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GCA174 Decoder for GCA145 to simulate Märklin / Fleischmann / Lenz standard turnout decoders.


By Marc Roede and Peter Giling


Many software control programs do not have the ability to use the 7 control bits as a Byte.
They use the standard method for controlling Turnouts.
This board is only attachable to J5 of GCA145/MGV145, and simulates either Märklin, Fleischmann or Lenz turnout controllers, depending on the programmed firmware
For each brand name, there is special firmware available, see below, under 'The board design'.
It receives the standard codes and converts that to 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 board design

Address selection

The adresses are identical to the Märklin / Fleischmann / Lenz standard, depending on the programmed firmware.
N.B. Besides the short manual, as presented here, it is also adviseable to consult the original manual for your turntable dekoder.


GCA174MR 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 GCA174MR.
By connecting J2 pt 3(+) and 4(-) to any device like S88 units or derivates,
Settings in Rocrail , see Position sensor
This drawing shows how to connect GCA147MR.

Led signals

GCA174MR is priovided with 8 leds for indication of decoded turntable position.
The led with indicatio 'wr1' shows the write signal ( in Rocrail, look for 'new Position Flag' ), to inform GCA145 to start responding on the given position.
The led with indication 'rdy1'indicates that GCA145 has returned the 'turnrtable ready' signal.

Extra modification on blue coloured GCA145(MGV145) boards

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

gca174mr-en.txt · Last modified: 2018/11/12 08:56 by