SN65HVD234: Enable Line doesn't shut down driver

Hello Billy,

You should be seeing the device function normally whenever EN is high. That is, you should see toggling on the D pin transfer through to the different CANH/CANL outputs and reflect back via the R output. Is this what you are seeing, or is the R output always low? (Does the device ever function properly in your testing?)

Just so I can clearly understand the issue, can you please let me know the voltages at the CANH/CANL/R pins for the different possible states of the EN and D input pins?

Max

Hi Max,

So here are the states (Rs=0V both cases):

Disabled:
1) D=0.2V, R=3.3V, EN=0V, CANH = 3.3V, CANL = 3.3V
1) D=3.3V, R=3.3V, EN=0V, CANH = 3.3V, CANL = 3.3V

Enabled:
2) D=0.2V, R=0.0V, EN=3.3V, CANH=3.3V, CANL=0.4V
3) D=3.3V, R=3.3V, EN=3.3V, CANH=2.2V, CANL=2.2V

Is this normal behavior? I'm trying to shut down the driver such that CANH & CANL both equal to 0V.

Billy

Hi Billy,

The voltages you report for the enabled case are normal.

When the device is disabled, the CANH and CANL lines are made high impedance. If nothing else is connected to the circuit, in most cases I would expect the measured voltage at CANH/CANL to be near 0 V. If there are any sources of pull-up current, though, they may appear "stronger" than the disabled driver and result in a high voltage measurement.

Does your circuit match the image you posted above (Figure 19), or do you have any other components on the CAN bus? What are you using to measure the voltage? And, can you tell if the 3.3-V is being "driven" or is it a high-impedance bias? (You should be able to determine this by placing a relatively high pull-down resistance like 10 kOhms on CANH or CANL and seeing how much the measured voltage drops.)

Regards,
Max

Hi Billy,

Just checking in - were you able to make any progress towards resolving this issue?

Max