[ARM] SN75176 network failure

[haohaodk46]

Hi Everyone ,
I am facing a issue relate to SN75176 in network RS485.
In my Rs485 network. The master send request to all node in network. I have use TVS diode for ESD protection as circuit below. In some cases these some node faile and it cause this entire network getting stopped communication with Master. Seem TVS is shorted to GND. If I remove all TVS diode in all node then It work well . I checked communicate face to face 1 master - 1 slave ( in case TVS diode have not removed yet ) then it still work well but in network then it fail
Anyone can help me this issue ?
This is my RS485 design :

 

[FvM]

The bourns TBU protections devices are e.g. designed to disconnect surges up to 850V. You didn't tell however, if your application involves possible RS-485 bus overvoltage in this order of magnitude? Basic ESD protection is already provided by the RS-485 transceivers.

--- Updated Nov 9, 2021 ---

A higher protection level is achieved by transceivers with built-in fault or surge protection. You'll start with an analysis which overvoltage events are expected in your application.

 

[haohaodk46]

The bourns TBU protections devices are e.g. designed to disconnect surges up to 850V. You didn't tell however, if your application involves possible RS-485 bus overvoltage in this order of magnitude? Simple ESD protection is already provided by the RS-485 transceivers.

My application is use for Parking guide system . So is The protection not necessary ?

 

[KlausST]

Hi,

Thank for your comment. But why do i see almost use protection circuit
You can see application note as below :

Don´t you see:
* They are using bidirectional protection diodes, but you use unidirectional ones
* they are 12V / -7V ones, but yours are 6.8V / -0.7V.
* they don´t use them between both lines

--> look at the symbols. Read the datasheets.

Klaus

 

[FvM]

It depends. What's the network extension, are bus cables installed near to mains cables, is the logic ground of network nodes floating or connected to protective earth? How reliable is the power supply grounding, does it e.g. have multiple supply connections?

In a large range RS-485 network that possibly involves ground potential differences between nodes, galvanic isolation of RS-485 interfaces would be suggested. It's more effective in guaranteeing health and reliable operation of the network than most protection circuits with ground connected interfaces.

 

[haohaodk46]

Hi,


Don´t you see:
* They are using bidirectional protection diodes, but you use unidirectional ones

=> My circuit using SMAJ7.8CA , It is bidirectional diode

* they are 12V / -7V ones, but yours are 6.8V / -0.7V.

=> volate supply for transceiver is 5v so i use diode 6/8v . What wrong here ?

* they don´t use them between both lines

--> look at the symbols. Read the datasheets.

Klaus

 

[FvM]

I see the original problem "network failure" not directly connected to protection circuits, except for the incorrect usage of an unidirectional TVS diode between bus lines which is shorting the bus voltage in one direction in post #1 schematic.

--- Updated Nov 9, 2021 ---

My circuit using SMAJ7.8CA , It is bidirectional diode

O.k., why not using correct circuit symbol and component name in schematic?

 

[KlausST]

Hi,

I´m confused.

In post#1
* there are three diodes
* unidirectional
* 65V
* one line there is : cathode to GND ... and at the other line it is: anode to GND

In post #5:
* SMAJ6.8CA (which indeed is a bidirectional one. I admit i´ve missed to read the "C")
* voltage levels still don´t comply with RS485 standards
* .. unclear how many diodes you use

In post#25 you wrote:
* SMAJ7.8CA

*****
So I have to admit:
With 6.8V bi-directional diodes it functionally should work ... but it does not cpmply with RS485.

*****

=> volate supply for transceiver is 5v so i use diode 6/8v . What wrong here ?

The protection level does not generally depend on supply voltage.
It depends on specified voltage on the I/Os. "Absolute maximum ratings / Voltage range at any bus terminal"
--> TI SN75176A datasheet says: -10V ... +15V (independent of supply voltage!)

In opposite to:
Many other devices have internal protection diodes to VCC and GND, limiting the voltage to (about)
* GND -0.6V
* VCC +0.6V (here it depends on applied supply voltage. Means: if supply is switched off to 0V, then you must not exceed +0.6V at the according pin. If you supply it with 3.3V then it is 3.9V)
... but this is not the case here.
And this is urgent for a RS485 driver: Let´s imagine an unpowered node, where the supply voltage is down to 0V.

It´s urgent to read each devices´ datasheet. Because it differs from device to device.
Even different RS485 drivers may have different specifications.


Klaus

--- Updated Nov 9, 2021 ---

O.k., why not using correct circuit symbol and component name in schematic?

I fully agree. And requested this in post#6.

 

[haohaodk46]

Hi,

I´m confused.

In post#1
* there are three diodes
* unidirectional
* 65V
* one line there is : cathode to GND ... and at the other line it is: anode to GND

In post #5:
* SMAJ6.8CA (which indeed is a bidirectional one. I admit i´ve missed to read the "C")
* voltage levels still don´t comply with RS485 standards
* .. unclear how many diodes you use

In post#25 you wrote:
* SMAJ7.8CA

*****
So I have to admit:
With 6.8V bi-directional diodes it functionally should work ... but it does not cpmply with RS485.

*****

The protection level does not generally depend on supply voltage.
It depends on specified voltage on the I/Os. "Absolute maximum ratings / Voltage range at any bus terminal"
--> TI SN75176A datasheet says: -10V ... +15V (independent of supply voltage!)

In opposite to:
Many other devices have internal protection diodes to VCC and GND, limiting the voltage to (about)
* GND -0.6V
* VCC +0.6V (here it depends on applied supply voltage. Means: if supply is switched off to 0V, then you must not exceed +0.6V at the according pin. If you supply it with 3.3V then it is 3.9V)
... but this is not the case here.
And this is urgent for a RS485 driver: Let´s imagine an unpowered node, where the supply voltage is down to 0V.

It´s urgent to read each devices´ datasheet. Because it differs from device to device.
Even different RS485 drivers may have different specifications.


Klaus

--- Updated Nov 9, 2021 ---

I fully agree. And requested this in post#6.

I correct my shematic :