I wonder why is RS485_Rx is connected with two line drivers while RS485_Tx is connected with only one line driver. Is this some mistake in the schematics or is it some standard circuit for RS485?

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  • 1
    \$\begingroup\$ Please clarify what you mean. The pairs in red circles are typical differential RS-485 input and outputs. The pairs do not have the same names, so this maybe a node unit. \$\endgroup\$
    – user105652
    Feb 15, 2019 at 3:34
  • \$\begingroup\$ I am concerned about the red circle on RHS, namely RS485_Rx.. it is connected with two drivers U27a and U29a.. that confuses me.. \$\endgroup\$
    – scico111
    Feb 15, 2019 at 3:45
  • \$\begingroup\$ It was something of an answer so I had to post it as an answer. \$\endgroup\$
    – user105652
    Feb 15, 2019 at 4:49

1 Answer 1


U27a receive and U29a receive are isolated by R123 680 ohms. MPU should have control but this addition implies that both receivers may be active at the same time. In that case U27a will always dominate over U29a. If U27a is NOT in receive mode (it goes to high impedance if RE\ is high) then R123 allows data from U29a to be read. A simple and crude priority switch.

The resistor is there to prevent a short if both receivers drive the same wire with no current limiting, which is what R123 is really there for.


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