Electronic – arduino – Wire up multiple slave Arduino devices to a master Arduino device

arduinoserialwiring

I want to use serial communication between a master and multiple slaves.

I've already connected the RX of the master to the TX pins of both slaves, and vice versa (thus connected the TX pin of the master to the RX pins of both slaves). I also connected the GND pins.

But now I got weird characters when I open the serial monitor window.

Now how do I properly connect a master Arduino device with at least two slave Arduino devices using the RX and TX pins? Do I need to connect 5V pins?

PS: I see a lot of words like "SPI" and "I2C", and people start using terminology. But I'm not thát experienced, so please, if using the terms, explain them.
Another PS: I want the Arduino devices to use power from the master Arduino device.

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Now how do I properly connect a master Arduino device with at least two slave Arduino devices using the RX and TX pins? Do I need to connect 5V pins?

Poor-man's networking:

The circuit below is "naughty" in the extreme but can be made to work "well enough".
You can do the same thing using formal gates.

Top circuit:

Master + N secondaries.
Master is always heard by all secondaries.
Master can hear any one secondary.
Secondaries cannot hear other secondaries.
Multiple secondaries transmitting simultaneously corrupt both signals.

Master RX is pulled up by resistor.
Either slave can pull Master RX low by setting their TX low.
Only one slave can :"talk" at a time if corrupt transmissions are to be avoided. Max limited by time constant of R plus circuit capacitance.
Too low an R cannot be pulled down properly by TX.

Bottom circuit: All peers.

Any can send (one at a time).
All will receive anything sent.
Sender will receive own transmission and can judge if transmission was corrupted.

Useful for eg Pure-ALOHAnet or slotted-ALOHAnet operation.
Or for that matter token ring, round robin or most other network systems. Even CSMA/CD

enter image description here

Pullup resistor value left as an exercise for the student.

Polarity shown assume TX idle level is high.
Reverse diode polarities and use pulldown for TX idle is low.

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Electronic – arduino – multiple arduino communication (1 master, n slaves)

As I understand it you want to send different data to each of the slaves, but the slaves don't have to send data back.

I2C is an addressed bus, so if you assign a different I2C address to each of the slaves you'll need only two wires to send the data. If needed you can ask data back as well. The Arduino's AVRs have an I2C compatible serial bus. And you can extend to more than 3 slaves without extra hardware, up to a maximum of 127.

UARTs don't have addressing, so you would need either 3 UARTs (which the AVR doesn't have), or add external logic to switch between UART lines (which costs money). Each additional slave means extra cost. Not recommended.
edit
Like Chris says you can use UART to create a multidrop bus. And then you'll have to add addressing, which makes your UART work a bit like I2C, but then asynchronous, and without address matching hardware like the I2C has. So still not really an advantage. end of edit

SPI also uses shared lines for data: a single MOSI, and the MISO lines connected. To address each slave individually you'll need one SS (Slave Select) line per slave. So that's at least 5 I/Os: MOSI, SCK, 3 \$\times\$ SS, and MISO if you also want to read data from the slaves. Each additional slave adds 1 I/O pin on the master.

enter image description here

I think the I2C is the best solution, requiring the least number of wires. The protocol is a bit more complex than UART or SPI, but since the AVR has the hardware for it, it should be easy to use.

Electronic – arduino – Making two Arduinos talk over SPI

I took a quick look at the Arduino SPI library and couldn't find the attachInterrupt() function. I assume that you're using an extended library of some sort and I'll make the assumption that it works although I would like to see how it's implemented underneath.

A couple things that I saw:

This compiles? if (pos < sizeof buf) - it probably needs to be if (pos < sizeof(buf))
In the sample code on the forum, he has a key line right here that you're missing in your slave code:
```
// turn on interrupts
SPCR |= _BV(SPIE);
```

Without this your receive interrupt probably won't fire. I also don't see where global interrupts are being enabled although I have no idea if Arduino takes care of that for you under the hood. I also haven't bothered to look up what any of those registers mean, I'm trying my best to go by memory of AVR's SPI registers.

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Electronic – arduino – multiple arduino communication (1 master, n slaves)

Electronic – arduino – Making two Arduinos talk over SPI

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