Another major accomplishment was the installation of a Digitrax PM4 which acts as an additional circuit breaker and reversing section controller. Basically the layout is divided into 3 blocks. These are the continuous SP main and the TNSF yards at Sutter and Angels Camp, the TNSF main from where it diverges from the SP main all the way through Tuolumne at the upper end of the layout, and the various reversing sections. The PM4 provides additional short protection in these blocks so that if (ok, when) a loco derails only that block goes down. The PM4 also flips the polarity for the reversing sections as trains enter those blocks. With analog locos (those without decoders) this works fine at one end of the block but requires good timing on the throttle reverse button when hitting the other end. For decoder equipped locos there is no indication that the train has entered or exited the block, it just keeps on running in the opposite direction. Pretty cool for those of us who grew up carefully wiring layouts to avoid this very condition! For anyone who would consider using a PM4 on their DCC layouts I've drawn up a schematic for how I wired the beast, by far the most complicated mind numbing puzzle I've dealt with so far in this process. And here's a photo of the real thing. Hope it helps someone besides me! If anyone needs advice on this subject please send me an e-mail and I'll be glad to share what I've figured out.
