SN75176B: Frequent failure of SN75176B IC even with transient suppressors like MOV, re-settable fuse and TVS diode array

Prasad Janaswami

Part Number: SN75176B

Hi,

I have been using SN75176B IC for RS 485 over modbus protocol as a slave device. We use these devices on multiple locations in industry and it has been observed that the SN75176B IC is failing frequently

We have used MOV's (MOV10D), TVS diode array (SM712), resettable fuses as well as for protection. But still the IC's are failing not instantly but over the time.

Our voltage scheme is like

1. SMPS( 24V DC) ---- Isolated DC -DC converter (24V DC Input, 5V for controller and 5 V isolated output for SN75176 IC).

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

3. The SN75176 is connected to micro controller through opto couplers in between them.

4. The devices are connected in three wire configuration say.., RS+, RS-, RSG.

We have facing same problem on multiple locations. Please guide how to rectify it.

over 5 years ago

0 Hao L over 5 years ago

TI__Mastermind 25705 points

Hi,

How's the grounding of the SN75176 and controller? I would like to understand how the isolation works and if there is solid ground path to take the ESD current. Sometime the devices still get damaged with external protection, which might be due to the protection not diverting all current of overstress events from the IC. In other words, the IC still takes current more it can handle. To analyze this, we need to understand the electrical characteristics of the strikes. Another point is SN75176B was designed in 1980s. TI has released many new RS485 with better ESD protection capability. If you're interested, please check out this post.

Regards,

Hao

0 Prasad Janaswami over 5 years ago in reply to Hao L

Prodigy 20 points

Hi Mr. Hao,

Thanks for the reply,

1. SN75176 and controller's powers supply's were isolated using two different 5V and grounds and also the TX, RX, RE, DE were isolated using opto couplers.

Please suggest if there was any other alternative rather than going for new component.

Regards,

Prasad Janaswami

0 Hao L over 4 years ago in reply to Prasad Janaswami

TI__Mastermind 25705 points

Prasad,

I'm sorry I thought I had replied. As I said, it's better to understand the characteristics of the ESD events then boost up the protection. TVS diode and MOV work well under certain conditions.

Regards,

Hao