S88N decoder 8 channel model railroad block occupancy detector

This S88 decoder model railroad block occupancy detector can be used to automate your model railroad with software running on your computer.

The schematic has been based on traditional S88 block detectors with a 4014B shift register with one major change: there are no 4044B quad latches used. These latches do not add any value in a scenario where one must detect block occupancy by measuring current usage. The 4044B latches have been replaced by a simple RC pair. In my opinion the RC pair adds simple filtering of interference signals. You do not want to detect ghost trains by the very fast triggering of the 4044B latch.

The RC pair can be found behind the optocoupler. The optocoupler (or opto-isolator) gets it signal from a diode bridge rectifier. The rectifier has been shorted at the positive and negative poles, this creates a small voltage difference for the optocoupler.

The diode bridges are specified for 1,5A continues usage. This should be more than enough for sections for H0 model train track. It survives short circuits because the DCC booster cuts the power when such an event occurs. The diode bridge (W02M) can withstand 40A for 8,3ms, or if the short has a lower maximum current the duration can be longer.

The S88N decoder has 2 independent pairs of 4 detector inputs. The 2 pairs can be used in different booster regions or when you have a section of a track that switches polarity (such as a Y-loop). Or the pairs can be tied together to make an 8 channel detector.

Detected block occupancy signals are transmitted to the command station via UTP cat 5 cables.

The maximum number of S88 decoder inputs is 496 per string. These are typically configured in multiples of 16 inputs (31×16) or multiples of 8 (62×8).

More information on the S88N specification can be found here: www.s88-n.eu

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