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Original question:

SN65HVD30-EP: Full duplex not working

SN65HVD30-EP: SN65HVD30 Full Duplex Multiple Slave Not Working

Ahmad Reza
Prodigy 20 points
Part Number: SN65HVD30-EP
Other Parts Discussed in Thread: SN65HVD30, SN65HVD35, SN65HVD32, SN65HVD33

Tool/software:

Hi

I am doing a project that has one master and 60 slave. my MCU is STM32F103 and my data is in UART form. My connection mode is full duplex

and the baudrate is 115200. In each time, my master send a request with ID and that slave responses to it.

At first we connect one slave to RS485 bus and all things was good and system was working. When we connect second slave, the A B differential voltages decreases but connection is still ok. When we connect third slave module to the bus, the bus voltages decreases more then before and now the connection get lost. we used oscilloscope and saw that three data packet is transmitting on A-B but the SN65HVD30 can't verify the data to transmit to the  micro (Master's MCU). In other expression, the master request data steel is receiving correct and all three slave see that but the master can't see the responses.

My question is that how we use this IC (SN65HVD30) for Full duplex ???

when a slave don't send a data, we want that the SN65 output be high impedance but this IC is not like this. So how in datasheet is written that this IC is 1/2L and max slave connection is 64 !?

Should we use the SN65HVD35 that is full duplex and has RE DE pins to be high impedance other slave to not driving the bus ?

I tried any change in RT resistor and series resistor and was not responsive. I will thank to anybody that help me.

  • +1
    Guru 316690 points

    On a bus with multiple senders, all transceivers that are not currently sending must be disabled by pulling the DE pin low. A transceiver without DE pin (like the SN65HVD30/32) can be used only for point-to-point connections between two devices.

    (And an RS-485 bus must not have more than two termination resistors; the resistors go at the two ends of the bus.)

  • +1
    TI__Genius 13472 points

    Hi Ahmad,

    Clemens makes some excellent points. If multiple devices are trying to communicate on the bus at once, the signal can be degraded. SN65HVD32 permanently has its driver and receiver lines enabled. SN65HVD33/34/35 should be used so that the driver and receiver enables can be controlled externally. 

    I would check to ensure only two termination resistors (120 Ohm each) are being used. These should be at the farthest ends of the cabling.

    Additionally, I would consider removing the 10 Ohm resistors to improve the differential voltages. These are not the direct problem, but removing these will certainly help. 

    Best,

    Ethan

  • 0 in reply to Clemens Ladisch
    Prodigy 20 points

    Thanks' for your response Mr Cleamens Ladisch I'm very happy that you and  "Ethan Sempsrott" answer to our problem quickly !!!. My problem resolved.