Hello, I'm trying to connect two (more later) Megas for now.
I read about the I2C pullups and signals relation here.
So apparently because every Mega has 10k external resistors on SDA/SCL, then do I understand it correctly that the sum Ohm value is too high for I2C communication between them without physically working around the resistors on the pcb?
I tried to follow this code and I'm getting random digital ports 1/0 values and "-" skips from the slave mega.
When you connecting two MEGAs using I2C Bus, then the pull-up resistors are coming in parallel and the resultant pull-up would be: 10K||10k = 5k which will should not create any problem for the I2C Bus operation.
There is also an internal pullups resistor. You can connect about 5 Arduino Mega 2560 boards together. When you need to add more, you might have to remove those 10k resistors from the board.
The maximum current to pull SDA or SCL low should not exceed 3mA.
The I2C bus has three wires: SDA, SCL and GND. Don't forget the GND 
The maximum total length is 50 cm.
Do not use that example at: Rod's Electronics Projects: Extremely simple I/O port expansion using two arduinos and I2C.
It is old and wrong.
Can you do your project with a single Arduino Mega 2560 board ?
Thanks everyone for replies!
Well I'm trying to make a harness cable tester. I need at least 430 digital I/O pins where only any two pins needs to be accessible at a time. Pins also have to be both inputs and outputs just like on the Arduino board.
The reason I'm trying to do this with multiple Megas (I need seven for this) is that their chinese versions are sold very cheap here, having the genuine Atmega chip. Also I thought coding and no-solder wiring would be much simpler.
I guess I'll have to go with the suggestions here using groups of four MCP23017 expanders for each digital pin on board.
Another option I can think of is TX/RX serial ring or chain, but communication would be very slow I guess.
What about the SPI bus? Could it work for seven boards?
pururu:
What about the SPI bus? Could it work for seven boards?
Possible using external 74LS138 (3 to 8) decoder. Edit ( 3 to 8)
Or even a (3 to 8) decoder