Ethernet connection
NOTE: If you use the USB version, you do not need the Ethernet module and information below.
Use it only if you want to use LAN to connect the master board instead of USB!
You need to have working local network with DHCP router to ensure automatic connection (no need to set IP address for Arduino and PC). Just connect Arduino with attached Ethernet shield (or Ethernet module) to any free socket in your router.
When uploading RealSimControl firmware make set the "Ethernet" checkbox in the "Firmware Upload" window.
Ethernet modules
For connecting Arduino to the local network additional Ethernet board has to be used. This board is wired with Arduino and provide communication with your computer over Ethernet. To work with RealSimControl over LAN use either "standard" Ethernet Shield W5100 or its small W5100 module version.
The W5100 Ethernet shield and W5100 Ethernet module are the same functionally and both occupy 5 Arduino Mega pins (#10, 50,51,52,53) that cannot be used for input/output.
It's up to you what board you want to use for LAN communication in RealSimControl, but in my experience the small W5100 module is more preferable. It can be connected to Arduino, using a thin plume of 4 wires (to pins #10, 50,51,52) and allows you to have more comfortable access to all other pins.
Note that "classic" Ethernet shield besides, uses one additional pin #4 that is used as SS signal for SD card. You could think you can use it, if SD card is not used, but tests showed that it can lead to errors with data exchange and unpredictable behaviour.
But, you can use pin#4 for input/output, for that you have to isolate this pin (just bend the pin aside off the Arduino connector) and connect it to 3.3V ( the shield pin, not Arduino), as on the right photo.
Ethernet Module W5100 connection
This small and convenient module has connectors that should be wired to these Arduino Mega pins:
SS 10
MOSI 51
MISO 50
SCK 52
The GND and +5v connectors should be wired to respective Arduino pins.
Notes about "511 resistors issue" on Ethernet Shield
Sometimes you can get several similar W5100 Ethernet shields, but some of them work excellent and others work unstable (or refuse to connect at all) and one of the reason could be in a difference in components used in those boards that is described in some blogs and forums pointing on this problem and its solving in relation with matching resistors.
For example, I have got these two W5100 Ethernet shields that are look completely identical.
But if you will look closer to each of these shields, specifically in the area near of the Ethernet jack, you may see 4-resistor array that is labelled with “49R9” on one board and the same resistors labeled as "511" on the second shield:
Could it be the problem with connection?
Answer is - Yes and No!
If you look at this reference design diagram you can see that 4 resistors R3,R4,R5,R6 are 49,9 Ohm (marked as 49R9 on the board).
If manufacturer used 51 Ohm resistors instead (labeled 510 array) there will be no difference, the shield will work fine.
But, if this resistor array is mistakenly replaced by 510 ohm resistors (labeled 511 as on one of my boards above), there could be a problem, but not nessesary will be.
If you have such a board you can replace the 510 ohm resistors. If you can’t find the proper replacement resistor pack then you can to solder a 100 ohm resistor between pin 1-2 (Tx+/Tx-) and a 100 ohm resistor between pin 3 and 6 (Rx+/Rx-).
Experience
I have one shield that has 49R9 (49 ohm) resistor array - works perfectly (always in Auto ArdSimX mode)
Another shield has 511 (510 ohm) resistor array - and it also works perfectly (always in Auto ArdSimX mode)
On the other hand, the same shield with same 511 on-board stop working after few tests... Why so, I'm not sure, but fact is that both shields has 510 Ohm resistors.
