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Somfy ILT 30 Motors RS-485

Jim Stankiewicz
Prodigy 10 points
Other Parts Discussed in Thread: SN65HVD05

Having ridiculous intermittent problems controlling

St-30 RS485 Somfy part #1000658      I have a Crestron processor AV2 sending out RS-485 to 44 somfy shade motors in less than a 4000 ft wired network using cresnet wire for power and communication.  4 ten port somfy module boards and one 5 port somfy module board.  I put a 10k ohm resister in the last open port.   Terrible intermittent problem.  Somfy claims they can handle 255 RS-485 motors on less than 4000 ft wired network.   I can't control 44 motors without problems.   I am looking at your part:   Will this have any affect?  Also is there a problem with Somfy's understanding of Nodes what actually their motor network will work for RS-485 commands?  Thanks,

SN65HVD05 (ACTIVE)

High-Output RS-485 Transceiver 

  • 0
    TI__Guru 62910 points
    Hi Jim,

    From just a transceiver perspective, the number of nodes that can be supported is a function of the leakage current that a receiver (or disabled driver) present on the bus. Above a certain number of nodes, the amount of loading presented by all of these receivers connected in parallel would exceed what an RS-485-compliant driver would be designed to handle. RS-485 drivers typically support 32 "unit loads"; since the SN65HVD05 is a "1/2 unit load" transceiver, up to 64 of them can be supported on a common bus.

    Here is an application note that goes into more detail on the unit load concept in RS-485:

    www.ti.com/.../slla166.pdf

    Note that the lower-speed devices in this family (such as 'HVD06 and 'HVD07) are 1/8 unit load transceivers, meaning that up to 256 nodes can be supported.

    Beyond just the transceiver ratings, though, there are other practical concerns that could limit the number of nodes that can be used in a given RS-485 network. The wiring topology and termination schemes also strongly influence performance, especially at higher data rates. If the nodes are connected in a tree or star topology, then the transmission line "stubs" that are introduced will likely start to degrade the signal integrity before you would see any ill effects from receiver loading.

    Here is another application note that discusses RS-485 bus topologies, termination, etc. and how they affect link performance:

    www.ti.com/.../snla049b.pdf

    Regards,
    Max