I've seen in some schematics that the RX, TX, RE and DE pins are connected to VCC with a resistor. This is how the connection with the UART pins is made (see schematic below).


In some schematics, these pins are connected directly to the UART pins without being connected to the VCC:


What is the difference between these schematics?

Likewise, the A and B pins can be connected differently. If you look at the schematic below, pin A is connected to VCC with a resistor and pin B is connected to ground with a resistor.


However, in the second schematic, pins A and B are connected directly to the screw terminal. What is the reason for this difference?

  • 3
    \$\begingroup\$ There are very many good tutorials about RS-485, but one of the best is Bob Perrin's 1999 Circuit Cellar article The Art and Science of RS-485. Also very useful is Texas Instruments RS-485 Basics \$\endgroup\$
    – jonathanjo
    Sep 20, 2022 at 10:03

1 Answer 1


The resistors on the MCU side pins (DI, DE, RE, RO) set the default state of pin if nothing is connected or driving them to a defined logic state. It really has nothing to do with RS-485, as this is basics of digital signals - don't leave important (or any) signals floating.

For example when a MCU boots or resets, all the pins are floating inpus, until software is running and initializes the pins to correct states.

So without the resistors, it is possible that when the pins are floating in undefined state, a device may transmit a burst of garbage on the RS-485 bus. This may or may not be a problem in a system so it may or may not require special handling of the pins.

The resistors on the RS-485 bus side are biasing resistors. If no transmitter is driving the bus, the common mode voltage is rather undefined, and differential voltage is zero, which is not a valid logic state. The resistors try to bias the bus to valid common and differential voltages. But the example values are poor. Even in the case of a node being disconnected, the resistors keep the idle bus common mode at 2.5V and 30mV differential voltage.

The 30mV again may or may not be enough for all transceivers on the bus to detect it as the idle state, so while the resistors are in theory good to have, the actual values of 10k in the example are rather useless.

This is usually explained in basic RS-485 tutorials and application notes.

  • \$\begingroup\$ Addition: per RS-485 standard biasing voltage must be > +200 mV to be in idle "1" state, and there must be only one biasing resistor network on the bus. 120 Ohm resistor is termination, which must only appear at the ends of the bus. Any bus, even short one, must be terminated, because some RS-485 circuit may not function properly without them. \$\endgroup\$
    – Anonymous
    Jun 6, 2023 at 18:57

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.