BMD16N-SD

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 Fleischmann Twin-Center 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 S88 clock signal is filtered to counter false readings as much as possible. The BMD16N also features an improved rest 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 parts can be bought from Conrad, Reichelt (cheaper) or an electronics’ shop in your neighbourhood. 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 number
16	resistor 47 Ohm	403881 (100 pieces)
1	resistor 330 Ohm*	403199
18	resistor 10 kilo Ohm	404160 (100 pieces)
2	resistor 100 kilo Ohm	403490
1	resistor 220 kilo Ohm	403539
16	rectifier B80C1500	501441
1	LED 3 mm	184713
1	capacitor 100nF	453358
1	capacitor 10pF	451630
1	transistor BC517	154695
10	16 pin IC socket**	189529
4	ILQ620 optocoupler	153612
4	IC 4044	172871
2	IC 4014	172707
6	3-pole screw connector***	729957
2	connector strip****	741119
2	RJ45 western connector	716148

* If you connect the BMD16N to +12 Volts, you have to use a 1 kilo Ohm resistor instead.
** Only needed if you want to mount the ICs in sockets.
*** 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.

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.

The BMD16N-SD and the ECoS 1 and 2

ESU's ECoS central units read the S88 bus in a slightly different way. In exceptional cases this could lead to a shift while reading the status of the contacts. If you plan on combining the BMD16N or BMD16N-SD with an ECoS, you might consider leaving out the 10pF capacitor. You replace the 10K resistor below the capacitor with a wire. You can make this change later if you want to, by cutting away the capacitor and the resistor and by soldering a wire where the resistor was.

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.

Boards are available from the shop.

2008