TSS721A: Optocoupler application

Olcay Korkmaz

Part Number: TSS721A
Other Parts Discussed in Thread: TIDM-FRAM-WATERMETER

Hello all,

I found Water Meter Implementation with MSP430FR4xx application report and there is a TSS721A schematic in the document.

As you can see from schematic, transceiver is used with optocoupler in order to provide isolation.

One side of optocoupler goes common ground and another side of optocoupler goes exgnd. Where is the external ground? Where should we tie to exgnd?

Thanks.

over 8 years ago

0 Shubhendra Singh over 8 years ago

Intellectual 885 points

hi olcay Korkmaz,
EXGND is already tied to GND with a resistor named R18.

0 Ryan Brown1 over 8 years ago

TI__Guru**** 219527 points

Hello Olcay,

The point of the optocouplers is complete electrical separation/isolation between the sensor system and meter-bus application. Please see Figure 10 of the TSS721a datasheet for more information. Therefore in the above diagram, GND refers to the sensor system's ground whereas EXGND applies to the meter-bus solution. R18 may need to be left unpopulated, I am looping in the TIDM-FRAM-WATERMETER author to confirm the intended operation.

Regards,
Ryan

0 Ling Zhu2 over 8 years ago

TI__Genius 9570 points

Hello Olcay,

Ryan is right.

And for detail information, please refer to TSS721A datasheet.

Regards,
Ling

0 Olcay Korkmaz over 8 years ago in reply to Ling Zhu2

Intellectual 505 points

Hello Ryan and Ling,

Thanks for the explanation.

So, R18 should be left unpopulated, right? Can you please confirm whether the below schematic is correct?

0 Ling Zhu2 over 8 years ago in reply to Olcay Korkmaz

TI__Genius 9570 points

Pay attention to the inverted output TXI and the non-inverted output TX,the inverted input RXI or the non-inverted input RX.

0 Ryan Brown1 over 8 years ago in reply to Olcay Korkmaz

TI__Guru**** 219527 points

Hello Olcay,

Connections to U2 from the TSS721A's side do not appear to be aligned with the TIDM-FRAM-WATERMETER or Basic Optocoupler Application design from the datasheet. I also do not see Rload, please review your schematic further with reference to the provided resources.

Regards,
Ryan

0 Olcay Korkmaz over 8 years ago in reply to Ryan Brown1

Intellectual 505 points

Hello Ling and Ryan,

Yes, Rload should be added but can't i use both non-inverted TX-RX pins? I think it can be handled by software.

Thanks.

0 Ryan Brown1 over 8 years ago in reply to Olcay Korkmaz

TI__Guru**** 219527 points

Hello Olcay,

Use the non-inverted pins if necessary for your application. I'm just following the basic design provided with limited knowledge of the TSS721A.

Regards,
Ryan

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TSS721A: Optocoupler application