BMD16N-SD (version 1.2)

The BMD16N-SD is a feedback decoder with integrated current detection for the S88-bus. The s88-bus can be connected via network cables to greatly reduce the risk of errors. The BMD16N-SD is just like the BMD16N fully compatible with the s88-N standard. The board has been developed in dialogue with one the initiators of s88-N, Wolfgang Kufer. More information on s88-N can be found on his website. Or take a look at s88-N.eu.

The integrated current detection makes the BMD16N-SD very suited for providing feedback on 2-rail layouts. The BMD16N-SD can be used with any S88 based system, for example Märklin Digital, the Uhlenbrock Intellibox, the ESU ECoS and the LDT HSI-88. Thanks to its s88-N connectors, the BMD16N-SD can be connected to all other equipment bearing the s88-N logo.

BMD16N-SD advantages

The BMD16N-SD has number of interesting features. The decoder has 16 independent contacts. Each contact has its own input and output. That is twice the number of contacts compared to most other decoders. The BMD16N-SD is connected via standard S88 cables or UTP network cables. Network cables are much cheaper and are less sensitive to electrical interference. You can use cables that are over 5 metres long without a problem.

If you mount several BMD16N-SDs and/or BMD16Ns directly next to each other, a simple print connector or a few wires is all you need to connect the decoders. That will save you cables and money. The BMD16N also features an improved reset circuit. This eliminates possible errors even further.

You can decide yourself how you want to configure the 16 contacts: in two groups of 8, as 16 independent contacts or any combination of the above. You can also choose whether you want to fit your board with inexpensive strip connectors or sturdy screw connectors (only if you configure your board as two groups of 8 contacts).

Building the decoder

The BMD16N-SD is a do-it-yourself project. The professional boards are being produced in one of the top facilities in Europe. You can order them via the order form. The table lists the part numbers for Conrad. Alternative sources are Reichelt, Farnell and eBay. They are all standard components. The board comes with a printed manual that will guide you step by step. The board and the parts will cost about € 45,- all together. This about half what you pay for decoders from the big manufacturers.

Parts
Count	Description	Conrad	Reichelt
16	resistor 47 Ohm	1584374	1/4W 47
1	resistor 330 Ohm*	1417730	1/4W 330
17	resistor 10 kilo Ohm	1417697	1/4W 10K
2	resistor 100 kilo Ohm	1417735	1/4W 100K
1	resistor 220 kilo Ohm	1417665	1/4W 220K
16	rectifier B80C (round)	501441	B80C2000RUND
1	LED 3 mm*	184713	LED 3MM GN
1	capacitor 100nF	453358	KERKO 100N
1	transistor BC517 (or BCX38C)	--	BC 517
10	16 pin IC socket**	189529	GS 16
4	ILQ620/PC844/LTV844	153612	LTV 844
4	IC 4044	--	MOS 4044
2	IC 4014	--	MOS 4014
6	3-pole screw connector***	731891	AKL 101-03
2	connector strip****	741119	SL 1X36G 2,54
2	RJ45 connector	716148	MEBP 8-8S

* Optional power LED.
** Only needed if you want to mount the ICs in sockets (recommended).
*** You can only choose screw connectors if you plan to use the contacts in two groups of 8.
**** You only need one if you are fitting the board with screw connectors.

Check this overview for the parts too.

Please note the BCX38C! This alternative to the BC517 needs to be mounted the other way around!

You do not have to be a soldering virtuoso to assemble a BMD16N-SD, but some experience is advisable. The manual will guide you through the assembly step by step. The rectifiers on the board are suited for any H0 scale layout or smaller. They will also work in O scale, unless you are operating locomotives with really big power-consuming engines. In that case I would recommend replacing the rectifiers with bigger ones.

Connecting to S88 and s88-N

The feedback decoders are connected to the central unit as a chain. The order of the decoders determines how the contacts are numbered. The first decoder has contacts 1 to 16, the second one 17 to 32, etc. You can connect up to 31 decoders with 16 contacts to the S88-bus.

Connecting the tracks

The BMD16N-SD is typically inserted in the red wire running from the central unit or the booster to the track. The wire connects the input of the decoder with the output of the central unit or the booster. A second wires connects the output of the decoder with the right rail. The left rail is connected directly with the central unit or the booster via a brown or a black wire. You use a different contact for each feedback section (occupancy detector). The BMD16N-SD allows you to connect each contact to another power source.

Sections without detection

Sections without detection are connected via four diodes. If you want to do it professionally you can use the additional voltage reduction board. The rectifier bridge on the board creates the same small voltage difference as the BMD16N-SD does. The parts are: rectifier B80C1500 (501441) and 2-pole screw connector (729949). This ensures proper detection if a train enters a section with detection from a section without detection. This is a recommended practice not only with the BMD16N-SD, but with all decoders using current detection.

Reversing loops

The option to connect each contact to another power source comes in handy with e.g. a reversing loop. The reversing loop module has to be connected between the central unit and the feedback decoder. The module uses some power itself and if yo were to insert between the feedback decoder and the rails, the decoder would signal the track as ‘occupied’. The inputs of the contacts inside the reversing loop (which can be as many as you want) are simply connected to the reversing loop module. The outputs are connected to the track.

Märklin C and M track

If you are building a Märklin layout, your best option is a BMD16N with mass detection. However, it can be hard to isolate one rail from the other with C track; with the old M tracks it is outright impossible. In such cases you could opt for current detection and a BMD16N-SD. You simply put the section dividers in the centre rail rather than in the right rail. There is one little but... only locomotives and cars with a pick-up shoe - track power users - will be detected.

The BMD16N-SD and LocoNet

The BMD16N-SD is not a LocoNet decoder and cannot be connected directly to a LocoNet infrastructure. But with the LocoNet-S88-Adapter from Uhlenbrock (cat. nr. 63880) you can. Each Adapter can accommodate up to 31 BMD16N and BMD16N-SD decoders. You can connect mutiple Adapters to the same LocoNet, which means the number of decoders is virtually unlimited. You connect the decoder with the Adapter through either a standard S88 cable or a s88-N adapter and a network cable. Unfortunately the adapter is currently out of production, but a similar solution can be ordered from Rosoft (called the s88LN).

Testing

The BMD16N-SD can be tested with a few wires and a light bulb or LED (+ 1K5 resistor). Connect the decoder to the S88-bus of your central unit and call up the menu that displays the status of the feedback decoders. Connect the feeder terminal of the central unit with the group terminals of the decoder. Connect another wire to the ground terminal and attach the light bulb to it. Touch each of the outputs with a wire that's connected to the other side of the light bulb. At each output, the light bulb should light up and the feedback status should change to occupied.

Boards are available from the shop.

2008, 2012, 2014