When wiring multiple RS485 slaves to a master equipped with a RS232/RS422/RS485 interface*: is it safer to wire multiple point to point connections from the hub with their own termination resistor, compared to using one socket on the hub and daisy chaining them all terminating with a single resistor? The spec seems to say daisy chaining is a "dumb" parallel hard-wire, and I wonder if one receiver couldn't freeze the bus or even damage it (meaning the interfaces) if a failure occurs.
*: the interface is a 8 or 16 ports industrial USB to RS232/RS422/RS485 converter, to be selected
Best Answer
Whether you drive the chips in parallel or series, it's possible for a single failure (an input failing short) to disable the entire network.
Worse, if you build a star topology (a central hub with transmission lines radiating out to separate ICs) with each receiver having its own terminating resistor, the network will not work at all. Let's say that your connecting conductors have a characteristic impedance of 100 ohms. Putting a 100 ohm resistor at each end of a daisy chain will produce a DC load on your hub of 50 ohms (100/2). For a 2-volt swing on the bus, a load current of 40 mA (2 / 50) is required, and the hub is certainly capable of producing this, as are the trancscievers at each station. If you have 10 stations, you'll need 10 resistors, and the overall total load resistance is 10 ohms (100/10). The current needed by the hub to give 2 volts will be 200 mA (2 / 10) and I can pretty much guarantee that the hub won't do it. Not only that, but each of the bus stations need to do the same, and THEY won't manage it either.