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GCA124_8 Loc controller with function keys


By Peter Giling


This hand held throttle is very well supported in Rocrail.

The Hardware

The schematics
The PCB design and parts positions
The bill of materials
N.B. Only complete ordered kits will be supported!

Jumper settings JP1

Therer are two different incrimental switches.
The different is the way the code is made.
It results sometimes in having the speed control working the wrong way.
Since I never have been able to figure out which is which, the GCA124-8 is equipped with two
jumpers, where the function of the speed control can be reversed.
So if its working opposite, just position the jumpers 90 degrees from wthe actual position and
you will be fine.

Assembling instructions

A few remarks have to be made to help you being succesfull with the production of this FredI.
Fix the empty PCB with screws in it's future housing, and use it to drill the exact positions of the pushbuttons, SW2 and leds.
Use the pcb as a mould for holes of pushbuttons, leds and SW2.
Pushbutton positions have a little center hole for this purpose.
All other parts can be mounted after that.
The pushbuttons and leds are mounted at the backside of the PCB.
Please take care not to use the center hole of the leds, because that one is only a drilling mould.
After a serious check, connect the pcb , without IC's inserted, to LocoNet. Check if 3,3 volt is correct between pt 7 and 8 in IC1 socket.
The processor in the kit comes with set id number. After insering IC's and startup, the three leds wil indicate a loop constantly.
After pressing all buttons, tuning the knob, and pressing it, the lo0p will stop, and GCA124-8 is ready to go.

Testing before inserting IC1 and IC2

After assembling the pcboard, it is better to test the 3,3 Volt power to the processor, before inserting the integrated circuits.
Just connect GCA124_8 to LocoNet and switch Railpower ON.(Important!)
Between points 7 & 8 of IC1 should now show 3,3 Volt.

Short startup instructions

  • Insert chip, and connect GCA124 to LocoNet.
  • Three leds will show slow running lights.
  • Be sure to have power On in LocoNet.
  • Press all, buttons sequently, including pressing and turning speed knob.
  • If that all goes well, the FredI will respond with leds in fast running light.
  • Disconnect GCA124, and reconnect again.
  • Now red led is lit.
  • Prepare a loc in Rocrail for dispatching. See below: Rocrail and GCA124-8
  • Press shift button (bottom one) and speed knob together.
  • Green led will be on, you've succeeded.

Rocrail and the GCA124-8

  1. For setting up a Fredi in Rocrail look at: setting up a LocoNet Slotserver
  2. Leave the Interface ID in the loc setup empty: LocDialog Interface Tab
  3. Fill in the LC IID and the DP IID in the : Rocrail general Tab
  4. Fill in the IID of the Command Station that controls the locs (LC IID) : Loconet Interface Tab
  5. Tell the desired loc to dispatch, look at : Throttles

Important notes

It is important that Rocrail also has a central system connected.
GCA124-8 cannot be used without Rocrail to dispatch a loc directly via Intellibox.
After dispatching a loc via Rocrail, Intellibox which will act correctly.


Firmware can be found here for download:

Example with GCA101

ID number

When there are multiple units connected inside the same LocoNet network, each unit needs a different
ID number.
With a suitable programmer for Atmel chips, it can be programmed.
This ID number of two bytes is located in EEprom at the follwing locations:

EEprom offset part of word
0 High-byte
1 Low-Byte
gca_gca124-8-en.txt · Last modified: 2023/10/13 16:12 by phg