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There have been two major families of cartridge tapes on the 3B2 family, and they are not at all interchangeable. We'll describe them here in case you're sorting through what's in front of you.
The first tape drive available for the 3B2 family was the 23 megabyte Cartridge Tape Drive, and these are not compatible with any of the later SCSI drives. It was standard equipment on the 3B2/400 and was available in the XM expansion module for the 3B2/300, /310 and the /400. As far as we know, the 3B2 is the only system that can read these tapes.
These units use the CTC driver and are accessed via the device /dev/rSA/ctape1. There is a special ctccpio command that is able to put the device into streaming mode, but we've seen enough problems with this command that we prefer to use the standard cpio command for this even though it's a bit slower. A noticeable feature of the non-streaming mode is the very annoying back-and-forth running of the tape while reading: we've heard it described as "wheezing".
A particular oddity of these tape drives is that because they "look" like a (very large) floppy disk drive, the drive provides a block device interface (/dev/SA/ctape1), in addition to the expected character (aka "raw") device interface (/dev/rSA/ctape1. This means that it's possible to mount a tape drive as a filesystem. It works, but it's so painful that it's hard to convey just how slow this is.
Our particular unit is a Cipher FloppyTape 525 (model 960273), and it is branded with AT&T part number KS-23165. It uses a floppy disk interface with the familiar 34 pin card-edge connector as found on older 5-1/4" floppy drives. A distinguishing mark of this drive is the flip-up lever that is used to engage the tape after insertion: most of the drives shipped with the 3B2 were of this type. We understand that there is also a similar drive made by Wangtek that looks very much like the SCSI units.
Our understanding of these drives is that they should not use the same DC600 cartridge tapes as do the 60 and 120 MB SCSI drives, but after recently reviewing the 3B2 service documentation we are no longer of this view: AT&T says that DC600 tapes can be use in all their drives. This is good news: we've been telling people for 10 years that they have to use special tapes. Oh well.
It is possible to add a 23 MB tape drive to the 3B2/600 and newer units: we simply install the CTC I/O card (CM195H) into the 3B2/600 and install the CTC driver. We've done this on our system, though the mechanical arrangements are "suboptimal". Because the 23MB drive doesn't really have any provisions for external cabling. the only real way to do this is to mount the 23MB unit inside the cabinet and put the displaced SCSI tape drive into an external case: then use "regular" external SCSI cables to connect them.
This "floppy tape" interface is used in other computer systems, including many on the PC. As such, the Linux floppy tape project might prove fruitful for those determined to read and write many tapes on their own systems. We are not even close to motivated enough to try this ourselves, but we'd surely love to report on the experiences of others.
NOTE - I have one of these 23MB tape drives on my 3B2/600 and am happy to translate them, time permitting.
The 3B2/600 and later models (including the low-profile /522) all came with SCSI tape drives of either 60 or 120 megabyte capacity, and these are standard units as found in many other pieces of equipment.
We believe that AT&T only used Wangtek drives, and they are of the type where you push the tape in, hear a click, and let go: now the drive is engaged. To release, just push again and the tape pops out. The drives from AT&T have a small "60" or "120" stamped on the faceplate to report the capacity.
These drives all use the ST01 driver, and the device name is /dev/rSA/qtape1. Not surprisingly, there is no block device.
A 120 MB drive can read a 60 megabyte tape but cannot write one. Neither of these drives can read or wrote the 23 MB format. Sorry.