Feedback Modules – LocoNet or S88?

I’m in doubt: Uhlenbrock feedback modules cost 350 DKK for 16 ports. And it is even worse with Digitrax. They cost more than 1000 DKK for 16 ports. And I need many ports, when I start building the real layout. Maybe even 2 or 3 ports per block, so that the PC can “see” the difference between the different parts of a block: Approach, Brake and Stop.

LocoBOD can offer a totally different and lower pricetag. LocoBOD can be investigated without any cost by downloading the source code, installing WinAVR and see if it compiles. If I by that am able to generate a HEX file, I could buy an Atmel microcontroller and Reichelts best USB thing to push the HEX file into the microcontroller and see if that gives me a cheap LocoNet feedback module.

As an alternative, I could explore the possibilities for DIY S88 modules. Paco has a proposal, that looks easy and cheap. Not much more than 1 DKK per port.

And who says I cannot avoid noise problems, if I use proper shielded cables or just very short cables? Viessmann are now delivering S88 cables in lengths of only 15 cm with their S88 modules. It used to be a meter. And they now recommend placing all S88 modules right next to the command station.

Of course, there are also other LocoNet possibilities. For example LocoIO from However, it does no seem possible to download HEX file or source code. One has to buy a pre-programmed PIC. And I am not sure that I can find a diagram either, som maybe buying a kit or a finished board is the only possibility.

After a lot of research, I found S88N. See and The better solution seems to be coming along.

Alright, the described solution with it’s SMD components is overly complicated implementation with many components and options. But the Danish page proposes some simplifications that would make this possible to build with simple means and spending little money.

On there are many other very exiting projects. And everything is PIC based and include both source code and HEX files. In addition, it is possible to buy PCBs and kits. See also And Paco is also having a bit of S88N:

S88N utilizes standard Ethernet (RJ45) cables and connectors. And in addition, it is PICs, that interfaces the data bus, and not shift registers. Maybe most important, the data bus is galvanic from the command station as well as from the track. That requires a separate transformer >5V.

Back to LocoNet. One of the very first considerations was if I should build my own command station. I was mostly considering MERG. And a part of that system is this simple 16 port LocoNet I/O module with HEX file for PIC:

Why not revert to that module and just make it with standard LocoNet connectors? OK, it is somewhat irritating not to be able to get the source code. And possibly, there will be some trouble with CV programming the module, since it is not known by the Uhlenbrock command station. But that programming may be possible through the RocRail program, which is free, and which I have already installed. But I am not sure that I can find the CV definitions anywhere.

The original (that seems to be an improved version of the original original) at utilizes normal LocoNet connectors. A study of the diagram must determine if the same HEX file can be used, i.e same PIC connected in the same way. Note that something is mentioned under “software versions” about a change from 16F873 to 16F883.

Primo november 2014:

After many difficult considerations, I have come to the conclusion that the various flavors of LocoIO is a bit too closed and secret world. Digitrax has patents. And everybody else are also protecting their software.

One can find a HEX file to a PIC. But the source code is nowhere to be found. And I am also nervous that it is going to be difficult to set the module up (CV programming). It is probably only possible to set Uhlenbrocks own modules up from IB-COM. And I would be forever bound to JMRI or RocRail, who both promises possibilities to configure LocoIO, or is it only a certain flavour thereof?

I am therefore going with S88-N.

After a bit of pleasant communication with, I got him to make me some PCBs. And I feel very convinced, that I am going to make it work.

S88-N has however the limitation, that it cannot be configured at all, i.e. prel due to bad electrical contact between a locomotive and the track either have to be corrected by a capacitor in the current sensor or by software, such as RocRail (if it has that facility) or by modifying the S88-N PIC code.

Postscript: I have not experienced any problems regarding prel or noise or anything else with my S88-N system.

New year 2015:

I have now been building an S88-N-P module on a piece of veroboard. It took some time to design the board, which is a simplified version of the module from dif´ The S88-S-N module is a mix of a power supply, galvanic isolation between track, S88-N system and command station as well as a set of buffers.

img_2242My S88-N-P module

s88-n-p-diagram-del-1Diagram forS88-N-P – part 1

s88-n-p-diagram-del-2Diagram for S88-N-P – part 2

s88-n-p-veroboardDesign for S88-N-P on veroboard

Note that there is an improtant detail missing from the design above, which can be seen on the picture above: Ground from the track need to be connected to GND_BUS on the diagram if you do not use current detectors, but utilize Märklin contacts. This is necessary, since the S88-N-P module has it’s own separate power supply and since everything is galvanic isolated from the command station. The connection is by the wire, which can be seen on the picture being connected to pin 11 on IC2.

In addition, I have been building an S88-N module on one of the PCBs I bought from – S88 modul-N PIC628 with 16 inputs.

My only three problems were:
– a defect LED (re-use of a re-used very old component)
– a few problems with getting the PIC HEX file from to work in a 16F627A PIC. I solved it by making anew HEX file from the source code, which is also available from, but giving MPASM the option to build for 16F627A.
– drilling holes in the PCB from I tried with a regular Bosh drilling machine fixed in a drill stand that I bought. But it does not work at all with 0,8 mm drills.

An ordinary drilling machine is not particularly precise. The drill is not turning around it’s own center, but around some point outside of of the drill.

The solution was to buy a Proxxon machine with a drill stand. It works. And now I own a machine that can be used both for drillilng, cutting, grinding and more. I have already been using it for several purposes.

I chose Proxxon and not Dremel, because all the reviews I could find, pointed me in that direction. And also because Dremel is made of plastic and Proxon is made of metal. Go to your hardware shop and pick up the two machines and just feel the difference. I need to say that I may be very hard on Dremel without ever having owned or even used one.

I am very pleased to have been building an S88-N module that works perfectly. It has cost me a couple of days, a Proxon drilling machine as well as a little money for components and PCB. But most of all, it is a lovely feeling, because I feel I have achieved something. And I am now able to produce more S88-N modules for a very modest price and effort. COOL!!!


An S88-N module on a PCB from