ISO3086T: RS485 interface only working in one direction

Hi Andrew,

Thank you very much for the reply.

IPC communication to device OK, 

R =1.9V, RE =DE =3V , D= 400mV

when communicate from device to IPC, A & B receive signal from device.

But there is no any respond of pin R, RE, DE & D.

R=1.9V, RE=DE=3V, D=400mV

Look like my circuit design always stay at IPC to device stage.

Thank you.

Best regards,

Ku

Hi Andrew,

basically my circuit design was follow Dual Isolated Half-Duplex RS-485 Repeater as reference.

the only different was both ISO3082DW ic i change to SN65HVD72 and ISO3086T ic.

Best regards,

KU

sn65hvd72.pdfISO3086TDW.pdftidu590.pdf

Hello KU, 

What is the data rate of your application? The app note that you have mentioned uses the ISO3082 which means it is intended for much lower data rates. 

Hi Andrew,

according to ISO3082 reference design summary features, these design data rate up was to 100kbps .

The data rate i set in tera term was 9600.

ISO3082 design feature:

So in order to change the ISO3082 ic to ISO3086TDW ic for my design.

i need to reconsider all the value of resistor and capacitor of my circuit to cope with ic ISO3086TDW?

Hello Lee, 

The components should be the same since the data rate is not significantly changed from the reference design. Please allow me another day to review in detail. 

Best,
Andrew

Hi Andrew,

I already removed 1kohm resistor R219 & R222 per your suggestion.

Also modified my connection of J22 pin A connect to ISO3086T's A pin, and J22 pin B connect to ISO3086T's B pin.

unfortunately the result still same , communication signal send from device (J22) to IPC (J23) still NG.

But communication signal send from IPC (J23) to device (J22) is OK.

I had capture the waveform for each IC pin during OK and NG condition for your reference.

From the output waveform of J23 pin A & B, signal are distorted .

Please refer to attached excel file.RS485 connection issue 1.xlsx 

Best regard,

Ku

Hello Ku,

Thank you for the updated schematic. The design is still missing some of the critical components that set the timing for the auto direction. The output of 2A and 2Y should match the signal on the R pins with some slight delay which is used to switch the direction (using RE/ and DE) for each bit (as shown below).  

1. Two 10k pull-up resistors are missing
2. The 300-ohm resistors should be connected between the inverter inputs and the R pins. Then a capacitor to GND. 
3. The 5.1k resistors are 50-ohms which would change the direction timing. 
4. Section 2.2 Data Flow Control of the reference design goes into a lot of detail about the importance of the resistors.
   • After review, I realized that R219, R228 and R222 may need to be added back to the design later using the values in the design below. 
   • However, this can wait until we have corrected function of the auto detection circuits. 

I have marked them on the schematic. 

Best,
Andrew

Hi Andrew,

according to your No.2 comments :  300-ohm resistors should be connected between the inverter inputs and the R pins. Then a capacitor to GND. 

It was different from reference schematic below. As can seen, there was no dot between 300-ohm  the inverter inputs and the R pins. 

I will modified my design exactly same as the reference design by 

-add 10k pull up to both the R pins.

-change the 49.9-ohm to 5.1k-ohm.

Will let you know the above result in my next reply.

Thank you.

Best regards,

Ku

Hi Andrew,

I already modified my schematic accordingly same as reference design,

-add 10k pull up to both IC U10 R pin & IC U9 R pin.

-change both R223 and R224 to 5.1k ohm.

-change R225 & R226 to 300 ohm

-insert R219 & R222 with 560 ohm

unfortunately the result of RS485 communication signal from device (J22) to IPC (J23) still failed.

According to waveform i took , the signal of U10 pin R, RE , DE and D look OK.

But U10 output pin A & B waveform become distorted & suppose OK condition A& B signal are inverted,

but now was not inverted.

Hi Andrew,

 As I understand the problem, Communication from J23 to J22 is play. However, communication from J22 to J23 has problems, correct? 

-Yes, only J22 to J23 communication had problems.

U10 pin 5 must have a direct connection to ground. Is this fixed? 

-When U10 pin 5 direct connect to ground, condition became more worst , communication from J23 to J22 also became NG.

U10 become not working at all, when signal send from J23 to J22 / from J22 to J23.

Input signal to U10 pin A & B but there is totally no output pulse from U10 pin R, (pin R =5V, RE=DE=0V , D= 5V).

When signal send from J22 to J23, pin R=5V, RE=DE= 0 pulse, D= 5V pulse all look still OK at this stage. But there is no any output pulse being detected at U10 pin A & B.

Regards,

KuWaveform when short C314 to GND.xlsx

Hi Andrew,

Since i almost try all the possible method but still not working for my communication from J22 to J23.

and it look like mainly signal lost or distorted after go through U10 (SN65HVD72).

Do you think is it i have use the wrong U10 ic?

Thank you.

Best regards,

Ku

Hi Andrew,

Finally my RS-485 communication working fine in both direction after

-short C314 direct to ground (U10 ground pin 5 direct connect to ground)

-SN65HVD72 , U10 Vcc change to 3.3V.

Actually i try before change the U10 ic Vcc to 3.3V but communication from J22 to J23 still NG because U10 ground pin 5 was not direct connect to ground 

at that moment , C314 was still inserted.

Thank you very much for your great support to solve my design issue.

Best regards,

Ku

Hi Ku,

Thank you for being patient and updating me on what worked for the final solution. I will close the thread now.

Please feel free to ask a new question for your next design!

Best,
Andrew