Read output from another arduino

Im working on a project which needs to have 4 rfid readers and I suspect that its not possible to have multiple readers per arduino(?).
So my plan is to use five arduinos and have four arduinos connected (starformation) via their digital output to the fifth arduino which could summerize the four inputs? Can the fifth arduino be used to detect a HIGH signal output from the other fours?

Sorry for my bad english and possibly bad explanation...

First and most important, it may be possible to use a single Arduino for the 4 rfid readers and if so that would be much the simplest solution. Post a link to the rfid reader datasheet.

If you do need to send data to another Arduino …
What data do you want to send
How do you propose to send your data via a digital output (and I don’t mean to imply that is impossible)

I assume the multiple Arduinos are close together and a wireless link would not be justified.
You could use SPI to transfer data - it is designed for multiple devices.

Any Arduino can easily detect a HIGH or LOW from any other Arduino. But you do need to have the GNDs connected.

…R

Robin2:
First and most important, it may be possible to use a single Arduino for the 4 rfid readers and if so that would be much the simplest solution. Post a link to the rfid reader datasheet.

If you do need to send data to another Arduino …
What data do you want to send
How do you propose to send your data via a digital output (and I don’t mean to imply that is impossible)

I assume the multiple Arduinos are close together and a wireless link would not be justified.
You could use SPI to transfer data - it is designed for multiple devices.

Any Arduino can easily detect a HIGH or LOW from any other Arduino. But you do need to have the GNDs connected.

…R

Yes, if it would be possible to only use one arduino it would be the best and easiest solution. Im planning on using four rfid-RC522 shields. And this is the datasheet for MFRC522chip. http://www.nxp.com/documents/data_sheet/MFRC522.pdf
If thats possible please tell me how. =)

The readers will be a few meters from each other in cable length so as you thought, no need for wireless.

Thank you. So if it’s not possible to use only one arduino I will use 5 and share their GNDs between them and use only a HIGH or LOW signal to communicate. Since the information only needs to go one way.

edit* Fixed the pic. Sorry.

Image from Reply #2 so we don't have to download it. See this Image Guide

...R

The board seems to be able to use SPI and any Arduino can communicate with several spi devices. The SS pin on each device needs to be connected to a different Arduino I/O pin.

I don't know if there are limitations on the distance for an SPI connection.

...R

The readers will be a few meters from each other in cable length

If even possible, you will have great difficulty using SPI or I2C with long cables between the reader and the Arduino.

You could in principle use the serial TTL (UART) for long connections.

If you only need a single HIGH or LOW reading from each combination of Arduino and RFID reader then a simple wire connection between digital pins should work.

Using a serial connection between one Arduino and multiple RFID readers brings with it the need for multiple serial ports on the receiving Arduino (and a Mega has only 3 spare USARTs) or, if all the readers feed a single USART, there needs to be some means to prevent two or more from talking at the same time.

I guess you may be able to use a Mega and use SoftwareSerial to create a 5th serial port - leaving Serial0 available for communication with the PC.

Or, perhaps you could site the Mega beside one of the readers and use SPI to communicate with it and the 3 serial ports to communicate with the other 3 readers.

...R

Thank you for all your answers. I googled it a bit and Ill think I go with 5 arduinos. The other options is a level beyond my skillset. Yet. =)