User Tools

Site Tools


GCA101: LocoBuffer-UDP


Your train is going into the next generation .. with ETHERNET!
This will be the ultimate solution for communication between Rocrail and LocoNet-I/O®.
Almost each in the market available computer now-a-days is equipped with an Ethernet connection.
The USB does not always seem to be the perfect solution as substitute for the RS232 Comm port.


  • 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

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 useful.

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 a lot of experience with networking.
One important remark 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 unusable.


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.
On board power switching power supply with 3 Amp output to be able to supply a large range of Loco I/O modules MGV50 I/O modules.

The necessary transformer should have minimum of 52 VA.
Power supply : between 15V and 30V dc or between 12V and 24V ac.
Transformer or adapter MUST be dedicated ONLY for GCA101. No other connections should be made to transformer!
Ignoring this can result in damage of GCA101 or connected periphery.

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.



The GCA101 is provided with a programming connector ICSP1.
To program the Atmel chip you will need a programmer.
A programmer with suitable software is the mySmartUSB-light.

On Linux flashing the firmware is easy with avrdude:

avrdude -F -pm168 -cstk500v2 -P/dev/ttyUSB0 -Uflash:w:MGV101_UDP_TEST.hex:a

The programmer ID is "mySmartUSB-Light" firmware version dependent: stk500v2 or avr910/911.
For Windows, just use the software which comes with the mySmartUSB-light.

One other programmer, which comes with a very large library wfor many different branches and types (including all Microchip processors) is MINIPRO TL866A.
At this moment available from GCA.
Ask Peter

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 Purpose
JP1 15 mA Current source Loco-I/O line
JP2 +12V to Railsync+

Current source, what is that?

Click here for a brief explanation about it.

New development for GCA101

GCA101 also can be used with TCP.
The standard address for that is
Processor should be changed to ATmega328P.
Software is developed in Spain by Dani Guisado.
Firmware is downloadable here: ​
Website of Dany is also interesting :

Mounting instructions

  1. First start with all resistors and IC sockets.
  2. For socket of U2, it might be necessary to cut the middle bridge in the socket, to free space for C7.
  3. Then small capacitors and small elco's.
  4. Then all connectors, leds, fuse and dip-switch.
  5. Before mounting VR1 AND VR2, first fix the cooling block with two screws.
  6. After that place VR1 and VR2 in the pcboard, adjusting the wires in the right position, so that metal surface lies flat to the cooling and fix them with the clips.
  7. Be sure that all IC's are flat to the surface of the cooling, cut the wires at the bottom at 1 mm outside the pcboard surface, and solder the wires.
  8. Take a double check on the right types of VR1(LM2576t-12) and VR2(LM2575T-3.3)! They might sometimes be very hard to read.
  9. At last fix bridge rectifier GR1 and C1,CE1. The picture above shows clearly that the wires of GR1 are slightly bended, in order to clear space for C1. In case the total power consumption of the connected LocoNet is no more than 2 Amp, one of the two big capacitors can be omitted.
  10. Before inserting U1, U2 and U3, test power 3,3 Volt on pins 1 and 8 of U3.

Rocrail Setup

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.

mgv101-en.txt · Last modified: 2021/05/17 19:06 by phg