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SN75HVD05: Example of RS485 schematic when using 2 wire cable

g.f.
Guru 15510 points
Part Number: SN75HVD05
Other Parts Discussed in Thread: PROFIBUS, SN65HVD20, THVD1450

Hi,

Our customer are planning to use RS485 driver SN75HVD05D.
They have question about SG(Signal Ground).

In common case, RS485 cables all nodes using differential signals (+/-) and
as well as SG, three wires in total.
But in their end customer's system,  it is using the cable which have only two wires.
In this case, it is necessary to stop the explicit connection of SG to the cable and
take measures to match the SG level of each RS485 circuit as an alternative.
(Their cable length is up to 350 meter)

We understood that two wire connection is not the common use case in RS485,
but we need to use two wire cable, so if you have any example circuit of RS485 to use 2 wire cable,
could you please give us an advise?

For reference, PROFIBUS seems to have an example to handle with 2 wires.
In that case, it seems that SG of each RS485 circuit is loosely grounded with high resistance of mega ohm to match the SG of each 485 circuit while suppressing the influence of GPD.
It would be nice if you could also give us the 2-wire experience with TI products.

best regards,
g.f.

  • 0
    TI__Guru 62920 points

    Hi g.f.,

    The two-wire connections can work as long as the ground potential differences do not cause the voltages seen by each transceiver (with respect to its own local ground) to exceed the common-mode operation voltage range of the device.  For the 'HVD05 device this range is -7 V to +12 V, which aligns with the minimum requirements of the RS-485 standard.  Larger GPDs could be supported by transceivers with wider common mode ranges - for example, the THVD1450 or SN65HVD20.

    Note that each RS-485 receiver presents a high (but not infinite) resistance from the bus to its own ground.  So, if the grounds between systems are all totally independent and "floating" with respect to one another then the leakage currents that occur through the RS-485 receivers should help the grounds to drift towards similar voltages.  I believe this is similar to what you mentioned regarding providing weak grounding at each transceiver.

    Max

  • 0 in reply to Miguel Robertson
    Guru 15510 points

    Hi Max,

    Thank you for supporting me and I'm sorry for the delay.
    I understood.

    best regards,
    g.f.