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According to the standards, RS-485 networks should always be wired in a daisy chain configuration. This allows for two termination resistors on each end to prevent signal reflections. However, people often end up with star configurations or worse, stars on stars. The trouble with a star configuration is terminating it. You can only have 2 termination resistors on an RS-485 network and they are supposed to go at the extreme ends of the network. But in a star, you generally don't have 2 ends, you have many.
If you can't get by without using a star on your network, experiment with the placement of your termination resistors. Also, upgrade the 75176 ICs on your board (if that's what they use) to MAX483 ICs from Maxim which are slew rate limited to prevent signal reflections in non standard RS-485 configurations.
If your device uses another type of RS-485 IC, contact Maxim to find their equivalent device with slew-rate limiting.
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Home Automation FAQ : 
RS-485 Networking Overview