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Topic: UART to RS485 half duplex (Read 3 times) previous topic - next topic

Nick Gammon

Hi Nick, I understand your rationale for using the 680R's on A,B as you show, but that's not quite the same termination
as the d/s for the LTC1480 shows. I would think the 120R would quench the noise even on a floating line.

On the page I referenced I also have this statement:

Termination resistors

If the transceivers are not at the ends of the cable termination resistors are probably necessary. Something like 120 ohms, connected between the A and B cables, at each end only, stop the signal reflecting back along the cable.

Nick Gammon

The datasheet doesn't seem to mention the 680 ohm resistors, but I found a reference here:


They are called bias resistors apparently, and it seems the chip itself has a certain amount built in.


My understanding is that the RS-485 bus is to be centered about 2.5 V because the signalling is differential and it was for longer wires that the terminator is required to damp the ringing so the data transitions don't over run each other and they are really a necessity at higher baud rates.

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Jan 21, 2013, 10:26 am Last Edit: Jan 21, 2013, 10:29 am by hammamed Reason: 1
Hi everybody,
is it required, in Nick scheme, to share the GND between the two ships when wire length is about 1000 feet (so in total we have 3 wires; 2 for A/B and one for groud)? I tested the two cases with 4 feet wire length, and everything works (no remarks or difference). but I have to know now the number of wire to order a cable suitable for my project (whose the distance is about 1000 feet).

in internet, sometimes I found that the GND must be shared, and sometimes they said that the RS485 receiver compares the voltage difference between both lines (A/B), instead of the absolute voltage level on a signal line (and this prevents the existence of ground loops)

The second question is concerning the baud rate: I used 28800 as mentioned in Nick sketch. but for 1000 feet and with 30 bytes to send (to slave) and 15 bytes to receive (from the slave), what baud rate can guarantee maximum transfer and without error (How can I calculate that)
Think's a lot for your responses

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