Acknowledge Detector Variant 1

This circuite can be used for detection of Basic Acknowledgments form a Digital Command Control (DCC) Decoder following NMRA RP9.2.3 (Service Mode). It can be used together with (or to modify) DCC-MB or TMWDCC.

(ACKDET01.GIF)

Notes

• You can invert the output signal by exchanging the inputs of IC1A (simply exchange R2, R3 and C1, C2)
• If you don´t need an Optocoupler between the track and the PC remove R1 and connect IC1A,2 direkt to U10A,1
• I have tested this circuite with a Digitrax DH84G Decoder and a current-limiting-resistor (Resistor R1 of the TMW4 board) of 120Ohms. The Voltage at the track has been approx. 15V. It drops to approx 3V when the decoder gives the acknowledge pulse. A puls of approx. 35mV and 2.5ms can then be measured over R3. This gives an high-level-pulse of approx. 2ms at the input of U10A. With TMWDCC (Version 1.11) I have successfully read back values form the decoder.
• The calculated DC-input-levels of the comparator IC1A at different track currents are:

Track Current / A	level at pin 5 / V	level at pin 4 / V	difference /  mV
0.000	0.0495	0.064	14
2.123	1.0396	1.051	11
3.191	1.5347	1.5449	10

copyright 1997 by Martin Pischky (martin.pischky@fernuni-hagen.de)
( ACKDET01.HTM / creation 970215 / version 970216 )