I'm using several MFRC522 modules which communicate to an arduino with the SPI protocole. The distance between the arduino and the modules will be between 1 and 2 meters.
I'm worried about the distance, I'm not sure of the quality of the SPI signal for a such distance. I would like to know your opinion about using wire like RS422 or RS485.
If I have to use this wiring, could you explain me how to convert SPI to RS422 ?
Thank you ,
The SPI can work at 2 meters just fine, although at reduced speed. If you do not absolutely need maximum possible clock you can wire your modules and gradually increase speed to find out error-free maximum.
As a side note, SPI is not the best protocol to use off-board when you have multiple slaves, because wiring CS lines quickly makes it cumbersome.
Using I2C or dI2C simplifies wiring but requires somewhat more complicated transceiver circuits. Using I2C-to-1wire bridges simplifies this as well but at cost of greatly reduced speed.
UART also works at 2m without problems. You can extend it much further by converting UART to RS-485 or LVDS. The problem with using UART, however, is that it does not have addressing mechanism to support multiple modules. If your controller has enough UART ports for all modules then wiring them individually instead of using any bus would be best option.
Note that MFRC522 chip supports all three protocols, SPI, I2C and UART.
As the answer from Maple suggests, there are alternative approaches, but if you want to send SPI over RS422 wiring as mentioned in the title of the question, then there is an approach suggested by Texas Instruments in their document Extending the SPI bus for long-distance communication.
If you want a simpler circuit (and especially if you have a shared power supply between devices) then you might be able to get away without the ISO7242c isolator on the far end. In theory, this should work with other RS422 transceiver chips also as alternatives to the SN65C1167.
One reason for doing it this way is if you need to stick with SPI protocol communication (e.g. you want to extend the signal length of an IC that talks SPI, and you don't want to add your own microcontroller to translate the signals to something else).
