB-8500 in
EXB-8500 format 48 C000h 1 400h
EXB-8500 in
EXB-8200 format 2,160 21C000h 184* 2E000h
EXB-8200 2,160 21C000h 480 78000h
EXB-8200SX 2,160 21C000h 184* 2E000h
* The short filemark written by the EXB-8500 in EXB-8200 format is equivalent to the short filemark written by the EXB-8200SX.
End of Data (EOD)
When writing data in EXB-8500 format, the EXB-8500 writes an end-of-data (EOD) mark to indicate the location of the last data on tape. The EOD mark is automatically written when the EXB-8500 receives one of the following commands after completing a WRITE or WRITE FILEMARKS operation:
■ LOAD/UNLOAD (1Bh)
■ LOCATE (2Bh) in the reverse direction ■ REWIND (01h)
■ SPACE (11h) in either direction
The EOD mark is overwritten when additional data is written to tape.
Note: The EOD mark is not used for EXB-8200 format.
3.2 Streaming and Start/Stop Modes
The EXB-8500 includes a 1-MByte data buffer that enables it to operate as either a streaming tape device or as a start/stop tape device. The mode of operation depends on the rate that data can be transferred between the initiator and the EXB-8500. If the initiator can sustain a minimum transfer rate of 500 KBytes per second, the EXB-8500 operates in streaming mode. If the initiator cannot sustain this transfer rate, the EXB-8500 starts and stops the tape automatically.
In start/stop mode, the motion threshold can be used to fine-tune the starting and stopping of tape motion. In streaming mode, the reconnect threshold can be used to fine-tune the rate of disconnects and reconnects between the EXB-8500 and the initiator.