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POST No. 21580
RS-485 termination
2009-02-19 02:13:00
Hi,

I'm designing a robot that uses a couple RX-64 servos, and I'm designing my own RS-485 interface. On the RX-64 data sheet, it makes no mention of the bus termination resistor (typically 120 ohms) that is supposed to be (according to the MAX485 data sheet) between D+ and D- on each end of the bus.

Should we be using a termination resistor next to the MAX485, and should we be terminating the other end of the bus as well?

Thanks,
Jon

2009-02-19 02:13:00
2009-03-04 07:15:00 Scott McClure
A terminating resistor prevents reflections down long wires (when a wire is long enough that it appears to become a transmission line). The terminating resistor value should be equal to the characteristic impedance of the cable for the given frequency that you are using - somewhat hard to measure unless you are buying nice cables. Most shielded cable though with a datasheet will list a nominal value that you can use for a terminating resistor. This becomes important at longer lengths... Shorter length connections (under a couple feet) should probably be fine without one.
2009-03-04 07:15:00
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