To control the lights in a control trailer an extra decoder is needed to avoid a cable through the train or to the railcar.
There are only very few function-only decoders. This may be due to the fact that it is identical except for the motor drivers and therefore can not be much cheaper. As the quantity sold is smaller, they are often more expensive as a cheap standard decoder, which can be used for standard applications. In Europe the LE103XF from Lenz is the ideal choice.
As a standard control trailer will get its own address to be consisted with the loco used, the control trailer of a railcar may be assigned the address of the motored unit. This avoids the need for consisting.
If the loco decoder in the railcar is operated in 128 speed step mode the decoder in the control trailer has also to understand 128 speed step packets. The packets for function control are independent from the selected mode, but the direction bit is only available in the speed packet. As the packets for 28 and 128 speed steps are different, decoders with a maximum of 28 speed steps - like the old LE103 - can not understand the 128 speed step packets and have no information about the current direction. Therefore a 128 speedstep capable decoder should be used in the control trailer, if the railcar uses such a decoder.
The other way round, 28 speed step decoder in the railcar but a 128 speed step decoder in the control trailer, is no problem, as all 128 speed step decoder can interpret 28 speed step packets.
Another problem is the programming of the decoder in the control trailer. Parameters may be written without problems but for the acknowledge i.e. the reading of the decoder the motor is needed as a load. As there is no motor in the control trailer, it is replaced by a 150 Ohms resistor. This draws the 60 mA stated in the RPs at about 10 Volts or more at the rails.
During normal operation this resistor would normally be come quite hot as about one watt has to be dissipated at full speed with 14 Volts on the track. To avoid this extra power and heat, the programmable speed table is used by setting bit 4 (Lenz used to number it as 5!) in CV 29. All entries i.e. CVs 67 to 94 are set to zero, so the resistor is not powered during normal operation. For safety the analogue operation should be disabled (bit 2 = Lenz bit 3 in CV 29). The resulting value for CV 29 is 18 for short and 50 for long addresses.
Still a resistor for more power could be used, as it stays cooler during programming due to its larger thermal time constant.
The first picture shows an LE103XF in the frame of a VS98 usually carrying the light tubes. I dropped the interior lights, the upper lamp of the headlight is a 1.5 Volt micro bulb driven via a 5 V voltage regulator (78L05, the black bit) and a resistor of 240 Ohm. The bulb sits in the shrink tube on the left. It is glued to the roof right behind the lens in the body and prevents any light inside the body. The voltage regulator carries two SMD capacitors avoiding oscillations of the regulator.
The 150 Ohm resistor needs not to be that large, but the roof does not melt that fast if the decoder was not programmed correctly. With an LE103XF CV 2 has to be set to zero as well to insure zero voltage at the resistor in the lower speed steps.
The second picture shows an LE103XF in the base of a Kato 815 (ex ESA 150). The direction sensing switch has to be disabled by cutting the traces to the wiper areas. The resistor for reading has been soldered directly to the decoder.