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SN75LBC176: Default input resistor on D input pin.

Part Number: SN75LBC176

Hello, 

For a design in production, we use a  two SN75LBC176 connected to a micro-processor for a RS422 communication link (UART). One component is configured in TX mode only and the other component is configured only in RX mode.At power up all the processor i/o are in input mode, after power-up the i/o are configured in Input or Output. During the power-up stage i need that the TX line shall be at high level (default UART state). In the processor i/o pad there is no weak pull-up. My question is about the SN75LBC176 datasheet. I try to find if there is a weak-pull-up or not but unfortunately I don't find the answer in the datasheet, Could you help me to find the information. At power up some board are at low level for the D input and other board are at high level. It seems to be link to the SN75LBC176. Could you help me to solve this issue.

  • Hi,

    Yes, D pin of SN75LBC176 has weak pull-up resistor. Please refer to the schematics of inputs and outputs on page 3. Can you explain a little more about "at power up some boards are at low level and some at high level"? Do you mean during the power ramp-up, the voltage of D pin could be high or low? Is it possible to capture the waveforms? After all, may I know the reason why you chose this device? TI has many updated 8 pin half-duplex RS-485, like THVD1500, THVD1550 you may want to check them out.

    Regards,

    Hao

  • Hi,

    Thank for you reply. Is it possible to know the weak pull up value. In the table driver parameter page 5, it is written -100µA min for Iol or Ioh, do you think that the input resistor is 5v/100µA so Rinput min is 50KOhms ? The design is in production since 3 years without problem. The design work well with ALH1 lot number written on the component,.With the new lot ADR3, the defautl level at power-up is now low instead of high. With the low level the system doesn't work (UART). With the previous lot the design work, at power up the default level is high. Could you help me to understand the difference between the ALH1 and ADR3 lot numer. The processor connected to the SN75LBC176 has the same lot number.

    This component was choosen at the begining of the project since 5 years ago. If we want to change to a new reference, new qualification will be needed.

    Best regards

  • Hi,

    Do you have the 7 digital marker? I consulted TI quality team and they haven't seen similar issues about SN75LBC176. Your estimate about the pull up strength is correct. Do you know the driving capability of UART pin? You can do the experiment by pulling the current out of the pin or adding external resistors. You could do the same experiment for SN75LBC176 too. Another idea is to check the DC voltage of the UART pin with ALH1 or ADR3 to see the difference.

    Regards,

    Hao