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Original question:

THVD1510: Hot Swap Application

THVD1510: Load option

Shunsuke Nagata
Genius 13370 points
Part Number: THVD1510
Other Parts Discussed in Thread: SN65HVD485E,

Hi,

My customer think the system which connects 128 node and the node consists with THVD1510 and SN65HVD485E. 

The load option of THVD1510 is 1/8 unit load options ( Up to 256 Bus node). And the load option of SN65HVD485E is 1/2 unit load (64 Nodes)

New platform will use THVD1510 and Previous model use SN65HVD485E.

1. Does Previous model not use in the system which connects 128 node?

2. I cannot find the characteristics gap between two devices in Datasheet specification, What is differences among 1/8 unit load and 1/2 unit load? Is it output current?

Regards,
Nagata.

  • 0
    TI__Guru 62920 points

    Hello Nagata-san,

    The number of nodes supported by a given transceiver depends on its input leakage current (sometimes specified as bus input current or receiver input current).  You can read more about that concept in this blog:

    https://e2e.ti.com/blogs_/b/analogwire/archive/2016/03/08/rs-485-basics-how-to-calculate-unit-loads-and-the-maximum-number-of-nodes-on-your-network

    Basically, RS-485 defines the driver characteristics (like the output differential amplitude) based upon an assumed resistive load of 54 Ohms.  That load is intended to encompass two 120-Ohm termination resistances in parallel (giving 60 Ohms equivalent) as well as 32 "unit loads" also in parallel.  A "unit load" is defined in terms of an I/V characteristic where the drawn current at 12 V is at most 1 mA and at -7 V is at most -0.8 mA.  If a device has input currents half this (i.e., 0.5 mA at 12 V and -0.4 mA at -7 V) then it would be considered half a "unit load" and you could have twice as many of them on a bus without overloading the driver circuits.

    In this case, THVD1510 is a 1/8 unit load transceiver so it can support 256 nodes and still fall within the RS-485 standard loading conditions.  SN65HVD485E is only 1/2 unit load, though, and so connecting 128 in parallel would cause a load condition beyond the RS-485 standard's guidelines.

    In this case, one way you could compensate for the higher transceiver loading would be to slightly increase the termination resistance values so that the equivalent resistance between A and B remains no smaller than 54 Ohms.

    Also note that the effects of an effective load resistance below 54 Ohms may not be so severe.  An RS-485 driver is designed to support a +/- 1.5-V worst-case differential voltage amplitude at this load condition.  As the load resistance decreases, this output voltage level will decrease slightly as well (due to the additional load current flowing through the non-zero output impedance of the driver).  RS-485 receivers are designed to properly sense high/low logic states on the differential bus based on thresholds in the +/- 0.2 V range.  This means that the signal amplitude would need to drop significantly in most cases before data errors occurred.  (Exceptions here include very long cable transmissions where significant signal attenuation is expected.)

    I hope this is all clear - please let me know if you have further questions.

    Regards,
    Max Robertson