5-2.7 Density Code 28H - Fujitsu M2488 Product Manual

TAPE UNIT PARAMETERS
Table 5-13. Page 10h - Device Configuration Parameters Field Description (Continued)
BYTE BIT VALUE
11-13 060000h
14 0-7
5-2.7 Density Code 28h
Until now, when sending Mode Parameters with a Mode Select command, the initiator could only set
00h, 09h or 7Fh in the Density Code field in the Block Descriptor (byte 00h). The Mode Parameter
data returned by a Mode Sense command would always return the value of 09h in the Density Code
field. According to the ANSI SCSI-2 standard, a Density Code value of 09h referred to 18Track for-
mat tape densities. The M2488 product tape drive is capable of reading both 18Track and 36Track
format tapes, but will only write in the 36Track (36T) format. Since the ANSI SCSI-2 standard had
not defined a Density Code value representing the 36T format, the M2483 and the early M2488
drives used Density Code 09h to represent both the 18T and 36T formats.
Since the ANSI SCSI-3 standard has defined a Density Code to represent the 36T format, the M2488
drive will now differentiate between 18T and 36T densities by using two different Density Code val-
ues. Configuring feature mode bit 6 (0x40) in byte FT4 (see the M2488 User's Guide) will cause the
M2488 to associate Density Code 09h with the 18T format, and Density Code 28h with the 36T for-
mat. If this feature mode configuration is not performed, the M2488 will use Density Code 09h to
represent both 18T and 36T formats.
For information and instructions on configuration settings, refer to the Configuration chapter in the
M2488 User's Guide.
5-14
The Buffer Size at Early Warning field indicates the value, in bytes, to which the tar-
get reduces its logical buffer size when writing between early-warning and physical
EOM.
This field is reported as not changeable on a MODE SENSE command, and the
default value is 60000h.
00h When set to 00h, the Select Data Compression Algorithm field indicates that the
controller will write in EDRC uncompressed format. Uncompressed format does not
encode the customer's data with the Binary Arithmetic Encoding hardware, but does
combine multiple customer records into a single tape block.
01h When set to 01h, this field indicates that the controller will first encode customer
data using the Binary Arithmetic Encoding hardware (compress the data), and to
also combine multiple compressed customer data records into a single tape block.
NOTES:
1) It is permissible to modify the data compression algorithm mode away from BOT
and in between write operations; however, buffered write data is automatically syn-
chronized to tape prior to activating the new mode.
2) For backwards compatibility reasons, the values 83h and 84h will be accepted in
this field and treated as 00h and 01h, respectively.
CG00000-011503 REV. A
M2488 PRODUCT GUIDE
DESCRIPTION
April 1997