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MGV84s Quad Servo interface with standard relays


By Peter Giling

Latest news..............

If you ever plan to copy and build this interface, you could also refer to the newest version MGV136/MGV137 combination.

This board replaces MGV81,MGV84 and MGV84s.

The new interface will be available as factory made printed circuit.

Next news is that also for MGV84s, new software (freeware) will be available shortly, making it possible to adjust left an right angle of the servo, the running speed, and relay choice.

This is all done while the MGV84s remains connected as usual to MGV50.

No extra connection or anything is to be made. Only new fromware needs to be programmed in PIC chip.

Adjustment pots and jumper will be without function after installing the new firmware.

Implementation of this application in Rocrail is scheduled.


Some turnouts have a way of sudden change of the frog, which does not allow the polarity to change in the middle position of the motor.

In that case the use of MGV84 is impossible and MGV84s is the solution.

MGV84S uses standard cheap relays, and will switch power to the frog after the servo motor has stopped.

The small disadvantage is that for each turnout one relay should always be on.

The interface will switch off power to the frog, and only will repower it after the new position has been settled.

The rest of the unit, including the way to adjust all positions, is mainly the same as MGV84.


For each turnout there are three individual settings to be made:

  • Speed of the motor.
  • The total angle, adjustable between 10 and 180 degrees.
  • The position of the relay. (normal or reverse)


  • Relay for each turnout frog
  • Feedbacks for each servo.
  • Isolated input and feedback, to avoid ground loops.

The prototype

The hardware

The schematics mgv84s-sch.pdf
The pcblayout + parts placement mgv84s-pcb.pdf
Connections diagram mgv84s-dwg.pdf


Basic, assembler and .Hex file

Basic program is compiled with Picsimulator IDE (see

Bill of materials

Components list mgv84s-bom.pdf

Connection to MGV50

refer to:

The Switch and Input address should be numbered according your own plans.

In general consider that connections are made to MGV50 1 to 8 (=J5) OR 9 to 16 (=J6).

The first 4 are MGV50 command outputs, the second 4 are MGV50 inputs.


Suppose MGV84s is connected to MGV50(J6). (see example below)

We want to command Servo's 1 to 4 to be activated with Portnumber 52, 66, 103 and 104.

We want the feed-back of the turnout position with servo's 1 to 4 to be number 121, 122, 123 and 124

This means that on MGV50 I/O #9-12 must be programmed as switch with number 52,66, 103 and 104,

and MGV50 I/O # 113-16 should be programmed as input with number 121, 122, 123 and 124.

Making adjustments

To set these parameters in the standard software, follow the next steps:

1  - Make all necessary connections to MGV84s.
2  - Shut off power to MGV84s.
3  - Shut off all 4 inputs (if already on)
4  - Connect servomotors to MGV84s
5  - Set jumper **jp1** to 'pgm' position.
6  - Power up MGV84s.
7  - MGV84 is now in adjust mode, Led3 is lit constant.
8  - Activate 1 of the 4 inputs.
9  - The corresponding servo will now run up and down.
10 - The speed of motor can be changed by **P1**. (clockwise is faster)
     NOTE: changing the speed will only occur after the motor has been in the left limit position.
11 - The angle of the motor can be adjusted by **P2**. (clockwise = smaller angle )
     NOTE: changing the angle will only occur after the motor has been in the left limit position.
12 - On of the two relays for the turnout frog will be switched on after the servomotor has reached its final position. 
     Which of the two relays is used can be reversed by changing position of jumper **jp1**
13 - When motor runs to satisfaction and the right relay is activated, inactivate input. 
14 - The parameters will be stored and the motor will stop in left limit position.
15 - Proceed, if necessary, with the other servo motors, starting at **item 8**.
16 - Before activating an other input, and before powering off, wait until previusly adjusted servo has come to rest.
17 - After all servo motors are set, shut off power, and put jumper **jp1** in nml position.
18 - Then turnon MGV84s power.  MGV84s is now ready for use, led3 will flash.
mgv84s-en.txt · Last modified: 2018/11/12 08:56 (external edit)