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pratikpanchal44
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November 17, 2023
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ST3485 Communication

  • November 17, 2023
  • 1 reply
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Hello Team,

We have one project where we customer used the ST3485EIDT IC for RS485 communication.

By looking the schematic design we see that DI pin is connected with UART Tx pin but its series resistor has been removed.

We checked the functaonlity and found that device is able to do the Transmit and Receive.

We are unable to understand logic by exploring the datasheet.

Kindly do the helpfull if something needs to impliment in the design or not.

 

 

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Best answer by Peter BENSCH

Welcome @pratikpanchal44, to the community!

In your schematics, R133 is not populated (DNP = Do Not Populate), so DI is connected to GND via R123 (0 ohms). However, transmission during communication only takes place via the line drivers for input signal 1, and via the pull resistors R93 and R94 for input signal 0:

  • When idle, the UART signal has the value 1, which becomes 0 through U34 and deactivates the output of the ST3485 (switches to high impedance). Now only the mentioned resistors R93 and R94 have an effect and pull A to high and B to low, which is received as 1 on the opposite side.
  • A 0 (low) in the UART communication (e.g. start bit) is inverted to 1 by U34 and activates the output of the ST3485. However, as DI is fixed at 0, output A is pulled low and B is pulled high, which is received as 0 on the opposite side.

Summary: the communication will work for short distances. However, you should experience increasing problems as the distance increases because the pull resistors cannot completely replace the impedance of the line drivers. Figuratively speaking, you turn on the tap to push water into the pipe when sending a 1. However, when the tap is switched for reverse direction, the water is no longer sucked back, but slowly drips past the tap.

I would recommend to take another close look at the circuit in the overall concept of your system (distance to be bridged, data rate, data to be transmitted, etc) and thoroughly rethink the control of the ST3485 on /RE, DE and DI.

Hope that helps?

Regards
/Peter

1 reply

Peter BENSCH
Peter BENSCHBest answer
Technical Moderator
November 17, 2023

Welcome @pratikpanchal44, to the community!

In your schematics, R133 is not populated (DNP = Do Not Populate), so DI is connected to GND via R123 (0 ohms). However, transmission during communication only takes place via the line drivers for input signal 1, and via the pull resistors R93 and R94 for input signal 0:

  • When idle, the UART signal has the value 1, which becomes 0 through U34 and deactivates the output of the ST3485 (switches to high impedance). Now only the mentioned resistors R93 and R94 have an effect and pull A to high and B to low, which is received as 1 on the opposite side.
  • A 0 (low) in the UART communication (e.g. start bit) is inverted to 1 by U34 and activates the output of the ST3485. However, as DI is fixed at 0, output A is pulled low and B is pulled high, which is received as 0 on the opposite side.

Summary: the communication will work for short distances. However, you should experience increasing problems as the distance increases because the pull resistors cannot completely replace the impedance of the line drivers. Figuratively speaking, you turn on the tap to push water into the pipe when sending a 1. However, when the tap is switched for reverse direction, the water is no longer sucked back, but slowly drips past the tap.

I would recommend to take another close look at the circuit in the overall concept of your system (distance to be bridged, data rate, data to be transmitted, etc) and thoroughly rethink the control of the ST3485 on /RE, DE and DI.

Hope that helps?

Regards
/Peter

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