Receive line in SN75HVD3082

Hi Davis,

When the driver is enabled it will continuously drive the bus to whatever state the D pin is in. Since you are using a half-duplex device, the driver output lines and the receiver input lines are tied together. Page 9 of the datasheet shows that if the DE pin is tied high the driver will always be driving the A & B pins differentially.

 Where in the datasheet are you referring to that says the receive lines should be high impedance when the driver is enabled? The receiver and driver are controller independently by the DE and /RE pins. If you tie the DE and /RE pins together then either the driver or the receiver will be enabled.

 Please let me know if you have any more questions.

 Thanks,

 John

Hi John,

              I have tied DE and /RE pins together.  When I enable the driver by driving DE high, according to the internal circuit diagram, R should be high impedance, isn't it.  This figure I have extracted from the datasheet.  But I am seeing that R is going low when DE-/RE is driven high.

Hi Davis,

You are correct, if the /RE pin is tied high the R output should be disabled and it should be in a high impedance state. Do you have a schematic that you could share showing your set-up?

Thanks,

John

Hi John,

                The schematics is attached above.

Thanks.

Davis

Hi Davis,

I am no expert at the Si8431 device, but I think the reason you are seeing the low on the OUT3 pin is because of the way the device is designed. On page 20 of the datasheet there is the following table:

I feel like there is one line in the table that is missing (with "X", "H", "P", "P"). But, from looking at the table it looks like if the input is floating "X", the EN pin is high, and both supplies are active, then the Output will be low. Can you check if the R pin is actually driven low? Are you just assuming the pin is low because the OUT3 is low?

Let me know if you think I am misunderstanding the table.

Thanks,

John