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SN65176B: What is the voltage and current?

Jerry Hsueh
Expert 2730 points
Part Number: SN65176B
Other Parts Discussed in Thread: SN75176B, THVD1450, THVD1500, THVD1550

Hi TI

1. If SN65176B chip pin1, pin4 serial port RX, TX signal is 3.3V level, can it be directly connected to SN65176B chip without level conversion?

According to specifications, D, DE, / RE signals larger than 2V are high levels.

2. What is the A/B foot current of SN65176B chip? There are two references to current in the specifications.

What would the current be if 256 loads were fully loaded according to RS485 protocol?

Thank you

Jerry

  • 0
    TI__Guru 62910 points

    Hi Jerry,

    SN75176B could receive serial port data from a 3.3-V MCU as long as the VIH level of 2 V were exceeded. For signals going from the transceiver to the MCU, you should verify the VOH and VOL levels against the VIH and VIL levels of the MCU and make sure it is 5-V tolerant. If not, you could use a level shifter or a p2p transceiver operating off of 3.3 V (such as THVD1450).

    The two output current specifications refer to different states - the higher current is with the driver enabled and the lower current is with the driver disabled.  The driver-disabled current would be what you would consider when finding the maximum number of nodes allowed. The RS 485 standard specifies up to 32 unit loads, and each SN75176 represents one unit load. So, you could have 32 of these devices on a bus to remain within the standard guidelines. If you needed a higher node count, you could use a lower-current (higher input resistance) device like THVD1550 or THVD1500, both p2p with SN75176 but supporting 256 nodes (as well as a number of other performance improvements such as ESD and noise immunity and VCC/temperature ranges).

    Regards,

    Max

  • 0 in reply to Max Robertson
    Expert 2730 points

    Hi Max

    Thank you very much for your support.

    Our circuit is shown in the following figure.

    If there are 32 loads, what is the current on the AB line?

    Because it is necessary to determine the package of two 10R resistors and 120R resistors.

    Jerry

    Thank you!

  • 0 in reply to Jerry Hsueh
    TI__Guru 62910 points

    Hi Jerry,

    To determine resistor power ratings, you should take into account the worst-case (highest) currents.  These will occur when the driver circuit is active.

    The most current would flow through the series resistances when there is a fault condition like a short-circuit on the RS-485 bus.  In this case, the current could be as high as 250 mA.  If there is no short-circuit, though, then the maximum current would be when the driver output is at its maximum level of 5 V.  In this case, the current through the 10-Ohm resistors would be 5 V / (10 Ohms + 60 Ohms + 10 Ohms) = 62.5 mA.  (I used 60 Ohms here assuming that there would be another 120-Ohm termination placed elsewhere on the bus.)  If other nodes do not have the 10-Ohm series resistances but could still product differential output voltages as high as 5 V, then the maximum current through the 120-Ohm resistor would be 5 V / 120 Ohms = 41.7 mA.

    Let me know if this isn't clear or if you have any further questions.

    Regards,
    Max