ISO1176: Fail to transmit signals from bus side to MCU side

GuanMian,

More details will be needed to find the root cause. Were there any system level events like ESD or surge that could have happened? ESD may have happened if there are physical connectors on the bus. Is the customer able to provide waveforms of the bus as well as the other pins of the device? Has power been verified to both Vcc1 and Vcc2?

Respectfully,

Lucas

Hello,

Schematic attached as below.

1. ESD and surge may be excluded in this case. Since the resistances between pins in bus side (VCC2 - GND2, A - GND2, B - GND2) are measured and they are the same as good chip. Besides, the series bus resistors R86/R87 are also ok.

2. The waveform on customer's site is ok for A/B bus lines. One suspicious item is the EARTH signal is noisy on site.

3. The power between both VCC1 and VCC2 is ok, they are provided by internal independent regulated power modules.

What is your opinion?

Thank you.

Guanmian

Guanmian,

ESD events can still appear with the voltage happening across the isolation barrier instead of Vcc2 to GND2. What is the resistance between GND1 and GND2? Does the system work if a new ISO1176 is placed?

Respectfully,

Lucas

Hello,

The resistance between GND1 and GND2 is open, the same as new one. Also the capacitance between GND1 and GND2 is similar as new one, approx. several tens PF.

The system works if a new ISO1176 is placed.

Best regards

Guanmian

Hi Guanmian,

I'm Manuel Chavez, a member of Lucas's team. Please allow me to respond while Lucas is on business travel this week.

Thank you for the previously shared information. Can resistances between Vcc1, R, D, RE, /DE, and GND1 be measured or waveforms captured on these pins of the non-functioning ISO1176? So far the bus side of the device seems okay, so this will help analyze the MCU side.


Respectfully,
Manuel Chavez

Hello，

Thank you for support!

The resistance measured as below, it is almost the same between failure chip and new chip.

Please note that they are on-board measuring as previous schematic shown, but without two power modules.

Resistance between      Failure ISO1176         New ISO1176
VCC1-GND1(1-8)          2k                                2k
R-GND1(3-8)                 18.9k                           18.9k
/RE-GND1(4-8)              9.8k                             9.8k
DE-GND1(5-8)               9.8k                             9.8k
D-GND1(6-8)                 18.9k                           18.9k
PV-GND1(7-8)               2k                                2k

Attached you can also find the waveforms between them.

Best regards
Guanmian

Hi Guanmian,

You're welcome! Since the impedance measurements were made on-PCB, external components likely affected the measured values, but they do not show any short circuits which is what we were looking for.

Thank you for including oscilloscope captures; in these waveforms, RS485+ is the positive bus line, A, correct? In order to narrow down the scope of this failure, please check whether the old unit can drive the bus like in CH3 and CH4 waveforms on the "New ISO1176" captures.

Since these units are being returned from the field, we suspect damage on the bus (A, B) lines.


Thank you, and have a great weekend!
Manuel Chavez

Hello,

Thanks for your patient tracking.

RS485+ is the positive bus line A. The old non-damaged unit can drive the bus like "New ISO1176" captures.

I de-assembled a damaged ISO1176 and compare the impedance with new one, the result is following.

Hi Guanmian,

Thank you for providing these impedance measurements! Our team suspects this behavior is due to local damage on side 1, likely to an overvoltage stress on Vcc1 or PV. Are there known waveforms that could exceed 7V on the MCU side of the isolator?

Please also confirm whether the damaged ISO1176 can drive the RS485 bus like in "New ISO1176" captures.


Respectfully,
Manuel Chavez

Hello,

I think the overvoltage output from internal isolated regulator is low but can not be eliminated, lots of products are sold but no one is failed because of this regulator.

Is there any possibility that the overstress from bus side(e.g. FE signals is noisy on site, common mode voltage is not stable) that will damage the pins on MCU side in this case? Since another damaged chip (shown above), the impedance measured on-board is the same as "new ISO1176", but also can not transmit signals from BUS side.

It is also confirmed that the damaged ISO1176 can not drive the RS485 as "New ISO1176".

Thanks

Guanmian

Hi Guanmian,

Thank you for sharing this information. Is there a high amount of current drawn by Vcc1 or PV of the failing units? Based on earlier impedance measurements, the problem could lie on the MCU side of the isolator. What is the impedance between Vcc1 and PV of the good and failing units?

It is understood these damaged devices cannot drive the RS485 bus lines. Damage from overvoltages or transients on the bus lines would not damage the MCU side, so if /RE and D pins operate properly, the RX output on the bus side could be damaged. Please attempt to capture a waveform similar to the one below on damaged units. If the bus-side is damaged, which can be prevented in the future with protective diodes on A and B lines, the waveform can be duplicated except with the sections circled in blue held to a constant:



Please share an image of the test waveforms if possible. We will await your update.


Respectfully,
Manuel Chavez

Hello,

The impedance between Vcc1 and PV is open @ failure ISO1176, is ~50KOhm @ good ISO1176.

So the PV pin is damaged? Because of the overvoltage of VCC1? Any other possibilities?

P.S. The PV pin is connected to VCC1 by a 0-Ohm resistor in our product (you can see the schematic R91), I just de-assembled this resistor and measure the impedance.

The waveforms have been posted in previous reply, attached I explain it in more detail.

Best regards

Guanmian