Keep in mind that you only need a Powermaster if you want to control your conventional engines using the Legacy 990 remote.
You can run your conventional engines just fine with the ZW's, even with the Legacy 990 base connected to the layout. The 990 is used to control Legacy/TMCC command controlled engines. The 990 base does not supply any power to the track - it only supplies a signal to control Legacy/TMCC engines. The power to the track is still supplied by the ZW transformers.
It can be a hard concept to grasp at first, but think of it this way - command control engines require about 18 volts to run properly, whereas conventional engines run best at about 10-12 volts. Therefor (for all intents and purposes), you cannot run a command control engine and a conventional engine on the same track at the same time because if you set the transformer voltage at 10-12 volts for the conventional engine, the Legacy/TMCC engine will not operate correctly and if you set the transformer voltage to 18 volts, the conventional engine will fly off the track.
So, once you hook up the 990 base to the track, if you want to run your conventional engines, just put them on the track and use the ZW transformer handle to operate the trains at the voltage you select. Even though hooked up to the layout, the 990 base signal does not interfere with the conventional operation. If you want the convenience of being able to run conventional engines using the 990 remote, then buy a Powermaster and integrate it into the system. It allows you to vary the voltage to the track for running conventional engines using the 990 remote, much the same way as if using the transformer handles on the ZW.
If you want to run your Legacy/TMCC engines, then put them on the track; turn on the 990 remote; and then turn on the transformer and set the voltage to 18 volts.
Hope this helps.
