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Pin Connection

Thread Summary

The user is designing a multiprotocol communication port using the LTC2870 for RS232, RS485 half-duplex, and RS422. The final answer confirms the connection scheme is correct but advises against floating unused pins. The user seeks clarification on whether it is safe to leave Y, Z, A, or B pins floating at the terminal side in unused modes and asks if Analog Devices recommends external pull-up or pull-down resistors for noise prevention and EMC issues. The user also inquires if the internal circuitry isolates these pins in unused modes and requests best practices for handling multi-function I/O pins in multi-protocol applications.
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Category: Datasheet/Specs
Product Number: LTC2870

Hello dear all, 

I am designing a multiprotocol communication port for my board and ı want to put 3 option for it, RS232, RS485 Half duplex and RS422. In my design, I want to permanently connect TX and RX pins of my controller to transciever's TX and RX pins. ANd also ı want to connect transciever's output to my ouput terminal in a permanent order. When i read the datasheet of the product ı am confused because some of the schematics are not clear enough and the document does not show how to use it for my usage purpose. I only want to be verified before i go for my design.  This is what i want to do: 

          -  I connect uC's pins to transcievers pins,  like TX-->DY and RX-->RA. The connection will be directly connected.

          -  For terminal side of the circuit there are 3 different case. RS232, RS485, RS422.

Terminal Pin Configuration

Terminal Pin   RS232     RS485 Half Duplex     RS422          LTC2870 Pins

         1              TX                       A                        Y      -->               Y

         2              NC                      B                        Z      -->               Z

         3              RX                      NC                     A      -->                A

         4              NC                      NC                     B     -->                B

         5             GND                 GND                  GND   -->              GND

If i do configuration of the terminal like above would it work or not? 

When i read the document i understood that i have to design like that but unfortunatelly some of schematics made me confused. 

Could you verify my usage scenario does work or not?

Regards

Burak Özdemir 

  • Hi Burak,

    Your connections look fine for RS232 and RS485 half-duplex and full-duplex (RS422). You can verify your connections with below simplified driver/receiver input and output table. I noticed only one RS232 driver is used. We recommend providing a valid signal for all input pins. Please do not float the unused inputs.

    Best,

    Jane

      OUTPUT
    INPUT RS 232 RS 485 HD RS485 FD
    Driver DY(TX) Y Y/A Y
    DZ Z Z/B Z
    Receiver A RA (RX) RA (RX) RA (RX)
    B RB    
  • Hello Jane thanks for your answer. But the answer cause a new question. 

    About the empty inputs of receivers; As i mentioned before i have three mode in my design

    • RS232 (full duplex): Y (TX output) and A (RX input) are used. Z and B are not connected to the terminal signals in this mode.

    • RS485 half-duplex: Y and Z are used as the differential bus. A and B are not connected in this mode.

    • RS485 full-duplex: All four pins (Y, Z, A, B) are used as two separate differential pairs. 

    When switching between these modes, it means that sometimes some input pins (A, B, Z) will be unused and left floating at the terminal side.

    According to the LTC2870 datasheet:

    • There is no “do not float” warning or note for Y, Z, A, B pins in the pin descriptions or in Tables 1–10.

    • There is no mention of required external pull-up or pull-down resistors for these I/O pins.

    • There are some internal resistors pulled down in some of the schematics and i also saw some details in the tables but i could not understand. 

    My question:

    • Is it safe to leave these unused Y, Z, A, or B pins floating at the terminal when not used in a particular mode?

    • Does Analog Devices recommend any external resistor (pull-up or pull-down) for these pins to prevent noise pickup or EMC problems during mode switching?

    • Does the internal circuitry tri-state/isolate these pins in unused modes, making floating safe? Or, for maximum robustness, should I add high-value resistors (e.g., 1MΩ to GND) to all input pins?

    • Are there any best practices for handling these multi-function I/O pins in multi-protocol applications like mine?

    Any guidance from AD engineers or experienced designers would be greatly appreciated!

    My schematic is below. İt has not completed yet. 

  • Hi,

    We typically recommend not leaving input floating. However, inputs of the receivers can be floating for LTC2870. The RS232 receivers include an internal input resistor (5k typ.), so the inputs of the RS232 receivers are internally pulled low already. Modern RS485 receiver typically provides failsafe function. The LTC2870 features true failsafe so there is no need to use external bias resistors on bus lines. Both RS232 and RS485 receivers in LTC2870 would pull the output to high even if the input is floating, please refer to the Receiver Outputs session on datasheet. Your initial post did not mention DZ’s connection, and I want to make sure this input is not floating. Your schematic shows TVS diodes are added on the bus lines for EMC protection. Please be aware the TVS diode SM712 is asymmetrical, -7V to 12V, and the RS232 standard allows loaded driver output signal swing between +/5V to +/-15V. The LTC2870 RS232 drivers have a max. +/-7.5V output voltage, therefore the standoff/working voltage of the TVS diodes should be greater than +/-7.5V, or the min. reverse breakdown voltage should be >7.5V. In addition, please be sure the clamping voltage of the TVS diodes is less +/- 15V (abs. max.) to avoid damaging the part.

    We have an apps note for the multiprotocol transceivers LTC2870/LTC2871, please see link below. The apps note not only explains the features of the LTC2870/LTC2871, but also includes different applications.

    https://www.analog.com/en/resources/technical-articles/multiprotocol-transceivers-combine-rs485-and-rs232-in-a-single-device.html

    Best,

    Jane