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Effect of the READ Option on DCE Bit in MODE SENSE Data

The value returned for the DCE bit in MODE SENSE for the READ

option depends on the type of MODE SENSE data being returned and

the tape’s format, as shown in Table A-14.

Type of MODE SENSE

data returned...

Actual tape format

8200

8500

(EXB-8505

only)

8500c

(EXB-8505

only)

8200c

Page 0F only

0 0 current* 1

Page 0F and

Block Descriptor

0011

* If the tape is written in 8500c format (8Ch), the DCE bit can be 0 or 1, depending on whether the

EXB-8505 is currently compressing data. This is because you can turn data compression on and

off away from LBOT. For other Density Codes, changing DCE away from LBOT is illegal.

Similarities between the WRITE and READ Options

The WRITE and READ options function identically when the default

writing format matches the tape’s actual format. For example,

regardless of the EEPROM option setting, you can legally change the

Density Code for the tape only when the tape is positioned at LBOT.

This is illustrated in the following example:

Example 1 Suppose you want to write a tape in 8500 format with

an EXB-8505. To do this, you would follow these steps:

1. Load the tape and position it at LBOT.

2. Issue a MODE SELECT command and set the Density Code

in the Block Descriptor to 15h (8500 format). This sets the

default writing format to 15h.

3. Issue a WRITE command to write data to the tape. This sets

the tape’s actual format to 15h.

After completing step 3, suppose you issue a MODE SELECT

command with the Density Code set to 15h. Since you are not

attempting to change the tape format or the default writing

format, the EXB-8505 does not return an error. This result is what

you would expect and it does not depend on the EEPROM option.

Table A-14 Value returned for the DCE bit in MODE SENSE for the READ option

A-32

EXB-8205 and EXB-8505

(Standard and XL)

510503

Appendix A EEPROM Options

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