MTH’s new WTIU question from a (so far) non DCS user

The reason for passive mode was basically around 2 different themes:

#1 you had an old, early revision TIU that did not have fuses, and was highly risky for burning out the traces of the board since there are no fuses. Running passive mode would not run  any current through the fixed channels of the TIU- thus bypassing the risk.

#2 you had high current draw requirements so much so that running in passive mode bypassing current around the TIU made sense even with later versions that had fuses.

Just as a note- another weak point of the TIU was the banana jack terminals and the wire to ring terminals inside being loose creating a self destructive meltdown scenario on the terminals when higher current are passed through the TIU.

So- the WTIU would be subject to the same rules or scenarios: You either are running high (7-10Amps of current on a regular basis) and you don't want short circuit currents (12-20Amps) to pass through the TIU or don't want the possible risk of melting the terminals.

Passive mode typically requires inductive chokes put in series with your power supply (transformer) to prevent your transformer from degrading the DCS signal strength when using passive mode.

It's this simple of a concept. DCS signal is a high frequency digital signal riding on the power. In passive mode, your transformer, the TIU output channel, and the track would all be in parallel. The choke is in series at the transformer to choke off the DCS signal from basically being "shorted" by your transformer source. A choke is high impedance (resistance) to high frequency, but as you near DC or a slow 60Hz AC is lower resistance.

Note also, when running passive mode- we often ALSO supply the TIU with aux power in to keep the TIU logic powered regardless of channel status. So I would expect similar usage and external aux power when using the WTIU- because you want to keep the wireless portion booted and running constantly- rather than waiting time after you first turn on track power.

@Vernon Barry

Thanks for the speedy and detailed explanation!

@Vernon Barry posted:

So- the WTIU would be subject to the same rules or scenarios: You either are running high (7-10Amps of current on a regular basis) and you don't want short circuit currents (12-20Amps) to pass through the TIU or don't want the possible risk of melting the terminals.

Passive mode typically requires inductive chokes put in series with your power supply (transformer) to prevent your transformer from degrading the DCS signal strength when using passive mode.

The jury is still out as to if the WTIU will work in passive mode.  I don't know if anyone has even broached that question yet.