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THVD1400: THVD1400 design confirm

Guangyan Liu
Intellectual 2130 points
Part Number: THVD1400

Dear Expert ,

Good day!

Our customer  want to use THVD1400 to make a 485 network system. There are more than 200 loads, and the transmission distance is about 28 meters. The load is evenly distributed. The resistance of the number line is 0.17 ohm/meter. The resistance of the power cord is 0.05 ohm/meter. The maximum power of each load is 0.12w. Now we want to know. Can the 485 chip work normally in this kind of network? What is the sink voltage that THVD1400 can withstand in normal operation? (only one driver)

BR,

Leon.liu

  • 0
    Guru 285410 points

    This is the kind of network that RS-485 is designed for. The THVD1400 supports 256 nodes, and the total resistance of about 5 Ω is negligible.

    What do you mean with "sink voltage"? The THVD1400 works with bus voltages from −7 V to 12 V, and will not be damaged by bus voltages up to ±16 V.

  • 0 in reply to Clemens Ladisch
    TI__Mastermind 25705 points

    Leon,

    Please refer to Figure 5-1 Driver Output voltage vs Driver Output Current and Figure 5-2 Driver Differential Output voltage vs Driver Output Current in the datasheet for the drive capability. 

  • 0
    Prodigy 30 points

    Hello! I saw it on the Internet. The reason is that the node 485 chip and the load on the 485 network share a power and ground network, especially the ground. The ground on the node and the ground on the source driver are not at the same level, because the current flowing back from the node After the ground network, the ground network has resistance, so the power ground of the drive end and the ground of each node have voltage, that is, the ground of the 485 chip and the power ground of the drive end have voltage. Looking from the 485 chip end to the source drive end, it is this The side loop should also have this voltage drop, and the voltage drop between the normal chip and the 485 network is very small, so that current will be poured into the 485 chip and returned to the source ground to meet the condition that the voltage drop is the node location voltage drop. . The voltage of the node ground is related to the power consumption of the load on the network and the resistance of the ground wire. This voltage should always exist. We want to know how much ti's 485 chip or THVD1400 can withstand such a voltage and still work normally? Thanks!

  • 0 in reply to ping wang
    TI__Mastermind 25705 points

    It would be helpful if you draw a diagram. From my understanding, you're referring to the ground potential difference between the nodes in RS-485 system. Some discussion can be found in section 11 of this design guide. 

    https://www.ti.com/lit/an/slla272d/slla272d.pdf

    RS-485 signaling is differential, therefore communication still works with limited common mode voltage.