SN75176B: U.L. Concept - Measure Receiver Line input current

Hello,

I'd expect the input currents to be low even with the device powered as long as the driver is disabled. To start, can you please verify that VCC is 5 V and DE is at 0 V? Does the issue show up when A or B is shorted to a test voltage while the other terminal remains floating?

Max

Hello Max,

thanks a lot for your quick reply and looking into this. Highly appreciated!

To answer your question I checked VCC and DE and the results are as follows when using the circuit outlined in a) Isolated Power Supply / Arduino (Atmega 328p pu microcontroller / SN75176BP:

• VCC --> Local_Ground: 5.02 V
• DE --> Local_Ground: 0 V

Everything is working fine when applying Test Voltages ranging from 3.0 V up to 7.0 V while having one of the terminals shorted to GND. As soon as I apply Test Voltages 9.0 V and 12.0 V the "issue" starts (Device get's extremely hot and finally get's damaged).

As well no issue observed when applying Test Voltages 9.0 V and 12.0 V to either A or B terminal while keeping the other terminal floating (so no connection to GND).

On top I build a "small" RS-485 bus having one SN75176B running as

• SENDER (RE = DE = HIGH (5.0 V)

and another SN75176B running as

• RECEIVER (RE = DE = LOW (0.0 V))

Signals generated on SENDER's D-Input (HIGH and LOW) can be well seen seen/received on the other device running as RECEIVER.

Best Regards,
Peter

Hello Max,

I need to correct my statement/answer made above in regards to

Everything is working fine when applying Test Voltages ranging from 3.0 V up to 7.0 V while having one of the terminals shorted to GND. As soon as I apply Test Voltages 9.0 V and 12.0 V the "issue" starts (Device get's extremely hot and finally get's damaged).

In fact the statement/answer should be:
Everything is working fine when applying Test Voltages ranging from 3.0 V up to 5.0 V while having one of the terminals shorted to GND. As soon as I apply Test Voltages >= 7.0 V up to 12.0 V the "issue" starts (Device get's extremely hot and eventaully get's damaged).

Best Regards
Peter

Peter,

What you have seen really puzzled us. If the driver is disabled, the device should have input current in the order of uA. However I cannot figure out what went wrong. To debug, could you start from the very basic setup, for example, providing a single DC supply to the device and repeating the test? Please make sure the pin voltage is reference to the same ground as the device.

Regards,

Hao

Hello Hao,

thanks a lot for your reply. It puzzles me as well as everything is working like a charm as long as the voltage is below 7.0 V. Anyhow I repeated the tests using the setup's listed above in a) and b) having even resistor R1 removed - same result as in my initial problem statement. Maybe worth to mention that EVERYTHING ELSE is working as it should and as well the measurement circuits provided in the datasheet are working like a charm.

Could it be due to the power supply I'm using? It's a regular power supply with PRIMARY 100V - 240V ~ and SECONDARY 3.0V / 4.5V / 5.0V / 6.0V / 7.5 V / 9.0 V / 12.0 V DC as mentioned above.

Any schematic of how an Unit Load test circuit should look like would be highly appreciated.

Best Regards,
Peter

Hi Peter,

I wouldn't expect fail-safe functionality to have a significant effect on input currents like you are reporting.  Plus, SN75176B does not feature internal fail-safe biasing.  So, I think something else is going on here.  But if you're happy with the solution, maybe it isn't worth debugging further.  By the way, if you wanted to change to a different single-channel transceiver, I'd recommend a newer device like THVD1550.  SN75176 was developed in the mid-80s, and there have been great strides made in integrated circuit technology since then.

Regarding the DE/RE control lines - it sounds like you have them correct.  Some users prefer pulling /RE high when sending messages to block the echo, but that is a personal preference.  (Having the echo enabled can be useful in detecting transmission errors if the software implementation allows for it.)  The most important thing is that the DE line be high on at most one transceiver at a time.  You want to avoid having multiple nodes sending at the same time - this would corrupt the messages.

Max

Hello Max,

thanks a lot for your detailed response. In regards to

Plus, SN75176B does not feature internal fail-safe biasing.

you are absolutely correct - my apologies for that. Was mixing it up with

The driver differential outputs and the receiver differential inputs are connected internally to form differential input/output (I/O) bus ports that are designed to offer minimum loading to the bus when the driver is disabled or VCC = 0.

My intention was to get familiar/play a bit/gain some further knowledge with RS485 Transeivers in general and the SN75176B transceiver looked to me as the ideal candidate (as well when looking at the price of a single device and given the fact how many devices I destroyed by now it seemed to be a good decision). Same is true for having DE and /RE control lines enabled at the same time - I just wanted to make sure that my understanding is correct and the according HIGH and LOW signals are send correctly by the MCU towards the transceier.

As you mentioned above - overall I'm happy with the solution found. Maybe I will invest in a newer device as suggested by you.

Overall thanks a lot for all your answers, the time spend and great support! As well a big thank you to Hao.


Peter

Peter,

Thanks for your further investigation. We will order SN75176B samples for the bench testing.

Hao

Hello Hao,

any results yet from the bench testing?

Best Regards,
Peter

Peter,

I'm sorry for the delay. I just received the samples last week. I plan to update you early next week.

Regards,

Hao

Peter,

Thanks so much for your patience. I tested a few devices with high common mode voltage, but didn't see any issues. Here are the curve trace results for your reference. The curve trace machine feeds the voltage on the pin and records the current as the voltage varies.

In this test, the device is powered off. You can see the leakage current is low until the voltage reaches +/- 20V.

Similarly in this test, the device is powered on but in receiver only mode. The results are similar.

Please let me know if you have further questions about the device. If you want us to test the parts you see abnormal, you can consider the return process through the vendor.

Regards,

Hao

Hello Hao,

thank you so much for your dedication, hard work and contribution to this topic (my problem). The tests performed by you show clearly that I'm doing something wrong on my end (as mentioned in my initial post). Would you mind to share  the schema/wiring/circuit you have used for determining the UL for SN75176B?

BTW: I'm using a digital multimeter for current/voltage measurements. I guess this shouldn't matter/causing the issue I face when the device is powered on and trying to determine the UL paramter of SN75176B.

Best Regards,
Peter

Peter,

You can refer to this app note (page 6-7) about the UL measurement setup.

www.ti.com/.../slla036d.pdf

You can put the multimeter in serial with a voltage source for the measurement.

I found a diagram online and hopefully it's helpful to you.

www.hagerty.com/.../

Regards,

Hao

Hello Hao,

once again a big thank you for all the links, efforts and time spend. Still donno what I'm doing wrong when trying to determine the UL for TI SN75176B whereas it's working like a charm when using

• Linear Technology RS485-Transceiver LTC485CN8
• TI quadruple differential line receiver SN75175

So I think with all the time and efforts spend this topic can be closed.


Best Regards,
Peter