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GCA101v2: LocoBuffer-UDP (TCP/IP is optional)


Hardware by Peter Giling

For the previous version of GCA101 please refer to this page


GCA101 was ready for an update.
Since dc power supplies are getting cheaper, this unit did not desire a special on-board power supply any longer.
So the new design is smaller and fitted with a very compact and good quality switching regulator for the 3.3V.
Furthermore, the complete design is only changed where it comes to power, resulting in a smaller unit.


  • solid GCA-Compatible power supply
  • isolated coupling between PC and Ethernet
  • on-board ICSP for easier development and self programming of ATmel processor
  • 2 x RJ12 connector for Loco-I/O periferie
  • 2 x D-sub9 connectors
  • clear on-board indication of present power and Ethernet/Loco-I/O functions
  • entirely compatible with all Loco-I/O PE commands
  • fully supported by Rocrail

Power supply

Only dc power is allowed.
Power supply : 12Vdc minimum 3 Amp.
GCA can provide you with a good and small 230V to 12Vdc supply.
Please refer to pricelist.

Ethernet cable connection

The GCA101 is equipped with a special Ethernet connector.
It provides an electric isolation , which means that computer and your trains are not electrically connected together.
The standard Ethernet cable can be one of the two types. Either normal or 'crossed' cables are useable.

Ethernet connection

Connecting GCA101 to PC is best done by means of a router in between.
So your home network will be one way to do it.
If you do not like this situation, use a dedicated cheap router for it.
Direct connection GCA101 to PC is possible but requires some experience with networking.
One important mark to old left-over routers:
Routers previously used at home with your previous Internet/ Tv provider, will experience problems with connection via UDP.
This is probably caused by the UDP channel inside the router, which should be opened by passwords/codes/pin numbers.
In this case, that router is unuseable.

One other problem might cause no connection in Windows.
The ethernet module in you pc might be not the only one.
In that case Windows sometimes has a problem to decide where LocoNet communication should be sent to.
Best way to solve this, is tot find out which inside ethernet adapters are not used, and shut them off in Windows.


Schematic design
Printed circuit + parts positions
Bill of materials
Gerber files
N.B. Only complete ordered kits will be supported!

Two processors on this board are installed.
One to connect Ethernet to the standard SPI and the second as interface between SPI and Loco-I/O.
The unit is equipped with two Loco-I/O RJ12 connectors and two D-con-Loco-I/O connectors.

Unit is available as board only or as complete parts package, including programmed processor.
Ready built units will not be available.

Thanks to Robert Evers & Fred Jansen who have put much energy in getting the system up and running, you will finally be able to run Rocrail and Loco-I/O without USB and/or RS232.


Led signalling (Firmware Version V2.1+)

The three green leds in front are indicating different functions.

LED Function
3 (left) ethernet receive / Loco-I/O transmit
4 (mid) Loco-I/O receive
5 (right) ethernet transmit

Jumper settings

Jumper JP1 Purpose
15 mA Current source Loco-I/O line

Jumper JP2 Purpose set
+12V to RJ12 1+2
RJ12 gets power from
Railsync (central station)

Current source, what is that?

Click here for a brief explanation about it.

New development for GCA101

GCA101 also can be used with TCP.
Processor should be changed to ATmega328P.
Firmware is developed in Spain by Dani Guisado and downloadable here: ​
Website of Dani is also very interesting :
Leds blink when waiting for a connection from Rocrail.
When Rocrail is connected, Led2 and 3 show Loconet activity (send and receive) and led4 Ethernet activity.
Standard Ethernet address:
Other IP addres can be pre-programmed in INO file before programming the chip.
These adresses are stored in EEprom from address &H0000 to &H000B :starting with GCA101 address, GCA101 sub address, gateway address.
On ordering kits, you can also inform me with your internal Ethernet addresses, so it can be programmed in the processor for you.
Communication needs to be set as LbServer in Rocrail settings > controller:

LED SIGNALLING with LN/TCP firmware:

GCA101 board
LED3 Ethernet receive continue blinking means waiting for connection
LED4 Ethernet send continue blinking means waiting for connection
LED5 Loconet activity
GCA101v2 board
LED2 LocoNet send
LED3 Loconet receive
LED4 Ethernet activity continue blinking means waiting for connection

Mounting instructions (Version 2)

  1. First start with all resistors and IC sockets.
  2. Then small capacitors and small elco's.
  3. Then all connectors, leds and fuse.
  4. Before inserting U1, U2 and U3, test power 3,3 Volt on pins 1 and 8 of U3.

Rocrail Setup (for multicast)

Firmware version: 2.10x

The communication between Rocrail and the GCA101 is by the User Datagram Protocol. Therefor only the LNUDP LocoNet interface type is supported.
Setup Rocrail with the exact values shown below except for the Local IP address which is in most cases not needed.
The GCA101 own IP address is not used in the Rocrail setup.

In case GCA101 is not the first controller
in your setup, do NOT forget the correct IID !


This is the Multicast address to use for communication and must be set to the FIXED value of
– The All Hosts multicast group addresses all hosts on the same network segment. –


The service port must be set to the FIXED value of 1235.


LNUDP is used as standard.
With different processor and firmware (see : ), LNServer
is possible with TCP address

Local IP

Only required if the PC has more then one network interfaces installed; In all other cases: Leave empty!
IP address of the PCs network interface which is used for communication with the GCA101 to make sure the traffic runs on the right one.

mgv101v2-en.txt · Last modified: 2024/03/21 22:27 by phg