Cant share dual-ported TU78s

[steve at dartvax.UUCP]

We have not succeeded in sharing our dual-ported TU78's between our two
750's (running 4.1BSD).  Either we are doing something wrong (likely) or
the driver won't support what we're trying (not unlikely).  If anyone
can tell us which is the case and maybe put us on the right track, we'd
be grateful.  What follows is a more detailed description of our configu-
ration and what we have tried.

We have a pair of identical 750's each of which has a TU78 tape with the
dual porting option.  Each TU78 is connected to both 750 thus:

		750-1 --------------TU78-1
		      .             .
                         .       .
                             . .
                              X
                           .      .
                        .             .
		750-2 --------------TU78-2

The TU78's are hung on the MASSBUS; the pertinent lines from our config
file (identical on both) are:

master		mt0	at mba? drive ?
tape		mu0	at mt0 slave 0
master		mt1	at mba? drive ?
tape		mu1	at mt1 slave 0
 
We have created special files /dev/mt0, -1, -4, -5, -8, -9, 12, 13, and
similarly for /dev/rmt*.

The problem is that although autoconfig seems to find the two drives at boot:

mba0 at tr4
mt0 at mba0 drive 0
mu0 at mt0 slave 0
mt1 at mba0 drive 1
mu1 at mt1 slave 0

we cannot read or write the other guy's tape drive from either machine.

Thanks in advance for help.
				Steve Campbell
				Dartmouth College
				...decvax!dartvax!steve