Yes, Alta 1553 products can use direct-coupling, but that is specified and built to-order. The default configuration for Alta 1553 products is for transformer coupling.
There are many limitations with direct-coupling and many bus reflection issues can be avoided with a properly terminated, trasformer-coupled bus. Alta prefers to support transformer coupling.
The following is an excerpt from Mil-Std-1553 with Notice 1,2,3, and 4, section 10.5:
Direct coupled stubs, as defined in 4.5.1.5.2 of this standard, should be avoided if at all possible. Direct coupled stubs provide no DC isolation or common mode rejection for the terminal external to its subsystem. Further, any shorting fault between the subsystems internal isolation resistors (usually on a circuit board) and the main bus junction will cause failure of that entire bus. It can be expected that when the direct coupled stub length exceeds 1.6 feet, that it will begin to distort the main bus waveforms. Note that this length includes the cable runs internal to a given subsystem.
