[HECnet] Can we hook this up to a VAX or something?

Oh no. A TU58 is a joke device that connects via a UART; the driver for it is a pile of software that speaks a packet protocol over that UART to send/receive the data. The TU56 is a classic PDP-11 style DMA device, hooked to a Unibus interface. It looks somewhat like an old style disk interface (like the RK05 or RP04) except that you first had to seek to the required block number (by doing read block number operations until the right number shows up). Also, a TU56 is reliable, unlike a TU58.