BillBoard origin:
USB-IF Association in 2014 published the Power Delivery v2.0 specifications, but also in the original USB Class defines a new classification Billboard Device Class v1.0.
What is a Billboard?
First we have to look at Type-C:
USB-C that is USB Type-C-type interface, a USB Association's new interface, with slender structure compared to traditional USB interface, USB-C, supports positive and negative plug, high power transmission, support optional mode (DisplayPort / Thunderbolt, etc. ) highlight the characteristics of high-quality output.
Interface form: both sides is the same, this feature is to meet the needs of both positive and negative plug.
需要说明一下：Type-C上所使用的USB-PD协议，与传统USB协议不同，USB-PD协议通常不用于数据传输，只是进行USB-C接口的属性设置(充电，Power属性，Alternate Mode等内容) 。简单来说，USB-C实际上只管接口的连接，type C 为了支援正反插由四组差分信号，Type C只去识别哪一面有USB连接，之后的USB传输它不管。
It is because Type-C highly integrated all the features shown above, a single host device or impossible to use so much power, so Type-C defines a pattern Alternate Mode﹝替代模式)to be selected.
USB-C的Alternate Mode
Alternate Mode is USB-C in addition to the traditional USB data transfer mode additional support - such as image data (DisplayPort) or lightning (Thunderbolt) (need to explain is not to say typeC support equipment will certainly support this feature, this It is an option option). USB Association presented a unique USB standard coding of 16bit (SID) for each standard selectable modes for the two sides to communicate supports this mode between host and device.
For example, host and device support DP (DisplayPort is the Video Electronics Standards Association (VESA) launched a digital video interface standard), then typeC can transmit data and USB data DP. This is because the plug typeC to support positive and negative, there are four high-speed differential signals, USB data signal using two pairs of differential transmission, DP using the other two pairs of transmission interference.
And BillBoard What is the relationship? ? ?
Billboard need to support the device, and its role in host and device communication failure is used to notify the host and user.
When Billboard appear? ? ?
For example:
When the notebook by type C, wants to connect to the display support display interface Type-C Type-C when using the connector, the display device may be displayed properly identify a laptop, it is in this case both Alternate mode (PD transmission) communication, and can display normally.
However, if the display is to support the Alternate mode (PD transmission), while the notebook is not supported, the two sides are connected by type C, host will recognize the display as USB Billboard device.
That is, only when the device is unable to enter the Alternate mode will jump Billboard.
On Device Manager you can see this is BillBoard Device Device tell the host.
Notebook lower-right corner will display the following information, which is used to tell the user.
第一种情况：host 不支持Alternate mode（DP）
第二种情况：双方都支持支持Alternate mode（DP）,但是连接时用C转A的USB去连接。
USB A口的线肯定是无法满足要求的，所以会提示如下信息。
