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SN75179B: Need help with termination setup

Chris Hartwell
Prodigy 10 points
Part Number: SN75179B
Other Parts Discussed in Thread: THVD1426, THVD1520

Tool/software:

We are using the SN75179B for RS485 and we think the termination resistors are not correctly setup.  During testing we see the transmit reflected on the receive pin.  We believe this fs from the R38 termination resistor.  We have the circuit setup for 122 ohms.  Does this IC get setup different from typical RS485 transceiver with a DE and RE enables?  I was looking through the datasheets, do we need to be doing the termination resistors separately on each drive (A,B) and receive (Z,Y) to power and ground for bias instead?  Any help would be appreciated.  Below is our current setup.

  • +1
    Guru 318050 points

    You must not leave the B and Z pins open. Anyway, the SN75179B cannot be used for a half-duplex RS-485 bus because its outputs are always active. You must use a transceiver with a driver-enable pin (e.g., THVD1520), and the DE pin must be set correctly by the MCU. (If you really do not have a DE signal, try the THVD1426.)

    An RS-485 bus should have exactly two termination resistors (like R38), at the two ends on the bus.

    A bus needs fail-safe biasing resistors (like R36/R44) only if there are receivers without fail-safe inputs on the bus, and then you need only one set of resistors on the bus. (THVD transceivers have fail-safe inputs.)