DD Motor Mite with RS 2.5 or RS4

The motor mite uses a standard R2LC. Pin 24 of the R2LC is the serial data line that gets connected to the audio board to trigger the sounds. If you have an ohmmeter you should be able to find the pin on the motor mite board that connects to pin 24 that is intended to be connected to the audio board usually with a 2 or 3 pin connector.

Pete

There are three wires from the R2LC to the RS4 boards, Ground, Track Power, and Serial data (referenced to ground).

The 4 connector normally only had 3 wires for RS 2.5.  The LCRU had a 4 pin connector that had Pin 1 AC center rail, 2 outside ground, 3 not used, 4 serial data.  The harness connected the two units.

RS4 is modular and did not need direct wires between the R2LC and the RS 4.0.  Normally the mother board for diesel had all the connections, or for steam  only the serial data was send to the RS Board in the tender.  The AC came from the tender pickups.   G

Well, since we're nit-picking...

If we want to be correct here, frame ground is almost always brought over in the tether except for the wireless tether.  A derailment that put track voltage on the frame would be instant death for the serial output of the R2LC if there wasn't a frame ground.

The most common four leads are ground, coupler, backup light, and serial data.

A couple of examples...

Not really nit picking John. Your statement that the R2LC supplies track power and ground is misleading. Track power and ground may pass through the motor mite board or directly from the track to the audio boards but the R2LC is not the source.

Pete

Well, yes, I suspect that most folks know that track power doesn't get generated in the R2LC!  

I should have said that the three are common to the R2LC and the RS4 boards.  However, GGG is correct, the center track voltage is supplied directly from the tender rollers for steamers, however the ground is referenced from the engine in parallel with the tender wheels.

John, What does referenced mean?  Also what frame are you talking about in the post previous.  AC center rail and outside rail are fed into the motherboards for modular and into the specific components if the original system.  The engine and tender frame carry the AC common.  Some engines using wire tethers used wipers to pick up AC outside rail for the tenders.  My Fleet of 28000 series TMCC/RS units uses the ground wire, because an internal tender ground wasn't used.  I don't see how a derailment puts AC on the serial data line.  A derailment shorts the AC input.

I was just trying to be specific to the original poster as 2.5 and 4.0 are different.  One is not modular, hence the need for direct wire connections between the LCRU and the RS 2.5 platform.  Done via the 4 connector 3 wire harness.   G 

It means that the ground for the RS serial data travels with the serial data.  While you may think that a derailment results in an instant short, that's not reality in all cases.  If the wheels manage to be on the center rail without shorting, even momentarily, that results in the ground reference for the serial data being a whole lot different than it should be.  That's how chips get toasted.

I am not sure how that is possible but ok.  I looked at the ground wire going over as more to do with continuity of power for the RS boards than complimenting the serial data.  There was a service note about using an additional ground wire for "control issues" what ever that means, but also stated it wasn't used in production.  Was a note for some of the early 2000 steam with TMCC/RS and wire tethers.

I guess the use of IR some how changes that, since the tenders are stand alone for power.  G

No chance of a feedback to the R2LC serial data with a wireless tether.

Since I've seen a number of cooked wires in passenger cars and even a couple of engines, I know that a derailment can pass a lot of current before anything trips.  If that happens to impress the track AC on the frame of the tender, with no ground that would be bad.

I suspect it's also simply a signal continuity issue, the ground from rolling wheels probably isn't as solid as bringing the ground reference right from the locomotive.  The poor ground reference would probably make the serial data a lot harder to read reliably.  That may indeed be the primary reason since I don't see any other protections in most of this stuff from the effects of a derailment.

John,  This is the part I don't follow you on.  The track ac is impressed on the frame of the engine and tender.

I have done pin outs on many TMCC mother boards and the AC common (outside rail) is just a buss on the mother board.  AC into the mother board and the AC common is distributed to a number of the pins used as the return path for the accessories such as lights, and couplers.  It is also used as an input to the R2LC, RS Power Supply, RS Board.

If you have a derailment, the least of your concerns is what happen to serial data, since the engine is shorting and isn't going anywhere.

The only damage I have had to repair on RS 2.5 and 4.0 boards is blown audio amps.  I am sure that has more to do with the AC spike effect on the 5VDC power supply and since AC is monitored for DC offset, I assume that is why the AC issues blow audio amps.  Same effect on the PS-2 audio amps.

K-Line is one manufacturer that uses TMCC equipment but has wipers and direct connections for AC Center Rail and Outside Rail in the tender. 

Of course we have strayed way off FMH's original question.   G

FMH, To be more specific in an answer for you, the MM instructions state J2 is the Ant and RS connection.  You just need to use a VOM and do a continuity check on J2 pins.  You need to find the pin that is AC Center Rail, AC outer rail, and Serial data (pin24 of the R2LC).  Those are the 3 connections to your RS 2.5 board.  G