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The GCA50 is based on the LocoIO design from John Jabour and can be used for all kind I/O depending on the drivers or sensors. Firmware is mainly LocoNet PE compatible
It differs from the LocoIO size and features other connectors for more robustness and flexibility, which was also the reason to start our own developments.
Without any extra satellite modules the I/O is 5V and only to use with low current LED's and reed switches, as well as opto-isolated sensors.
By using different (standard D-Type -) connectors, there was no chance of making fault connections, and also we could have our own choice of supplying the system.
GCA50 is full compatible with Loconet, exept for supply. Standard Loconet units are supplied by Railsync output, GCA50 requires DC supply.
All 16 lines on GCA50 can be used as input or output.
So switches (to start/stop all kinds of lights , leds, motors relays etc anywhere else in the LocoNet), pushbuttons (i.e. for turnouts) and Leds are to be connected directly.
The configuration for this all, can be done inside the rocrail program.

A wide choice of different interfaces are available (see The GCA Boards ) to connect servo_motors, InfraRed gates, current-detectors, turnout coils (single and double) etc.
For further info about programming GCA50, please refer to LNet I/O programming.

GCA50 pcboard

The build result

A friendly user of GCA-LocoNet boards has made this picture of his work:

Complete English manual



Older versions, like GCA50d, have U3 in reversed position (notch left).


Jumper Open Closed
JP 1 report input states at startup No report at startup

Use the Programming Dialog to setup the I/O.
Addressing type is PADA.

Reed switches and hall sensors

Read switches and hall sensors can be used as feed-back sensors.
However, since the contact information inside the GCA50 is arranged so that the unit is
low sensitive for distorsions, the minimum Pulse lenght to be accepted by GCA50 is 80 miliSeconds (0.08 second).
In addition to that, reed-contacts usually give more than 1 pulse when activated, known as 'contact dender'.
For this way of sensoring, a special pulse-extender is available.
For further information please look at GCA173.

A large variety of peripheral units to choose from

With GCA50 as a base in the LNet network, you have 16 IO ports, which can be used for input and output.
Output, coming straight from a microprocessor, is limited to 5V and 25 mA.
Just for driving direct connected leds, is is more than sufficient, but many devices in railroad environment need more power.
For that there is large selection of buffers, to get what you need.
These units can be straight connected to GCA50 with a 10-pin connector.
A few examples.:

  • GCA76 max 2A output for driving heavy loads like Maerklin turnouts,
  • GCA77 the smaller brother of it, with max 0,5 Amp or each coil.
  • GCA93 octal current detector with fused connections.
  • GCA94 octal very low loss current detectors for high currents.
  • GCA107 8 relais for multiple purposes.
  • GCA136 4 Servo motors with separate feed-backs for turnouts
  • GCA133 4 IR gates.
  • GCA173 Hall=sensor or reed-contakt inputs for feed-back.



Connector to J3(male), J4(female) Loconet DB9

The best suitable cable for this connection is cat5 Ethernet cable.

DB9 Pin LocoNet function Ethernet cable colour
1 GND blue + blue/white
2 Railsync - orange/white
3 Loco-I/O signal green
4 Railsync + orange
5 +V supply brown
6 GND green/white
7 n.c. not connected
8 n.c. not connected
9 +V supply brown/white

To avoid any problems, connect the wires according this list.

Changing wire colours could cause errors.

Standard full wired male to female 'extension' Rs232 cables are allowed, but up to a certain limit.

It is never experienced how many can be used, but a few do not disturb the communication, but it is by far not able to conduct the full 3 Amp of the GCA85 or GCA101!

Connector J1 + J2 screw terminals

LocoIO port terminal#
1 1
2 2
3 3
4 4
5 5
6 6
7 7
8 8
9 9
10 10
11 11
12 12
13 13
14 14
15 15
16 16
GND 17
+5V 18

Connector J5 10pin header

PSK header connectors

Some users seem to know better, and solder wires directly to the pins or the pc-board.!
That is totally unacceptable !!
Wires will easily break off, and cause a lot of damage in many cases.
So use connectors !!!!!
In order to ease making these cables, it is also possible to buy the necessary tool, to create the PSK cables.
The best wire to be used with PSK connectors is 0,25 mm².
This special plier will be sold for € 20,00 if ordered together with complete kits. crimpzange.jpg
Just ask peter.
Two possible options can be used for this connection. (see picture below)

Option 1 (left on picture)

This cable should be made with wire 0,25mm.

Qty Materials supplier
2 PSK254/10W
1 PSK-Kontakte (set of 20 pcs)
1 Psk-Crimpzange

Wires are NOT crossed or mirrored i.e. connect pt 1 to pt 1 , etc.

Option 2 (right on picture)

Flatcable 10x2 + connectors.
The side without the notch will fit easily.

Qty Materials supplier
2 PFL 20
1 Flatcable AWG 28-20G 3M
1 Crimpzange MWZ 214

Connector J5 10pin header

LocoIO port Pin #
+5V 1
1 3
2 4
3 5
4 6
5 7
6 8
7 9
8 10

Connector J6 10pin header

LocoIO port Pin #
+5V 1
9 3
10 4
11 5
12 6
13 7
14 8
15 9
16 10

Led functions

LED Meaning Color show indicates suggestion
1 power 5 V green always ON power OK
2 LocoNet red OFF No LocoNet activity
ON Loconet False GCA101 J1 set?
GCA85 SW1.1 ON ?
flash LocoNet activity

LED3 no longer exist since version 1.3.

Direct led connection to outputs

Configuring GCA50

The MGV50 needs a configuration for each of the 16 in/output connections.
This configuration is completely standard to Loco-I/O items.
Rocrail has a built-in possibility to do this.
First the GCA50 chip needs a dedecated adress, constisting of two numbers : Low adress and Sub adress.
Both can be numbers between 1 and 126.
With these dedicated adress bytes, it is remains possible to change any configuration of GCA50, while it is connected together with many others.
Input(feed-back)numbers and block detection numbers are possible from 1 to 2048.
Output numbers are possible from 1 - 2048 (FADA notation).


Be sure to mount the female and male d-connectors in the right position.
Male connector is with pins and mounted at the right hand side, female is with socket and mounted left.
The 6 AKL012-03 connectors must be assembled together, before mounting them on the pcboard, unless you are only to use J5 and J6.
In case GCA76 or GCA77 is used, connected to either J5 or J6, it is better NOT to mount the corresponding RP1 or RP2.
This will avoid that turnout-coils are responding on power up.

Direct connection GCA50 to Intellibox, Twincenter or Digitrax

The GCA50 is also suitable to make a direct connection to the above mentioned systems, without the need of GCA85 or GCA101.
The connection on IB for that is LocoNet T, because the B outlet does not have significant supply available.
To achieve this, a cable should be made with a RJ12 connector at one side and at the other side a d-sub9 Female connector

Connections to be made according the following list.

D-sub9 female pin (GCA50) RJ12 pin Function
1 2 or 5 Ground
3 3 or 4 Signal
5 1 or 6 +12V


On the connection T of the IB, the max. capability of the supply is only 500mA.
Twincenter needs update to Version 1.1 to enable setting of Interface to Syntax = LocoNet.

More power demand.

Some connected peripherals, like GCA107 with 8 relays, might need more supply,
resulting in overheating the VR1 regulator.
In that case, VR1 can be replaced by R-78e5.0, which can supply the needed current without overheating.
Qustions ? ask Peter.

mgv50-en.txt · Last modified: 2018/11/26 02:20 by rainerk