Autochanging tape drives used to be the stuff of science fiction among 3000 managers, but those days passed by before HP cut off making Classic 3000 MPE V systems. Just because an autochanger is a 3000 storage option does not make it automatic to program, however.
A question posed to the community by Ideal Computer Services Ryan Melander reached for help on programmatically controlling such autochangers -- to select a slot, and load the tape and come to ready. "I am trying to configure an old DDS3 auto-changer, one that I don't believe will unload and load the next tape," he said.
Gilles Schipper noted that the command ad ldevno id=hpc1557a path=?? Mode=autoreply configures the device, and to advance tape after use, employ the command (from Devctrl.mpexl.telesup) ldev eject=enable load=online
Denys Beauchemin mentioned HP's pass-through SCSI driver as a tool to drive the device's robot. The software was built by HP's labs and labeled as "not for the faint of heart" by engineers, but can assert a programmatic control over autochangers. Some third party programs such as Orbit's Software Backup+/iX can also do this work.
If ever there was a theme song for an autochanger at work, it would be a tune called Powerhouse. Children of the Fifties and Sixties will know it as soon as they hear it, if they've ever watched a Warner Brothers cartoon.
As for being able to control the robot itself, you definitely need to have the [HP] SCSI pass-through driver configured and loaded, and then you need a program to actually issue the IOCTL calls to the robot with the properly formatted SCSI commands. There was such a program a long time ago from a vendor, but that's all gone now.
Perhaps the high-test flutes and heavy octane horns of Powerhouse -- used in Duck Dodgers and the 24th and a Half Century -- can be put up on the MP3 player while fitting the driver to MPE. ("Oh drat these computers -- they're so naughty and so complex," says Marvin the Martian in one installment. "I could just pinch them.")
From our archives we found this advice from the late Jack Connor of Abtech, pointing to similar complex answers about controlling DDS changers.
Typically, there's a second SCSI port/address assigned for the transport control which allows the selection of specific tape. For MPE, stacker mode is typically selected, which tells the drive to just mount the next tape in line when requested. I don't know if the DDS autoloaders have a network connection available like the C7145NA DLT autoloaders do; with that device's web interface you can reload any tape, bypass a bad tape, and so on.
Back in 2011, John Pitman checked in to report that a much simpler solution to his changer's control needs popped up. "On re-examining my code for HPDEVCONTROL, I found I had catered for 1- and 2-digit device numbers in the string passed, but I had configured the drive as dev 777. This produced a string dev number of 77, which doesn't exist as a tape drive. Once I fixed this, it works like a treat."