> 3.2.3 | Different Usage of AT Commands on USB and ASC0/ASC1

> The following sections discuss AT commands that have different effects if used on the USB interface or the serial interfaces ASC0 and ASC1. For further detail see also [2]. Please note that there is no ASC1 interface available for XT65/XT75 modules.

> 3.2.3.1 | Bit Rate

> As mentioned in Chapter 2, the module supports a bit rate of 12 Mbit/s on the USB. This is the maximum bit rate shared by all USB devices connected to the same application platform. Consequently, if more than one USB device is connected the bit rate may be lower.
> The bit rate settings known from the serial interfaces ASC0/ASC1 are not relevant on the USB. Therefore, all bit rate settings made by the following AT command are ignored on the USB port:
> AT+ILRR (sets TE-TA local rate reporting)
> AT+IPR (sets fixed local rate)
> The same applies to the bit rates selected on the Modem Property pages of the Windows Control Panel. Although the settings are visible and can be changed, none of the selected bit rates is of use for USB.
> Despite that bear in mind that the bit rate selected on the Modem Property pages will be shown in the status display window of the Dial-Up network connections. This may be confusing to the user.
> Figure 5: Bit Rates

> 3.2.3.2 | Modem Control and Status Lines

> The VCOM USB driver (usbser.sys) uses the DTR control line for the signaling between DTE and DCE to indicate that the VCOM port is open and ready to receive data. This is independent of the selected hardware handshake mechanism, but can influence the communication for the used virtual COM port if the signaling state is changed explicitly.
> Besides, the signals RTS, DCD, DSR and RING can be used on the USB port for signaling purposes.
> DTE -> DCE
> DTR: DTR signals that the host is ready to send or receive data on the VCOM port.
> Behavior depends on configuration command AT&D too.
> RTS: RTS signals will only be transmitted if DTR changes too.
> Unlike legacy serial interfaces the DTR and RTS signals cannot be used to start the module.
> DCE -> DTE (Fastest response 128ms after triggering)
> DCD: Signaling depends on configuration command AT&C.
> DSR: Signaling depends on the configuration command AT&S.
> RING: Indicates an incoming call or URC (as configured by AT^SCFG; see [2])
> CTS: This signal is not available inside CDC ACM.

> 3.2.3.3 | Flow Control, Character Framing

> The direct flow control mechanism driven by hardware (RTS/CTS) or software (XON/XOFF) typically used for a legacy COM ports is not used by the USB VCOM port emulation, simply because it is not needed. USB already supports the native flow control via protocol (ACK/NAK technique). Therefore, the settings of the following AT commands are ignored if used on the USB port:
> AT\Qn (sets type of flow control)
> AT+ICF (sets serial interface character framing)
> AT+IFC (sets flow control separately for data directions)
> AT^STPB (transmits Parity Bit (for 7E1 and 7O1 only)

> 3.2.3.4 | USB and I 2C/SPI Usage

> On TC65 and TC63 modules it is possible to operate the I 2C resp. SPI (TC65 only) interfaces in parallel with the USB interface.
> This is not the case for MC75, AC65/AC75 and XT65/XT75 modules however. Here, the USB interface will be disabled as soon as the I2C or - where available - the SPI channel is opened using the AT^SSPI command. To activate the USB interface again after closing the I2C or SPI channel it is necessary to restart the module, either by switching the ignition line or entering the command AT+CFUN=1,1. The terminal program or host application connected with the USB interface must be closed before activating the USB (see Section 3.2.2).