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I am currently wiring 138 inputs into 2 MEGA2560 boards. I will be using all 8 ground/common pins on the 2 boards. Can I use a pin for input on one board and the ground on the other board? I would think so (but am not sure) as they are both grounding through the same wire in the USB hub.
Thanks,
John

Hi

At first.... THIS is not possible.....
A Mega got 54 Digital Pins ( D0-D53) and 16 Analog Pins A0-A15 .... 70 Pins together..... 2 Pins are INOP ( Dig 0 and Dig1) .... So Overall 68 Pins for use.
2 Megas ..... 68*2=136 ...... Its not possible within official regulations to use 138 Inputs with only 136 Pins !!

To Topic.

Technical your right... BUT there "can" be a problem.... e.g. if the USB Ports are not shared electrical.

The most easy way is to build a "Common Ground Potential"
Simply wire a empty GRD From Mega1 to a a empty GRD Pin from Mega2 ....... In a biger Build go forward..... E.g. Build a single Wire that is connecte to every Mega in the System.
In easy words.... If all GRD from all Megas are combined ( whatever in one Loop or parallel) Then you got "ONE" GRD for the whole system.

Note:
Its not needed technical for inputs to split your devices to the 4 GRD on a Mega or 8 GRD Pins on both. In Theroy you could combine all GRD wires to one single Point and into one GRD Pin. There exist systems who use 500+ Devices with only one GRD wire.

Cool, Your right. it's only 136 inputs. I was worried there for a minute as I had counted them all out to make sure there was enough. But then I remembered that was on the last itineration of the board. This one actually only has 110 inputs.
I have all the grounds daisy-chained to 8 ground wires from the panel. I can do a jumper between the boards when I put the connectors on the ends of the wires.
I also have a 4 port hub mounted in the panel enclosure so there is no worries about there being a single ground.
Thanks for your reply. Now on to the final phase of assembly.
John

Normaly there is no problem.... But believe me.... In the past we got really strange issues we could not understand.... After GRD sharing they was gone ( Whatever electrical we still think the sharing was there not needed anymore) ..... So its a good choice to do this.

Summary; Simply test your system. If all INputs do what you expect then all is fine.
If there is a problem give it a try and combine the GRD´s ....

By the way.... You say you have 2 Megas and 8 (2x4) GRD Wires.... The Share is no trouble. The MEGA got 5 GRD Pins overall.


2 On the Left Power line.... 2 on the Double Block below Pin 52/53 and 1 next to Pin 13

Should be no problem to connect a old Jumper Wire !