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I have a TMCC Action recorder module 6-14181

The manual says not to ground the U terminal of the input power connection but I want to know why not?  It works fine on a wall transformer but I would rather use another transformer I already have in place that has U connected to outside rail (ground for my entire layout).  Any schematic available that I could view so I can figure out why I shouldn't ground the U input would help a lot.  Thanks.

Text quoted from the manual:

1. Attach a wire to the Common/Ground/U terminal on your transformer and connect it to the

POWER/U terminal on the ARC.
Note! Do not connect this terminal to the outside rail.

Last edited by John R (Bob) Kelty
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I can definitely think of and understand a few reasons.

#1 Because this also plugs into your TMCC or or Legacy base that is generating an RF signal for the outside rails, we might be running into the issue of the U on the ARC module and the "Common" COMM of the serial data connector to the base. This could/would possibly kill off or short the TMCC signal.

Also, it appears the sensing outputs are not common and not connected or bonded to "U". Again, what appears to make this sensing unique (compared to the more recent blue LCS modules is the sensing inputs are not tied to a common U).

Here is a combined diagram showing both sensor and the base actually connected to the track- why "U" power source cannot be common


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Note that in a more modern setup, where you could use an LCS SER2, does (in theory) isolate the serial output.

Support manual for SER2

Again, what makes the SER2 different is the fact that with Legacy and Legacy Lite (CAB1L, and 990 set Legacy Base) is they need a power supply and Y cable to start the LCS chain. This is also partially why LCS since using an isolated supply then the LCS modules can be "common" to the track and not kill off the TMCC or Legacy signal. This separate power supply and cable system allows the modules to repeat the signal and be be isolated from the serial terminal of the base.

Specifically for the ARC, since it is a bi-directional com device (it not just receives commands, it sends them across the serial bus) it is a 3 wire TX and RX device.

And again, specifically an ARC 3 wire connection VS a receive only device 2 wire connection.


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All I'm trying to get at, ultimately, one way or another, you end up using a dedicated isolated power supply that is not common with U.

The old way, (black TMCC modules) just needed an aux AC transformers again completely isolated.

The new way (Modern LCS  Layout Control System- AKA blue modules) carefully considered their own power supply system that by default isolates them. This also makes the older black modules compatible when using a SER2.

Thanks everyone for the replies.  I am using a separate 1073 transformer now to power the ARC.  It also is powering some lights and turntable motor.  I use a SER2 which is feeding the data lines to the ARC.  And a wifi unit before that fed from an LCS cable mentioned in earlier reply that powers those 2 items via wall adapter.  All this seems to work as well as an SC-01 and 3 SC-02 modules (those are on wall adapters).  The 1073, type V, KW, and two ZW transformers powers the trains and accessories (all phased) and all have U terminals connected together with common ground wire which is also the outside rail of layout tracks.

I do not have any TMCC problems with Legacy and a command base with the ground going to Legacy ground post from U terminal of KW.  All of this is working fine (along with a homemade arduino board (that does similar to what the ARC does but has some randomness thrown in the programming).

I was hoping to find some electronics reason for not connecting the ARC U terminal to ground.  I'd like to see the schematic but ohmmeter check shows data comm isolated from U terminal so I tried it without problems.  Closest thought that makes sense was response that mentioned RF single feed to rail, but that happens anyway with Legacy box post (?).  And that the blue boxes have their own supply which is isolated as mentioned in previous response.  Anyway the entire setup works and I do not seem to need the wall transformer for the ARC.

I do not use any detector tracks but might in the future.  If I do, I pretty much will be back to where I was in the 80s with PC running Microsoft BASIC and some control boards.  Not thinking this is progress, just different and I can use phone but never do.  :-)

Next up is Block power Controller (using with turntable tracks) that I'll pick up later this week...   :-0

Thanks everyone for the responses.

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