# required uart pins?

**URL:** <https://forum.sierrawireless.com/t/required-uart-pins/1758>\
**Category:** General\
**Created:** [December 3, 2007, 1:18pm UTC](https://forum.sierrawireless.com/t/required-uart-pins/1758 "2007-12-03T13:18:09Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![ndon](https://avatars.discourse-cdn.com/v4/letter/n/8e7dd6/32.png) [@ndon](https://forum.sierrawireless.com/u/ndon)\
**Post date:** [December 3, 2007, 1:18pm UTC](https://forum.sierrawireless.com/t/required-uart-pins/1758/1 "2007-12-03T13:18:09Z")

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Hi,  
I try to interface a fasttrack supreme GPRS modem via UART: Using a 9-pole connector from a PC everything works well but using only 3 pins (RxD, TxD, …) from a uC I don’t get any data. Is it possible that some kind of flow control is required for interfacing and that communication will only work with the 9-pole connector?

thanks,  
Niko

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**Author:** ![awneil](https://avatars.discourse-cdn.com/v4/letter/a/49beb7/32.png) [@awneil](https://forum.sierrawireless.com/u/awneil)\
**Post date:** [December 3, 2007, 5:34pm UTC](https://forum.sierrawireless.com/t/required-uart-pins/1758/2 "2007-12-03T17:34:33Z")

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> [@ndon](#):
>
> Is it possible that some kind of flow control is required for interfacing

Yes it is - search for the word “mandatory” in the Fastrack Supreme Manual…

> [@](#):
>
> and that communication will only work with the 9-pole connector?

It can be fooled - provided you’re sure that your comms will work without flow-control:

See: [viewtopic.php?f=3&t=419&p=3498&hilit=rts+cts#p3498](http://www.wavecom.com/modules/movie/scenes/forums/viewtopic.php?f=3&t=419&p=3498&hilit=rts+cts#p3498)

But note that you must have hardware flow-control for downloading:  
[viewtopic.php?p=6344#6344](http://www.wavecom.com/modules/movie/scenes/forums/viewtopic.php?p=6344#6344)

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**Author:** ![ndon](https://avatars.discourse-cdn.com/v4/letter/n/8e7dd6/32.png) [@ndon](https://forum.sierrawireless.com/u/ndon)\
**Post date:** [December 11, 2007, 1:34pm UTC](https://forum.sierrawireless.com/t/required-uart-pins/1758/3 "2007-12-11T13:34:13Z")

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I tried to loop back RTS to CTS as described in your link. However, as soon as RTS and CTS are connected all lines have zero potential. Before only inputs had 0V to GND and outputs had +6V or -6V to GND. After connecting all lines (incl. RTS and CTS) have 0V to GND. Do have an explanation for that?