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Greensboro, MD
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would it be fatal (to the boards) if i ran a wire from the output of one arduino to the input of another. causing arduino 2 to do something when arduino 1 went high???
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Basically, there is no danger to connect output to input. Put resistor in series (200 OHm) if you make connection between two outputs by mistake.
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thanks for the quick response i have one arduino uno at the moment that controls a door and 3 sensors along with some satus LEDs this takes up all the 14 i/o pins on my board. i now want to add 2 keypads and 2 RFID readers with no more pins. so my plan was to have another arduino do the processing of the keypads and RFIDs then just send HIGH to an input on the board that actually does all the works with sensing door position and moving it.   do you think this sounds plausable??
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Hard to say, as you provide description with zero technical details:
- what board do you have? For Uno, analog pins work also as digital - 20 pins overall;
- there are could be options to multiplex or simply set in parallel your sensors, what kind?
- the same with keypads, RFID - always there is a way to minimize width of bus / interconnections.
In general, I'd use two boards only in case of high safety/reliability requirements for redundancy.
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nr Bundaberg, Australia
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There are at least 20 IO pins on most Arduinos.

But apart from that I'd use a shift register or IO expander before I went to a second processor unless there's a real good reason. 

_____
Rob
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Rob Gray aka the GRAYnomad www.robgray.com

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completly forgot about the 5 analogs pins.

Currently im using this setup

PIN | Setting | Use
5      Output    Alarm
1      Output    High for reed switch 1
2      Input    digital Read for reed switch 1
3      Output    High for reed switch 2
4      Intput    Digital Read for reed switch 2
6      Output    High for reed switch 3
7      Input    Digital Read for reed switch 3
8      Output    Control Pin for Motor Relay
13      Output    Status LED

Total = 9 Pins
if i add 2 RFID Readers that another 6 Pins. The parrelax has a TX RX and Enable PIN
and then the 2 keypads totalling 7 Pins a piece for a 4X3 That adds another 14 pins.
Now im at 29 Pins smiley-sad
I dont know how to use shift resistors or multiplexing.
any help is greatly appreciated.
thank you in advance
-Richard


I am using the UNO btw.
« Last Edit: December 12, 2011, 06:09:30 pm by rkn2008 » Logged

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Shift registers are great for multiple simple inputs or outputs, like your reed switches.  I guess the keypads could be interfaced that way too, although I've not tried.  Essentially, they turn 3 pins, which you use for serial communication with any number of registers, to control or read multiples of 8 (or 4) digital inputs or outputs.  Three 7-digit displays, for example, would take 24 pins but you can use just 3 and string 3 shift registers together to handle them.

There are some nice tutorials on shift registers:

http://www.arduino.cc/en/Tutorial/ShiftIn

http://www.arduino.cc/en/Tutorial/ShiftOut
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