SN65HVD10 Receive pin

Santhosh Ramani

Other Parts Discussed in Thread: SN65HVD10

Hello,

I am using the SN65HVD10 for RS-485 interface connected to my UART on LM3S9D90. The nRE and DE signals on the IC are tied together and driven by a output pin on the mircocontroller.

When the output pin is set to enable transmit ( which at the same time disables Receive) - the receive output (pin 1 on the IC) goes low. This inturn causes a Receive interrupt on the microcontroller (due to the start condition).

If I use an external pull-up on RO (pin1) the receive interrupt does not occur any more but the RO pin is at 2V instead of 3.3V. This tells me that the IC is still trying to pull the pin low but is not completely successfull. The pull-up resistor I am using is 4.7k.

Is this normal behaviour for the RS-485 chip? Do I need to use a stronger pull-up?

Thank you for your replies

Regards

Santhosh

over 14 years ago

0 Thomas Kugelstadt over 14 years ago

TI__Mastermind 28265 points

Santosh,

can you disconnect the receiver output of the HV10 from the input of the controller and then meausre whether the receiver is close to 3V?

Thanks,

Thomas

0 Santhosh Ramani over 14 years ago in reply to Thomas Kugelstadt

Expert 2765 points

Hello Thomas,

Thank you for you reply. The only way for me to disconnect the receive output would be to cut the trace and check. I am unwilling to do that as of now since I have to test the rest of the design and I do have the RS-485 working. I will do so when my preliminary test is complete. I am attaching the original schematic for this design, please note there is an additional pull-up resistor on the RO output.

Thank you so much.

Regards

Santhosh

0 Thomas Kugelstadt over 14 years ago in reply to Santhosh Ramani

TI__Mastermind 28265 points

Okay we will try to verify this in the lab.

regards, Thomas

0 Thomas Kugelstadt over 14 years ago in reply to Santhosh Ramani

TI__Mastermind 28265 points

Hello Santosh,

I'm assuming you are replacing the Intersil part with the HVD10?

Anyway, we tested the pull-up behavior in the lab and it pulls-up properly. But I see from your schematic that the problem could be at the receiver enable pin, /RE. Please reduce your pull-down resistor (R608) from 100k down to 4.7k. Internally the /RE input has a weak 100k pull-up. So you applying a 100k pull-down externally will half the voltage at this pin and in idle mode, the receiver really does not know whether it is ON or OFF. This could lead to strange leakage currents inside causing the receiver output to not fully conduct or fully disconnect.

Let me know how it works.

regards, Thomas

0 Santhosh Ramani over 14 years ago in reply to Thomas Kugelstadt

Expert 2765 points

Hello Thomas,

Yes it did work. Thank you very much.

Regards

Santhosh

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SN65HVD10 Receive pin