[SOLVED] reason of reverse current in circuit

[ravi.2k17]

hi all,

I am seeing reverse current in below circuit at DI (driver input) pin of rs485 transmitter. this is shown with red color.
what could be the reason of reverse current?
Here I have connected PIR sensor HC-SR01 to Arduino(currently LED only for testing) using RS485.

 

[KlausST]

Hi,

What do you mean with reverse? Show voltage and current direction.
What magnitude?

Did you read both datasheets? Post links.

Klaus

 

[ravi.2k17]

Hi,

What do you mean with reverse? Show voltage and current direction.
What magnitude?

Did you read both datasheets? Post links.

Klaus

PIR sensor data pin should send signal to input pin DI (driver input) however I am getting current from the input pin. if i connect a LED to input pin DI, it gets turned on. here is the datasheet of MAX485

https://datasheets.maximintegrated.com/en/ds/MAX1487-MAX491.pdf

 

[KlausST]

Hi,

This does not answer a single question...

Klaus

 

[ravi.2k17]

Hi,

This does not answer a single question...

Klaus

Let me try to clarify things once again.
As per the datasheet of Max485, pin DI works as input pin when we configure it as a driver/transmitter by shorting DE and RE and setting them to High. But in my case pin DI is sending current instead of receiving. I confirmed that by using a LED (not shown here) between DI and GND. If i am correct this input pin should never send any signal as long as Max485 is working in transmitter mode. Please see current direction in my below circuit.

Does current/voltage magnitude matter here? only knowing direction is not enough to guess that the current is reverse and something wrong in the circuit?

I have gone though both the datasheet of PIR HC-SR01 and Max485. Do you think I am missing something in Datasheet. I have referred the truth table of Max485.

Below is my circuit showing "Reverse Current Flow" and the truth table from the Datasheet of Max485.

 

[FvM]

But in my case pin DI is sending current instead of receiving. I confirmed that by using a LED (not shown here) between DI and GND.

I don't see how this LED circuit can indicate that DI is sourcing current. LED current can be sourced by sensor output as well. You want to place a series resistor and measure the voltage drop.

 

[ravi.2k17]

I don't see how this LED circuit can indicate that DI is sourcing current. LED current can be sourced by sensor output as well.

to confirm that i just disconnected PIR data wire and connected a LED between DI and GND and it was on.

 

[FvM]

DI input current is specified to be below +/- 2 uA. If you see higher current, you either connect the transceiver chip wrongly or damaged it before.

 

[ravi.2k17]

DI input current is specified to be below +/- 2 uA. If you see higher current, you either connect the transceiver chip wrongly or damaged it before.

do you think any thing wrong in connection for transmitter ? i have connected as per the truth table in datasheet.... DE, RE should be 1 (connected to 5 V) and RO should be non floating hence connected to GND. how can i figure out if the chip is damaged or connection is incorrect?

 

[betwixt]

Yes, there is a connection error - you have the receive output of the transmitting MAX485 grounded - you should leave it floating if not using it but that wouldn't account for current flowing from the data input pin.

Just double checking, are you wiring the ICs as shown in the schematic with that layout of pins or are you definitely using the pin numbers associated with their functions.

Brian.

 

[ravi.2k17]

Yes, there is a connection error - you have the receive output of the transmitting MAX485 grounded - you should leave it floating if not using it but that wouldn't account for current flowing from the data input pin.

Just double checking, are you wiring the ICs as shown in the schematic with that layout of pins or are you definitely using the pin numbers associated with their functions.

Brian.

you are talking because in the diagram DI pin location differs between transmitter and receiver. i just change the layout for display as in my software there was no option for overlap.
i am using with pin layout not pin number. plz do not get confused with that.

 

[KlausST]

Hi,

RO of the transmitter needs to be left open.

Klaus

 

[FvM]

RO of the transmitter needs to be left open.

Seriously, I don't understand why RO is grounded in the schematic, but because RO is floated by applying high level to /RE, it doesn't cause problems.

 

[ravi.2k17]

Seriously, I don't understand why RO is grounded in the schematic, but because RO is floated by applying high level to /RE, it doesn't cause problems.

might be i am not very sure about floating pin concept. since RO is not needed in transmitter, i though leaving this open will cause floating pin issue. please correct me if i am wrong.

not only this one, in receiver side i put DI to GND which is not used in signal receiving.
anyway removing RO (and DI also) from GND also has no effect.

--- Updated Nov 3, 2020 ---

Hi,

RO of the transmitter needs to be left open.

Klaus

i am still learning basics, i thought leaving RO would create floating pin issue. is not it true here? why do you think it should be left open?

 

[FvM]

Floating pin issues (like excessive quiescent current) exist for pins with input buffer, not pure outputs.

 

[ravi.2k17]

Floating pin issues (like excessive quiescent current) exist for pins with input buffer, not pure outputs.

then I am removing RO from transmitter and DI from receiver sides but unfortunately it is not fixing this issue.

 

[FvM]

then I am removing RO from transmitter and DI from receiver sides but unfortunately it is not fixing this issue.

As expectable. DI is an input an should be not left open.

 

[ravi.2k17]

As expectable. DI is an input an should be not left open.

later i also realized the same and kept DI it connected back to GND
but the main problem remains same.
so if my connections are now seem to be proper, is there a possibility that there is some h/w issues with rs485 transmitter or receiver?
what should be my next approach.

 

[KlausST]

Hi,

Does current/voltage magnitude matter here?

Yes, magnitude matters.
If we talk about nA or small uA then it´s simply leakage current.

If we talk about mA then there is a problem.
Then we need to see the complete test setup.
Is it simulation or real measurement?
PIR sensor connected or not?

From my experience with MAX485 and from the datasheet there is no significant input pin current. .

Klaus

 

[ravi.2k17]

Hi,

Yes, magnitude matters.
If we talk about nA or small uA then it´s simply leakage current.

If we talk about mA then there is a problem.
Then we need to see the complete test setup.
Is it simulation or real measurement?
PIR sensor connected or not?

From my experience with MAX485 and from the datasheet there is no significant input pin current. .

Klaus

to narrow down further i have removed the PIR sensor, added a LED and only concentrating on reverse current issue as of now. below is my test circuit. please see red color wire and direction.
for magnitude i have not measured but we are discussing for mA as this current is enough to light on a LED of 2.1 v and which draws current in mA.