Operate 2 isolated loops w/ 1 TIU?

@rtr12 posted:

For your DCS system, connect it as Barry describes above. To add Legacy, connect the Legacy output wire that goes to the track to the black terminal of your TIU loop 1 output, add a jumper wire from there to the black terminal of your TIU output to loop 2. Your Legacy is connected and ready to go. Power up and run some trains.

I am about to do the same but I wondered if there would be some issue in connecting the black terminals of the two independent TIU channels together.  I guess not.  Thanks.

There's no problem in connecting the black TIU output terminals together, that will be fine.  The red TIU output terminals should always be isolated from one another and never connected together in any way.   

Thanks.  That makes sense, but now that I am thinking about this, what happens when a loco passes over from a TIU1 block to a TIU2 block (i.e. half of the engine is on the TIU1 center rail and the other roller is on the TIU2 center rail), or is that quick enough to be inconsequential?

That,s why you leave a gap between the 2 loops by installing a insulated track pin! Alan

@DrTrainDad posted:

Thanks.  That makes sense, but now that I am thinking about this, what happens when a loco passes over from a TIU1 block to a TIU2 block (i.e. half of the engine is on the TIU1 center rail and the other roller is on the TIU2 center rail), or is that quick enough to be inconsequential?

Not a problem, DCS keeps doing what it's doing when it loses a signal.  I run my entire layout using all four channels of the TIU, and I can pass seamlessly between power districts on separate DCS channels.

I have a question: are the common (black) ports on the TIU connected internally or not? That would make wiring Legacy/TMCC into the TIU easier. Plus it would make sure that the transformers are in phase.

@Dylan the Train Man posted:

I have a question: are the common (black) ports on the TIU connected internally or not? That would make wiring Legacy/TMCC into the TIU easier. Plus it would make sure that the transformers are in phase.

No, the black terminals are isolated inside the TIU (only the very first version was common) just like the red. That said, you do not want to impose "large" voltage differentials like an out of phase pair of inputs or outputs either- there are electronics connecting this all together (it is one unit, all one circuit board) with maximum breakdown voltages so while the blacks terminals are not bonded together common, you also don't want 35-40+ volts differentially between a pair either.

Also, bonding doesn't make sure the transformers are in phase. Bonding necessitates that the transformers MUST be phased.

Again, the bond is not what makes or guarantees a transformer is in phase- but that same bond further means that if you are not in phase- really bad things can happen.

On my layout, all four of the PH180 transformers come to individual terminal strips for power and common.  All the commons are connected together at that point for all the transformers.  The accessory transformers that power switches and SC2 boxes are also tied into the same grounds.

@Vernon Barry posted:

No, the black terminals are isolated inside the TIU (only the very first version was common) just like the red. That said, you do not want to impose "large" voltage differentials like an out of phase pair of inputs or outputs either- there are electronics connecting this all together (it is one unit, all one circuit board) with maximum breakdown voltages so while the blacks terminals are not bonded together common, you also don't want 35-40+ volts differentially between a pair either.

Also, bonding doesn't make sure the transformers are in phase. Bonding necessitates that the transformers MUST be phased.

Again, the bond is not what makes or guarantees a transformer is in phase- but that same bond further means that if you are not in phase- really bad things can happen.

The fact is, most folks when building their track layout and just distribution and wiring normally have the layout and accessory bonded common. So even if the TIU internally is isolated, in a lot of folks systems and track plans, generally we wind up at it's common somewhere on the track layout wiring side of the equation.

Further- this is why using one transformer with multiple outputs generally is a good idea since with single multi output transformer (ZW-L, Z4000, even original ZW) The transformer also is bonded common on all outputs and they are all in-phase by  default from this single source.

Put another way, the instant you have more than one transformer-you become responsible for ensuring phase is correct. There is nothing automatic or guaranteed phase between any 2 separate transformers even of the same brand, even with polarized plugs. You must check and verify, and possible correct if found out of phase before connecting to the rest of the system.

On the bright side, it's pretty easy to bench check all the transformers for phasing.

Thanks everyone- a very useful discussion.

My main concern was that as a loco crosses from channel 1 to channel 2, the two red leads (center rail) will be in brief contact through the electronics of the loco.  I didn't know if this was bad for the loco or TIU, but if it was, I would have imagined that I would have read about it somewhere (in 'the book' or on the forum).  Still nice to have 'real world' confirmation.

@gunrunnerjohn posted:

Not a problem, DCS keeps doing what it's doing when it loses a signal.  I run my entire layout using all four channels of the TIU, and I can pass seamlessly between power districts on separate DCS channels.

That's impressive.

The only thing connecting two power districts together from different TIU channels is preclude getting any useful commands through.  The two channels each broadcast the DCS signal, and the timing of the signals isn't perfectly the same, so it just scrambles them.

@gunrunnerjohn posted:

The only thing [about] connecting two power districts together from different TIU channels is [that it] precludes getting any useful commands through.  The two channels each broadcast the DCS signal, and the timing of the signals isn't perfectly the same, so it just scrambles them.

Wonderful.  Makes sense.

Thanks again !

Sorry- one last question (I think) on this topic:

I have Lionel Fastrack and am using the 1 3/8" straight with an electrically isolated center rail (6-12073) but contiguous outer rails to separate my power blocks, rather than a piece of block section track (6-12060) where all three rails are cut.  Since the DCS signal travels only through the center (red) rail, I assume that this will be sufficient to isolate the TIU channels?  Or, do I need to electrically isolate all three rails?  Thanks.

Just the center rail needs to be isolated.  In most 3-rail layouts, including mine, all the outside rails are common throughout the layout.