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MSP430 UART to RS485 communication

Kuldeep Kulkarni
Prodigy 110 points
Other Parts Discussed in Thread: MSP430FG4618

Hi,

I am trying to connect MSP430FG4618 board to 'RS232 to RS485' converter for Modbus communication. Problem is that data sent from MSP430 UART, is not appearing at the RS485 side of 'RS232 to RS485 Converter', when observed on the DSO. Baud rate set is 9600, 18E

Following are more observations:

1. When Coverter is connected to RS232 of PC and Data sent from hyperterminal----> Data is received at the RS485 side of converter which can be observed on DSO.

2.. Data sent from MSP430 UART ----> Can be observed on the hyperterminal of the PC. [i.e serial comm of MSP430 is working]

3. When connected to PC, converter is port powered. No externer supply was required.

4. In case of MSP430 both approaches of connecting converter i.e Port Powered and External power supply +-12V were used. None of the approach worked.

Please suggest what might be going wrong.

 

 

  • 0
    Guru 227245 points

    RS485 is half duplex. That means both directions go over the same lines, only one direction can be used at a time. There needs to be some sort of flow control. Either each side pulls an 'enable transmitter' line as long as it is sending, or the converter determines this on its own, depending on its smartness.
    So what kind of signal converter do you use? A MAX485 chip or similar? Then the direction control is up to the endpoints. Or is it a microcontroller with RS232 and RS485 port? There are even separate control lines for sending and receiving, so you can go deaf if you're sending, or have your ears always open, even to yourself. However, if both sides have their transmitter on, then the stronger one wins.

  • 0 in reply to Jens-Michael Gross
    Prodigy 110 points

    Hi Jens

    Thanks for your reply.

    I am using 'RS232 to RS485' readily available device. It determines the direction of data flow automatically. As already mentioned above when it is connected to PC both Tx/Rx work fine. Problem occurs when I connect it to MSP430FG4618 .

    After few more experiments I am able to transmit frame from controller to converter. Frame can be observed at the A/B of RS485. But reverse is not happening.

    Shall I try with MAX3485 instead of converter. Do I need to raise signal level for modbus communication or levels which are generated by controller are sufficient for communication. Where can I find circuit diagram for MAX3485 interfacing?

    Thanks & Regards

  • 0 in reply to Kuldeep Kulkarni
    Guru 227245 points

    Kuldeep Kulkarni said:
    I am using 'RS232 to RS485' readily available device.

    Do you have a part no or a datasheet? We made such a thing ourselves based on a PIC some years ago. But if we need one again (and I see it coming), a 'readily available device' from stock would be nice to have.

  • 0 in reply to Kuldeep Kulkarni
    Guru 227245 points

    Kuldeep Kulkarni said:
    I am using 'RS232 to RS485' readily available device.

    Do you have a part no or a datasheet? We made such a thing ourselves based on a PIC some years ago. But if we need one again (and I see it coming), a 'readily available device' from stock would be nice to have.

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