KB837: Zephyr Xtra and Zephyr - Multiple Unit Locomotive Operations

This article was last updated on Sept. 5, 2011, 3:38 p.m. | Print Article | Leave Feedback


MU (Multiple Unit) Operations

It is not uncommon to see two, three or even four diesel locos pulling in consist at the front of a train. Sometimes, there is a locomotive added in the middle of the train or at the end of the train to give “helper service” to the head end locomotives. This is called Multiple Unit Operation or MU operation.

With DCC, each locomotive is separately controlled. But in the case of MU'ing we would like to have more than one locomotive address controlled by a single throttle to simplify MU operation.

Your DCS50 uses universal consisting to make MU operations simple and easy to do. This is the most flexible and realistic method of MUing available. You can add any locomotive to your MU, no matter what kind of DCC decoder is in your loco. You can even add an analog loco to your MU. You can add a locomotive to an MU in either orientation and in any physical location in the train.

When you send commands to the MU you use the address of the TOP locomotive to control the entire MU and the command station handles the rest of the commands to the other locomotives in the MU. As the address you will use to control the consist is not necessarily the address of the lead or head end loco, we use the term “TOP” for this special address. The TOP address can also be the train number.

You will not be able change the speed or direction of any individual loco that is part of an MU. Locomotive speed and direction for all locos in the MU are controlled by the TOP address.


To MU two addresses together with the DCS51 Zephyr Xtra or DCS50 Zephyr:

1. Choose two locos you want to MU together. Place them on the track. Make sure that both locos that will be part of the MU are headed in the same physical direction. Select and run the first loco to determine which way it is headed. Then select the other loco and run it in the same physical direction.  Once the second loco is physically moving in the same direction as the other, do not change the direction lever.

2. Select the address you want to use to control the MU. This is called the TOP address. To do this, press LOCO, enter the address & press LOCO again. For this example, we will use loco address 09 as the TOP address and we will add address 23 to the MU.

The display will show:

3. Press the MU key. The TOP address will remain in the display and the MU Indicator Dot will begin blinking indicating that you are preparing to add to the MU.

4. Use the numeric keypad to enter the loco address you want to add to the MU, in this case 23.

5. Press the t/+ key. If the MU you just set up is valid, the display will revert to the TOP address. You are now ready to run your MU. To run your MU, just turn the Throttle Knob to set the speed and use the Direction Control Lever to change directions. Use the numeric key pad to control the functions on the MU’ed locos.

If you want to add more locos to the MU, repeat the procedure.To add locos to an MU you must add them to the TOP address. It’s a good idea to keep an MU set up sheet like the following to help you remember which locos are MUed together:

If your MU could not be set up you will see the following error message:

This happens when you try to link to an address that is already part of an MU or link an address that is under the control of another throttle. For example if you MU'ed address 14 to the top loco 23 and then tried to MU another address to 14 you would get this error. If you wanted to add another loco to the MU with 23 as the TOP address, the right way to do it is to add that loco to 23, not to another address in the MU.


To remove a loco address from an MU consist:

On the DCS50, select the loco address that you want to remove from the MU by pressing the LOCO key, entering the address and pressing LOCO again. In this example, the TOP address is 09 with address 23 MU’ed to it. We will remove address 23 from the MU. The display shows address 23 with the Function and MU Indicator Dots lit.

2. Press the MU Key. The Function Indicator Dot will go off and the MU Indicator Dot will begin to blink indicating that you are MUing.

Press the c/- key to remove the loco address from the MU. The DCS50 will return to regular operation mode ready to run address 23 which you just removed from the MU.


MU of Mismatched Locomotives

Inevitably, in a multiple unit consist, one locomotive always seems to be running faster or slower than the others.  Even with locomotives from the same manufacturer, of the same model locomotive, there may be performance differences.  It could be a simple matter of one locomotive needing lubrication, or a minor mechanical defect, but speed matching multiple unit locomotives can be a challenge.

If all the locomotives in the MU have performance characteristics that are closely matched, you can run all the MU'ed locomotives in 128 step mode. If the units are completely mismatched (for example if you are running two different brands of locos with noticeably different operating characteristics), then we recommend speed matching the locos by programming the decoder with either Simple 3 step or Advanced 28 step loadable speed tables.

Likewise, it may be helpful to turn off the Back EMF speed compensation feature if necessary.

It is helpful to start with the slowest locomotive and then match the other locomotives' speeds to that locomotive.


Controlling Functions on MU'ed Locomotives

Even though an individual locomotive is part of an MU and you can’t control its individual speed and direction, you can still control its function outputs independently as follows

1. Select the loco that is part of an MU for which you wish to independently control functions. In this case we will select loco address 23 which we added to an MU with address 09. Press LOCO, enter the MU’ed loco’s address, press LOCO. You’ll see the loco address and the Function and MU Indicator Dots will be lit letting you know that you can control functions on this loco that is part of an MU.

2. Use the numeric keypad to control the functions. The changes will take effect immediately if the locomotives are moving, otherwise changes will take effect when you reselect the TOP address on the throttle.

3. When you are finished turning functions on or off for the consisted loco, simply select the TOP address and resume running the MU.

4. Remember, you will not be able change the speed or direction of any individual loco that is part of an MU. Locomotive speed and direction for all locos in the MU are controlled by the TOP address.

Note: You can also control the functions of a MUed loco from a different throttle that is connected into LocoNet or with one of the Jump ports of the DCS50, leaving the TOP locomotive selected on the main throttle of the DCS50. Function state changes will take effect immediately whether or not the loco is moving.

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