Overcoming the 'PARITY ERROR' situation when reading 4mm tapes produced on A/P.

4mm Tapes produced on A/P may cause problems when read on an mvBase system. Specifically, an error message saying 'PARITY ERROR' may occur when an attempt is made to read the first block.

The format on such tapes is unusual for 4mm tape. It uses 512 byte fixed-length blocks (physical not logical blocking) and 80 byte labels embedded in 512 byte physical blocks.

While this is unusual for 4mm, but this is exactly the format mvBase uses for QTR-INCH. Knowing this allows the following workaround to be effective.

Workaround

Since the 4mm tape appears to be written using a QtrInch-like format then we need to change the mvBase 'type' of the tape drive to QtrInch.

Using mvADMIN, delete and then re-add the 4mm tape drive, but this time when prompted for the "type" of this drive, specify Qtr-Inch and NOT 4mm. Since drive type specification in mvBase is soft and not determined by the hardware/driver, this will be successful.

Restart the mvBase server. The 4mm-tape drive will now show up as a "Qtr-Inch" drive under T-STATUS.

At this point, it should be possible to do ADD-ACCOUNTS, ACCOUNT-RESTORE and SEL-RESTORE from an A/P filesave or T-LOAD's from an A/P T-DUMP. It should also be possible to write A/P compatible 4mm tapes as well.

Limitations

This reconfiguration of the 4mm tape drive so that it is viewed by mvBase as a QtrInch drive has to be done in mvADMIN while the mvBase server is not running.

Also, while the 4mm drive is configured as a QtrInch drive, it can no longer correctly read or write standard format (mvBase compatible) 4mm tapes. So the tape device becomes a dedicated "A/P compatibility" tape device, at least until reconfigured via mvADMIN.

It should be understood that this issue deals with the PHYSICAL formatting of the tape blocks by the tape driver. There are still LOGICAL format incompatibilities between mvBase and A/P in File-Save which require the use of the "(C)" option on A/P systems and special procedures for handling large size items.