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Interfacing a MSP430 to a RS485 tranceiver

Brock
Intellectual 970 points
Other Parts Discussed in Thread: MSP430F2619

Hey guys; haven't found much info here when searching 485. I just want someone to verify my setup. I will be using a Max483 RS485 tranceiver(http://datasheet.octopart.com/MAX485ESA-Maxim-datasheet-51242.pdf) for communications with my MSP430(msp430f2619).

I have pin 3.4(USCI A0 transmit output going into pin 4(DI) of the Max483, pin 3.5(USCI A0 receive data input) going to pin 1(RO) of the tranceiver. For the driver/receiver enables I have it going to 2 general i/o pins. Is this configuration ok?

Also if anyone can point me to some info i may not be seeing on msp430/rs485 interfacing that would be awesome.

 

Thanks,

  • 0
    Guru 227245 points

    Brock said:
    Is this configuration ok

    Looks like. If you don't bother about incoming data if you're not sending, you can even connect the two DE/RE signals to one GPIO pin.

    There is even a way to automatically handle DE/RE if you know the baudrate: a retriggerable monoflop, triggered by the start bit, switches the direction to output and falls back 8 bit times after the last low  bit (requiring a one-byte dead time after sending). This can be used to transform normal UART connections into RS485.

    Some thing that's necessary but not shown in the Datasheet you mentioned above: on one side of the RS485 (and only on one), the data lines should be pulled to GND and VCC (e.g. using a 4.7k resistor, depending on the number of slaves). The resistor between the two dat alines should only placed on teh other side of the bus as termination. Or, if you make a star-like network with longer connections to the single slaves from the common bus, on each end of the star except for the end with the pullups.

    While RS485 is fairly robust, it is power-consuming too. High currents against high distortions is the main recipe. (for normal UARTs, the original formula was 'high voltage agianst distortions', but the high voltage has been dropped from +-12V to +-3V  over time)

    We use RS485 in out projects a lot (sensors and RF transmitter communicate through bidirectional RS485, polling mode now, but time slice mode is in the works) and bus termination is critical. Way more critical than I had thought.

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