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We are working on RS485 communication. We have designed the schematic as shown in the image. We are using TI's THVD1520 IC.

Is the logic of the circuit correct? We are concerned about the 2nd and 3rd pin connection. Do we have to pull up the pins?

  • \$\begingroup\$ You need 120ohm terminating resistor \$\endgroup\$
    – Damien
    Nov 22, 2022 at 8:18
  • 1
    \$\begingroup\$ It depends where it is connected, what controls it and how. You have a pull-down now, what do you want to be the default and why? It depends how you want to use the chip. \$\endgroup\$
    – Justme
    Nov 22, 2022 at 8:18
  • \$\begingroup\$ @Damien Why? If there are 20 or more than 2 of these on a bus, that would be wrong. \$\endgroup\$
    – Justme
    Nov 22, 2022 at 8:19
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    \$\begingroup\$ Your CON8 is missing its 3rd pin for GND. An RS485 bus is not a 2-wire current loop - it needs a common (GNS) node connected between devices in addition to the pair of signal wires in order to ensure that the common-mode voltage range is not exceeded. \$\endgroup\$
    – brhans
    Nov 22, 2022 at 13:47

1 Answer 1

  1. termination resistor is not a standard 100 or 120 ohm. TI or linear has a good, extensive app note on the different RS485 termination approaches(resistor vs. AC vs. active vs. split resistors vs ones I've forgotten). Read that to help understand why one is better than an another, since we don't know you application.

  2. it's not clear that the micro is on the same ground as the iso-gnd. This would be necessary unless the MCU pins go through the same isolations.

3)about pins 2 and 3. Tying DE pin low disables the transmitter.

Having ~RE low enables the receiver.

Since DE isn't tied to micro, you can never transmit. If you only need to receive, this isn't a problem.

  1. and a pet peeve suggestion: power and ground pins should be the arrows symbols. It very hard to read the schematic as drawn without them. Also always a good convention to follow it to put the supply on top, and the ground at bottom. To the experienced eye, having the +5V pin below the GND pin...well, it is like handing an architect a house drawing with the cement foundation on top and the shingles on bottom. Sure, you can still build it, but it hurts to look at. Power falls like rain from higher potential down to ground, so I like to see it drawn that way.

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