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GCA174 DCC/Motorola decoder for GCA145


By Peter Giling

Direct decoder for Motorola or DCC

This board is based on the design of
The firmware , copyright of, is available at
This board is only attachable to J5 of GCA145/MGV145.
It features the 7 outputs, needed to select position 1..48 on GC145/MGV145.
Also an isolated feed-back is provided.
Feed-back is free to be used with systems like S88.
The on-board opto-couplers isolate the didital rail signals and feed-back from the board.
Feed-back is very nice inside Rocrail environment, because it can signal Rocrail to start a train AFTER the selected postion has been reached.

The prototype

The board with opto-couplers
ready to be connected to GC145

Hardware files of protoype

The final board design

board in production, coming in short term

Hardware files for final GCA174 design

Address selection

Basicly, the firmware from comes with adress 1..7.
By pressing the button, the decoder is in learning mode, showing D1 to flash.
All you have to do now, is sending the first address from your central unit.
After that, just pwer off GCA145 and power on again.
The addresses of the next ports will be automaticly set in following sequence
One other option is to download hex_minipu from this page: .
It will give you the option to directly change adresses and store them, before programming the chip.


GCA174 has a feed-back signal, to inform Rocrail that turntable is ready to move trains on/off.
This feed-back signal is electrical (galvanic)isolated from the GCA174.
By connecting J2 pt 1(-) and 2(+) to any device like S88 units or derivates,
Rocrail will receive this port condition.
Settings in Rocrail , see Position sensor

Led signals

GCA174 is priovided with 6 yellow leds for indication of decoded turntable position.
The red led(D7) shows the write signal ( in Rocrail, look for 'new Position Flag' ), to inform GCA145 to start responding on the given position.

Connections of GCA174

Extra modification on GCA145(MGV145)

GCA174 needs 5 Volt which is by default not available from J5 on GCA145.
Therefore, this diode 1N4003 (or equivalent) should be mounted under J5 as this picture shows.
NOTE: The latest green pcb has this diode(D10) on board, so no further attention needed.

gca174-en.txt · Last modified: 2019/01/03 16:22 by phg