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SN65176B: LED Fixture

ikon
Genius 5355 points
Part Number: SN65176B

Hi Support,

1. As the end user won’t know or care about which node is the first node and last node in RS-485 network if they are 32 nodes.
Is that possible to communize all resistor value for every node?


2. MCU 3.3V and SN65176B 5V
MCU UART TX connects to D of SN65176B shall be OK
MCU UART RX connects to R of SN65176B we are not sure if the high-level output voltage damage MCU I/O as the MAX value below is not defined.

Thanks.

  • 0
    Guru 268470 points

    There must not be any termination on the intermediate nodes. If you do not have a fixed installation, and do not want to use jumpers/switches either, then all the nodes must automatically detect whether they have one or two bus connections, and use something like an analog switch to connect the termination resistor.

    As shown in figure 19 of the datasheet, the SN65176B has a 5 V TTL output. As shown in figure 4, it can go above 3.3 V. So you should use a 3.3 V transceiver with CMOS outputs instead.

  • 0 in reply to Clemens Ladisch
    TI__Guru 62910 points
    Hi ikon,

    Adding on to Clemens's comments - in some cases it is OK to operate an RS-485 network without termination at any node. This is generally the case when data rates are low enough that the rise/fall times of the differential signal can be kept lower than the propagation time through the cabling. Other cases can be made to work as well, although this is very system-dependent since different cabling configurations will give rise to different signal integrity impairments.

    Conversely, you should never operate an RS-485 network with standard 120-Ohm termination resistances at more than two nodes. This will overload the drivers and end up in smaller signal levels. In some cases you can implement "partial" termination with higher-impedance resistors at each node (keeping the total equivalent resistance of all the parallel terminations at 60 Ohms or higher), but this is limited in its efficacy.

    Also - in general, the maximum high-level voltage of a push-pull LVTTL output (like the one you mentioned) is the device's power supply voltage (VCC). Therefore, it is best to follow Clemens's suggestion to use a transceiver that is powered by 3.3 V instead (so that an external level-shifting circuit is not required).

    Max