When using a devclass with devt=Cartridge, 3590 devices can only read.  This
is to allow customers who used 3591's (3590 devices with the A01 controller)
to read those tapes with a 3590 (3590 devices with the A00 controller).
The 3591 device emulates a 3490, and uses the Cartridge devtype.  3590's
use the 3590 devtype.  You can do a Help Define Devclass, or check the
readme for information on defining a 3590 devclass, but it is basically the
same as Cartridge, with a devtype=3590.  The 3591 devices exist on MVS
and VM only, so the compatibality mode is only valid on these platforms.
On all other platforms, you can only use a 3590 with the 3590 devtype.

Carol Trible
ADSM Development