Frequent failure of SN75176

Hi Purushottam,


Would you be able to give any more details about your system? In particular:
What sort of voltages are present?
What does the bus termination scheme look like?
When the devices fail are they permanently destroyed or can they operate again after cycling power?
During normal operation are you seeing any unusual behaviors?
How are the protection devices configured?

Since the devices are failing over time, there could be some electrical stress on the part caused by unexpected voltages on the bus or the other device pins. Any additional information would be very useful in determining the root of your problem.

Best Regards,
Casey McCrea

RS485 Protection.pdf

Hi Casey,

Thanks for your reply and support. 

1. The voltage scheme is like this

    AC Supply (230V) --- SMPS( 24V DC) ---- Isolated DC -DC converter (24V DC Input, 5V and 12 V isolated output).                                                                             The grounds are shorted together for all voltage outputs.

2. We have connected 120 ohm resistor at the last device on the bus and at the start end we have connected PLC. 

3. The failed devices are permanently damaged. We need to replace the IC every time to start the communication again.

4. I didn't get you about unusual behavior during normal operation. What do you exactly mean.

5. Please refer attached file for protection device configuration.

The SN75176 is directly connected to micro controller without any opto couplers in between them. Sometimes we are facing the problem of damaging microcontroller also. 

The devices are connected in two wire configuration. Do We need to connect 100 ohm resistor in between Ground terminal on PCB and Earth terminal of AC supply.

Please let me know if you need any additional information.

Regards,

Purushottam Pawar

Purushottam,

The MOV10D201K are not adding any protection because its voltage is way too high.

The SN75176B has low ESD rating (1500V HBM) compared to our newer devices.

Hi Ronald,

What is your advice to resolve this issue. How do I figure out the reason of this failure.

Please guide.

Purushottam,

Do the other devices on the bus have problems or just one location?
If other location fails, do they have the same design?
Is there are correlation to events such as lightning storms, power glitches /failures, other power voltages on the same cable, user interactions (ESD), nearby Vandergraph generators or Tesla coils.

Hi Ron,

It has been observed that the slave devices are failing randomly at all locations. Let me give you brief scenario.

At one location there are seven slave devices installed at the field and all are communicating with master device (PLC). Baud rate is 9600 and distance is around 700 meters. At this place the IC's in slave devices are failing randomly at any instance of time. Even the failure of Master device which is CECO make PLC has been observed because of failure of communication IC. 

At second location again seven slave devices and Delta Make PLC is installed. But at this place there is failure in only slave devices. No failure in PLC since last one year. Baud rate is 19200 and distance is around 600 meters. 

At third location four slave devices and Delta make PLC is installed. Failure in slave device and no damage for PLC. 

All the slave devices are installed in LPG Gas refilling bottling plants at first and third location. And the slave device at third location are placed in closed control room besides master device.

I am not able to figure out the reason for this failure. 

All devices at all three locations has the same problem. All the devices has the same design except the difference of baud rate.

I am not sure about the  events which you have mentioned, but also note that the slave devices which was installed before our designed devices from another manufacturer are working without any problem. Those devices are also has SN75176B IC. So i don't feel it is the problem of IC.

If we think that if it is our designing fault then what are the parameters do i need to check at circuit level.

The UART pins of micro controller are directly connected to SN75176, and fail safe biasing alongwith termination resistor is provided.

RS485_Comm.pdf

Please advice. What are the precautions do I need to take to overcome this problem.

Purushottam,

Fellow employees have written some good application notes for RS485.
e2e.ti.com/.../videos__files

I suggest this application note: RS-485 Transient Protection
e2e.ti.com/.../download

Hi Ron,

Thanks for your suggestions. 

Does isolation in between micro controller and communication IC helps to improve the performance.

As per my understanding this kind of isolation is used to protect the micro controller.

I just want to confirm that my assumption is correct or not?

Purushottam,

The isolation protects the RS485 device. It is the high current flow in the ESD pulse that does the damage. Isolation prevents current flow.
If the voltage is high enough to jump the isolation gap, the current must not pass though the RS485 device.

I wrote this for TL431 shunt. Some of the concepts apply.
e2e.ti.com/.../designing-for-isolated-dc-dc-converter-shunt-safety

Hi Ron,
What if the RS485 bus is having only two wires(Data + and Data- ) and not having common ground wire.
Does it causes the failure of SN75176?

Hello Purushottam,

The common mode range and abs max voltage rating at RS485 bus terminals must be observed.

Best,
Michael

Purushottam,

It is fine provided that the absolute maximum voltage relative to the SN75176 ground is not exceeded.