# Open UART Interface on UART3 (USB)

**URL:** <https://forum.sierrawireless.com/t/open-uart-interface-on-uart3-usb/4318>\
**Category:** Open AT\
**Created:** [March 29, 2010, 1:53pm UTC](https://forum.sierrawireless.com/t/open-uart-interface-on-uart3-usb/4318 "2010-03-29T13:53:59Z")\
**Posts on this page:** 2\
**Page:** 1

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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:** [March 29, 2010, 1:53pm UTC](https://forum.sierrawireless.com/t/open-uart-interface-on-uart3-usb/4318/1 "2010-03-29T13:53:59Z")

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“Devices Services” or “raw” UART access:

> [@ADL User Guide forOpen AT® OS v6.21](#):
>
> identity:  
> This field is mandatory. It allows the service user to choose the UART  
> service provider it wants to work with. Setting this field to NULL generates  
> an error. Any non NULL value is considered by the UART service provider as  
> a pointer of a NULL terminated string. Supported string contents are listed  
> below:  
> • “UART1”: to get access to the Wireless CPU® UART1  
> • “UART2”: to get access to the Wireless CPU® UART2  
> • “UART3”: to get access to the Wireless CPU® USB serial port

So I should be able to use the USB (UART3) - yes?

> [@Sample - UART raw access - HTML file](#):
>
> UART Identity (to be supplied when Operation is set to 1)  
> UART1 : UART1 ;  
> UART1 : UART2 ;  
> UART3 : USB CDC.

So, again, I should be able to use the USB (UART3) - yes?

But, when I try it in that sample, I just get:

> [@](#):
>
> AT+WDU=1,“UART3”,115200
> 
> ERROR

😠

The return value from adl\_OpenDevice success is -1 - which is uncodumented 😠

In the trace, I see:

> [@](#):
>
> 18870.140 Trace DEV 1 usb\_cdc\_\_open wrong parameters

Which is typically unhelpful - as it gives no indication as to which parameter is “wrong”, nor how it is “wrong”! 😠

So, does anyone know the “trick” to getting this to work??

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<div class="post-metadata">

**Author:** ![awneil](https://avatars.discourse-cdn.com/v4/letter/a/49beb7/32.png) [@awneil](https://forum.sierrawireless.com/u/awneil)\
**Post date:** [April 1, 2010, 10:30am UTC](https://forum.sierrawireless.com/t/open-uart-interface-on-uart3-usb/4318/2 "2010-04-01T10:30:24Z")

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> [@awneil](#):
>
> does anyone know the “trick” to getting this to work??

The trick is to fix Wavecom’s sample so that it works properly!

The ADL User Guide states that UART3 (USB) supports the “DCE” role only; but the sample application is hard-coded to use only the “Null-Modem” role - therefore the sample cannot possibly ever work with UART3 (USB) ❗😠