Choice of multislave protocol for AVR system

[ScOe]

Multislave AVR system

Hi,

I was reading about multislave systems with AVR uC using different protocols, and came to a conclusion that i have no idea which protocol to use.

What i want to achieve is:
One master board avr 128, 5 slave boards with bunch of sensors and controlled by avr8. Max cable length would be 5-8m. I'm considering CAT5 or similar cables. Slave boards are passive which means that power is supplied by master board. Slave boards have to have ID's and need to support hot plug. Environment could be noisy, as in the same cable tube there is few other cables (TV, Ethernet, Telephone). Bit rate is not critical. My app is not time critical, so other slaves can wait while one is communicating with master.

Saying that, i was considering RS485 as it would be easiest to implement, but my design is star network that rs485 doesnt support. i2c was another consideration but i was reading that it has some problems with longer cables (not sure if 8m is in that category). CAN seems to be logical solution but it will increase the cost of every board, and that is what i want to avoid.

Any suggestions ?

 

[Tahmid]

Multislave AVR system

Hi,
What about SPI? 8m may be possible if you decrease the data transfer rate. And you can use as many slaves as you want, you'd just need additional Chip Select pins.
Tahmid.

 

[ScOe]

Re: Multislave AVR system

I was considering the SPI, but i dont see the way how to implement hot plug (it would need to ping all SS on regular bases to see if the slave is presented), and it suffers the same problems as i2c does with lengthy cables.

I'm not sure if twisted pair cable will help to achieve noise suppression. If i use eth CAT5 cable i will have 4 pairs of wire and will have SDA, SCL with grounds on each pair and left with 2 pairs for slave voltage supply.