Ersatz-11 includes support for attaching an external Q-bus or Unibus box to the PC, so that real PDP-11 hardware may be used with the emulated PDP-11 (in the "full" DOS and Linux versions only). This can be invaluable in cases where no supported PC equivalent for the PDP-11 hardware exists, or when the existing PDP-11 system includes custom hardware whose documentation has been lost, or when the equivalent PC hardware is simply too expensive to be worth the cost.
A unique feature of E11 is the ability to attach more than one bus adapter to the same emulated PDP-11 system. With proper PDP-11 software, this allows the creation of a combined Q-bus and Unibus system, or duplicate busses to alleviate bus congestion. This feature may also be used as a work-around for the BCI-2003/BCI-2004 cabling limitation of only two different bus request priority levels allowed on a single Unibus (the BCI-2103/BCI-2104 Q-bus adapters do not have this limitation).
Ersatz-11 takes care of routing device accesses to the proper place, so that emulated and real hardware can be seamlessly combined without the PDP-11 operating system being aware of the difference. A bus adapter can be configured to receive all I/O page accesses which are not emulated internally by E11, or it can be restricted to a particular range (or set of ranges) of CSR addresses. This makes it possible to "hide" PDP-11 devices at will, when experimenting with the configuration, without having to physically remove anything from the external bus box.