# SIM7600 SMS USB ok but TxRx not ok

**URL:** <https://community.sparkfun.com/t/sim7600-sms-usb-ok-but-txrx-not-ok/66804>\
**Category:** Miscellaneous\
**Created:** [September 4, 2025, 2:48am UTC](https://community.sparkfun.com/t/sim7600-sms-usb-ok-but-txrx-not-ok/66804 "2025-09-04T02:48:38Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![Oily](https://avatars.discourse-cdn.com/v4/letter/o/e56c9b/32.png) [@Oily](https://community.sparkfun.com/u/Oily)\
**Post date:** [September 4, 2025, 2:48am UTC](https://community.sparkfun.com/t/sim7600-sms-usb-ok-but-txrx-not-ok/66804/1 "2025-09-04T02:48:38Z")

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I can send SMS using a terminal on the USB connector but the Tx and Rx pins don’t respond to AT codes.

I can see data on the input with a CRO but there is no Ok response to AT.

Sending SMS without needing responses to codes doesn’t send SMS.

Is something inhibiting operation?

The red LED is on and the green LED is blinking every few seconds so that seems ok.

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**Author:** ![dilshana](https://avatars.discourse-cdn.com/v4/letter/d/f4b2a3/32.png) [@dilshana](https://community.sparkfun.com/u/dilshana)\
**Post date:** [September 4, 2025, 5:38pm UTC](https://community.sparkfun.com/t/sim7600-sms-usb-ok-but-txrx-not-ok/66804/2 "2025-09-04T17:38:31Z")

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What power supply are you using?

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**Author:** ![Oily](https://avatars.discourse-cdn.com/v4/letter/o/e56c9b/32.png) [@Oily](https://community.sparkfun.com/u/Oily)\
**Post date:** [September 5, 2025, 5:12am UTC](https://community.sparkfun.com/t/sim7600-sms-usb-ok-but-txrx-not-ok/66804/3 "2025-09-05T05:12:09Z")

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Thanks for your reply.

5V on the Nano and 5V on the SIM.

I have found a fault, dead nano data in.

Also replaced the SIM.

I now have replies to repeated AT but only when the SIM green LED is on. For about 2 sec every 10 sec or so.

This Sim is a different brand and only responds with data in at 5V level not 3V. Perhaps it uses 5 V logic level, previous brands had a note saying they used 3V data. (But 5V supply)

I’m sure I can get it all working from here, thanks.

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<div class="post-metadata">

**Author:** ![dilshana](https://avatars.discourse-cdn.com/v4/letter/d/f4b2a3/32.png) [@dilshana](https://community.sparkfun.com/u/dilshana)\
**Post date:** [September 8, 2025, 3:06pm UTC](https://community.sparkfun.com/t/sim7600-sms-usb-ok-but-txrx-not-ok/66804/4 "2025-09-08T15:06:37Z")

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Is it working properly now?

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<div class="post-metadata">

**Author:** ![Oily](https://avatars.discourse-cdn.com/v4/letter/o/e56c9b/32.png) [@Oily](https://community.sparkfun.com/u/Oily)\
**Post date:** [September 8, 2025, 8:32pm UTC](https://community.sparkfun.com/t/sim7600-sms-usb-ok-but-txrx-not-ok/66804/5 "2025-09-08T20:32:38Z")

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Yes, thanks.

Do you know why the SMS is only available for about 2 seconds about every 10 seconds? It is when the green LED is on.

Not a problem, just curious.