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I bought this TTL to RS-85 board and before connecting it to my Arduino I decided to double check (reverse-engineer) its circuit and I can see there's a 120 ohm resistor between the A and B outputs.

I know we are only supposed to terminate the end of the RS-485 bus, but this is a pretty popular board and it appears to terminate every single node of the bus if I use it.

How much of a problem can this be? Should I bother to remove the resistors? I guess it would load the signal more and more as I add more nodes, so when will this be an issue? Can this pull too much current from the MAX485 and fry it?

In case anyone is wondering, this is the resistor:

terminating resistor

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  • \$\begingroup\$ I would remove them. \$\endgroup\$ – DKNguyen Oct 8 at 0:36
  • \$\begingroup\$ That board may be popular but it is a crappy design. Any decent RS485 inteface will either have a jumper of switch option to turn the termination on or off. Some decent designs also do not have any termination resistors at all and leave it to the installer to wire in axial leaded resistors into the screw terminals at the end of the bus along with the wires of the bus. \$\endgroup\$ – Michael Karas Oct 8 at 0:44
  • \$\begingroup\$ One more crappy design: It doesn't fit any breadboard. Although it has the pins, its lenght goes beyond the size of 1 breadboard and at the second one (if attached to the first) the pins fall just between two rows (facepalm).....AND Vcc and GND are not terminais, but just pins, so I can't easily run Vcc and GND together with the differential signal wiring. \$\endgroup\$ – mFeinstein Oct 8 at 0:49
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RS485 networks should just have the 120 ohm terminations on each extreme end of the two wire bus. The RS485 bus driver is designed to handle the DC current flow for this pair of termination resistors.

If you build a small network of 3, 4, or maybe even 5 devices the driver will typically have enough drive strength to handle terminations at every node. You may very well pay the price of reduced differential transmit amplitude in the bus under these conditions which would translate to reduced total transmit distance where there is also attenuation in signals due to long bus wire resistance.

You should check the data sheet for the driver to comprehend what its rated drive current capabilities are. Then do not operate an RS485 network that exceeds that rating.

In the extreme case the RS485 driver will go into current limit on its output for chips that support that feature. Any overload on the driver will cause the chip to heat up. Some drivers may have an over temperature shutdown as well. For drivers without these safety features the chip will eventually get destroyed.

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