SN65HVD1780: Resistance Value between A and Gnd Pin

Hi Mohanraj,

I don't have typical measurements for this value. Variance between several k-ohm and a few M-ohm can occur based on the polarity of the measurement. When a meter measures resistance, it is applying a voltage and reading the current through the probes to calculate the resistance. Depending on which side of the circuit has the positive potential, the current may be smaller or larger depending on the orientation of diodes and FETs. The measured resistance will also vary depending on the operating mode of the part and how it is driving the A-B pins. The greatest measured resistance would be expected when the driver is high impedance (disabled) and the lowest would be when actively driving LOW. Do you still see these variation when keeping measurement polarity and operating mode constant between ICs?

10 ohms is much too small of a value for this measurement when idle and usually indicates damage to the part. Was this measurement taken when the device was active (possibly driving a GND potential)? Could you please share more information about the communication failure and the application in which SN65HVD1780 is being used? Any schematics or scope shots you are able to share would help diagnose possible causes. 

Regards,

Eric

Hi,

  Thanks for replying.

  The IC with 10 ohms is failed to due to wrong power supply connection which is rectified now. The negative terminal is connected to A pin wrongly which causes resistance of the A terminal is reduced to 10 ohms from 19Kilo ohms. 

 I am measuring the resistance value by keeping negative probe at ground pin and positive probe at A pin in both the ICs when there is no power supply. The value was same when i am checking it only in the IC alone.

The old IC(VP1780 8AM SJSL G4) which is working fine is showing 5.6Mega ohm and the one  new ic(VP1780 91M SJSL G4) which  has the value as 19 kilo ohms. We have used this communication ICs to connect multiple nodes(20) at 10 Meter distance.

Is this resistance change will cause any communication issue in the future or its just a normal thing? Is there any other ways to identify the communication is healthy like measuring voltage or some other ways?

Hi Mohanraj,

The resistance at the bus pins is not a typical way of identifying communication robustness. A common way of testing signal integrity is to attach scope probes to the various nodes of the system and measure the voltage that exists at the driving and receiving ends. Well designed systems will have a high differential at the receiver with steady voltages before transitioning and minimal ringing throughout the system. 

Are you encountering communication failures in your system or are you concerned about signal integrity in general? Is your system connected in a particular network topology? Star or daisy-chained? Please include termination scheme. 

Regards,

Eric

The above  image is the  schematic of my master controller board.

Slave:

We have maximum connected 50 nodes in daisy chain format. After some years we noticed that communication in one or two node failed and while servicing it we found that resistance(A and Gnd, B and Gnd) was reduced compared to original value(5.6Mega ohms).

Also if i measure the voltage across A and Gnd pin Its showing 3 to 5v peak(resistance value is 5.6Mega ohms) when normal and 2.1v at faulty board(resistance value is 1.8Mega ohms). 

But we recently we identified the fresh board (ICs)itself having the resistance value as 20 Kilo ohms but the voltage level is similar to 2 to 5V. My concern is will it causes any problem in future? Will this resistance value drops after some years?

Hi,

       In the IC its mentioned VP1780  8AM SJSL G4 - having resistance value as 5.6 M-ohms and IC - VP1780  91M SJSL G4 is having 20K resistance

       Please send me the email for sharing the purchase details.