Accessory Control

Perhaps “auxiliaries” would be a better word to describe such essentials as turnouts and signals. Having taken the decision to adopt DCC for the trains, the natural choice for accessories would seem to be settled. Not so! There is still a wide choice for control of accessories between analogue and at least three digital systems offered by MERG.

System Choices

For the remote operation of turnouts, signals, and other accessories under analogue DC, some form of relay control or a local power control system would be essential to avoid lots of heavy wiring. I considered various methods but I concluded that adopting standard DCC accessory decoders around most of the layout would promise to keep life simple. However, DCC doesn’t easily permit feedback from the layout which would be a serious problem for my hidden storage loops. For those I have constructed a mimic panel and I have used MERG’s PTP-Lite system for control and monitoring.


When I started to explore my options for DCC accessory decoders, I stumbled across a serious problem with the vague way in which the NMRA has defined them. Their standards and recommended practices have many pages defining the behaviour of mobile decoders for locos but are frighteningly brief on accessory decoders. Added to which I believe that more difficulties are probably caused by the many differences between North American and British railway nomenclature.

When an accessory decoder is first powered up at the beginning of an ops session, it resets its outputs to “off”. For my NCE cab this corresponds with the command for “N(on)” and the command “R(off)” turns the accessory output on. Unfortunately, this is the opposite way round from British railway nomenclature.

For a British railway operator or signal engineer, the normal default position for a signal should be “on” (at danger) and for a turnout, “normal” (straight) and this is what is required at the start-of-day-switch-on. So the problem can’t be solved by simply swapping the feeds to the point motor!

I eventually found the answer in the MERG steady state accessory decoder. This allows the outputs to be optionally inverted relative to the power-up sense. Thus, at power on, the decoder output goes LOW; on receiving a “R(off)” (turnout reverse or signal off) command, the output goes HIGH; on receiving a “N(on)” (turnout normal or signal on) command, the output goes LOW. This gives me the expected operation in accordance with British railway conventions.


Mimic PanelPTP-Lite is a relatively new layout control bus system introduced by MERG which seemed the best option to link my mimic panel to the hidden storage loops. It is a self-contained system that allows for control and feedback on one pair of wires and is very simple to set up.

My layout has two sets of hidden storage loops which are controlled and monitored from the mimic panel. This panel incorporates push-buttons to set the routes through the turnout fans at each end of the groups of loops. I have separate loops for the passenger lines and for the goods lines.

So that I know which train is in which loop, the system displays the 4-digit DCC address of the locomotive or multiple unit train occupying each of the loops. The displays here showing “Occ” are temporary indicators whilst I get the system set up. How I achieve this is described briefly here. The display modules respond to track occupancy detectors and handle the route selection in and out of each loop.


Train ControlIndexRolling Stock