While USB Type-C (USB-C) ports can enable video, data, and power over a single connection it doesn’t mean all these capabilities have been implemented on the host, device, or peripheral. Because USB Type-C is the name of the connector, and not necessarily the protocols, modes, or capabilities of it in a port configuration, it is not always clear why two USB-C ports can be physically connected (quite easy by the flippable connector) but do not function together. This paper explains why USB 3.0 is the only consistent protocol for connecting the bulk if not all USB Type-C hosts, devices, and peripherals, including docking stations, regardless of the USB-C mode implemented on the port.
There are various protocols that can be implemented to run over the USB-C connector, see https://en.wikipedia.org/wiki/USB-C#Alternate_Mode . At the time of this writing the three that appear most popular on hosts:
1) Thunderbolt 3 (TB3) or Thunderbolt 3 Alternate Mode
2) DisplayPort Alternate Mode (sometimes called DP 1.2a Alt. Mode)
3) USB 3.0 (aka USB 3.1 Gen 1 if USB Power Delivery is included)
Note, “host” refers to the primary display and USB data provider even though it may not be the primary power provider. A USB Type-C host is typically a PC, MacBook or Chromebook. A USB Type-C device is also often referred to as a peripheral or downstream device, however, a Smartphone can typically behave like a host or device in this context.
Thunderbolt 3 USB Type-C
Thunderbolt 3 is synonymous with USB Type-C TB3 Alternate Mode. Thunderbolt 3 (TB3) is not a video protocol but a 40Gbps data bus running over a Type-C connection. Thunderbolt 3 can supply power up to 100W but most practical implementation are currently at about 60W. Thunderbolt 3 is universal as a host connection, but not as a device connection because of its multiple modes of operations, similar too, but not to be confused with the USB Type-C Alternate Modes, even though it may provide host power.
TB3-USB Only Mode
If a USB device, often called a “USB Type-C” device is plugged in, a USB host controller inside the Thunderbolt 3 enabled system is activated, and the Thunderbolt 3 silicon PHY drives USB (2.0, 3.0, or 3.1) signals to the USB-C port. In this mode, a Thunderbolt 3 port behaves exactly like a typical USB-C 3.1 enabled connector or USB 3.0 connection depending on the device USB capabilities. USB does not contain video. In this mode a Targus DisplayLink USB based dock, like the DOCK160 is needed for video support.
TB3-DisplayPort Only Mode
If a DisplayPort display or adapter, or DisplayPort Alternate Mode dock like the Targus DOCK410 is plugged in, the Thunderbolt 3 enabled system will detect this, and switch the pins driving the USB-C connector to the DisplayPort alternate mode. Thunderbolt 3 silicon will then act as a router to send raw DisplayPort traffic from the graphics engine within the system out over the USB-C connector pins and pass that DisplayPort link directly to the display or adapter.
In this mode, a Thunderbolt 3 enabled USB-C port will support a single four lane (4 x 5.4 Gbps, or HBR2) link of DisplayPort. These four links run across the two pairs of high speed wires in the USB-C connector and cable. This kind of DisplayPort link can support a single, uncompressed display, at 4K resolution at 60 Hz.
TB3-DisplayPort and USB Mulit-Function Mode
Often referred to as USB Type-C DisplayPort Alt. Mode. In this alternate mode of operation, one of the high-speed connector pin pairs of signals will be dedicated to DisplayPort (now 2 lanes at 5.4 Gbps) and one to USB 3.1 This allows for a basic connectivity for data and display devices such as the Targus DOCK410. With two lanes of DisplayPort 1.2a resolutions to a single 4K2K at 30Hz are possible.
TB3-Thunderbolt 3 Mode
If a cable and device supporting Thunderbolt are plugged in, the Thunderbolt silicon activates its highest capability mode and configures four high-speed links at either 10 Gbps or 20 Gbps (depending on cable and device support) to support the Thunderbolt transport. This provides bidirectional data rates of 20 or 40 Gbps. See https://targusblog.com/2017/03/17/thunderbolt-3-usb-type-c/ for details about how this host Type-C port operates depends on the device being connected.
While it can be attractive to realize a 40 Gbps link and have power to 100W at the time of this writing TB3 docking stations were not quite viable. Implementations of TB3 relative to USB Type C DP Alt. Mode and USB 3.1 Gen 1 consume more cost and power. TB3 also requires pricy electrically marked active cables for 40Gbps operation at any practical link.
Thunderbolt 3 is universal as a Type-C host because it can operate with Thunderbolt 3, DisplayPort Alternate Mode, and USB Type-C devices. It is not a universal device connection because it does not operate with legacy USB only hosts.
DisplayPort Alternate Mode USB Type-C
Display Port Alternate Mode or just DP Alt. Mode is compatible with Thunderbolt 3. DP Alt. Mode can supply power up to 100W but most practical implementation are currently at about 60W. DP Alt. Mode supports two (not four) High Bit Rate (HBR2) lanes of a single DisplayPort 1.2a video stream, enough to support dual 1080P or a single 4K2K 30Hz monitor. The Targus DOCK410 DP Alt. Mode dock also supports an additional two lanes for a pair of USB3.0 hubs that in turn support downstream GbE, audio, and additional USB 3.0 ports.
The DOCK410 also supports DP1.2a Multi-Stream Transport (MST) enabling video daisy chaining from a single DP port, see https://targusblog.com/2016/12/20/optimizing-your-targus-dock410-experience/ . MST is not adding pixels to the stream, but a way of sharing the bandwidth of a DP1.2a stream. Therefore, the graphics capabilities of a DisplayPort Alt. Mode dock like the DOCK410 are no better than what can be supported with two lanes of HBR2 data, about 10.8 Gbps raw data rate, see https://en.wikipedia.org/wiki/DisplayPort#1.2 . Also, not all host graphics support DP 1.2 MST, see https://targusblog.com/2016/11/21/which-displayport-specifications-do-targus-docks-support/ where it is noted that the newer 2016 MacBooks do not.
Typical docking implementations like the Targus DOCK410 are relatively inexpensive when compared to TB3 docks because they can utilize inexpensive full featured passive USB3.1 Type C cables.
DP Alt. Mode requires connection to USB Type-C Thunderbolt 3 or USB Type-C DP Alt. Mode host to provide graphics and USB 3. The DOCK410 will not provide graphics or USB 3 when connected to a USB Type-C port implemented with USB 3 only. Because of this, DP Alt. Mode, like TB3 albeit at lower performance and price, is universal as a Type-C host because it can operate with Thunderbolt 3, DisplayPort Alternate Mode, and USB Type-C devices. But, also like TB3, it is not a universal device connection because it does not operate with legacy USB only hosts, even though it may provide USB Type-C host power.
USB 3 Type-C
USB 3.0 does not contain graphics capability natively, however it can carry graphics when used with a technology like DisplayLink.
Current state-of-the art for USB 3 docking utilizes DisplayLink’s DL6900 chipset, to enable dual Cinema 4K (4096 x 2160 at 60Hz with 24 bpp color) monitor output over the 5 Gbps USB 3.0 link . The DisplayLink adaptive compression algorithm software application running on the host platform (PC or MacBook) uses the CPU to compress the host graphics for use over the USB 3 data buss. This can take CPU cycles but usually not enough to notice as most graphics are memory bound and not CPU bound.
Typical docking implementations are relatively inexpensive when compared to TB3 or DisplayPort Alternate mode docks because they can utilize simple USB Type-A to USB Type-C conversion cables for data applications. All of Targus’ USB 3.0 docks can be connected to USB Type-C using the Targus ACC923USX adaptor or replacing the in-box cable with either the ACC924USX or ACC925USX as appropriate. However, by itself USB 3.0 does not have power and charging capabilities for USB Type-C hosts.
USB 3.1 Type-C often incompletely referred to as simply USB Type-C can also supply power up to 100W but most practical implementation are currently at about 60W. To have a dock that will connect to most, if not all, USB Type-C ports and supply host, device and peripheral power, support video, and high speed data consider the Targus ACP71 or ACP77 dock with the ACA41 USB Type-C multiplexer. This configuration can also be used to support legacy PC power using Targus’ patented tip system.
Using an ACA41 with the Targus ACP77 supports:
1) Dual DisplayPort audio/video up to 2K (2560×1600 at 60 Hz)
2) USB Type-C Power Delivery for Type-C Host power (up to 60W) and legacy PC power to 90W.
3) USB3.0 and BC1.2 device power
4) Gigabit Ethernet
5) Stereo audio In/Out
Targus has implemented the DL6900 in both the DOCK160 and DOCK177. The DOCK177, and its sister product the DOCK171, can also provide power to legacy 19VDC barrel type hosts up to 90W. The DOCK171 and DOCK177 can also support USB Type-C power and charging and data by combining its power and USB 3.0 data outputs using the forthcoming Targus ACA44 to create a 60W USB Type-C 3.1 Gen 1 device connection.
USB is the most universal Type-C host data connection because a USB data connection is available on USB Type-C in any implementation albeit Thunderbolt 3, DP Alt. Mode, USB 3.1 Gen 1 or 2, and other Alt. Modes. For this same reason it is the best device the most universal device connection because can even operate with legacy USB only hosts. It can also be implemented to supply USB Type-C power in device mode.
As shown in the chart below, the bulk of the enterprise/productivity applications are best provided for by in expensive Targus USB docks, even on USB Type-C.
Any Targus DisplayLink based USB 3.0 dock will work with any USB port, be it USB Type-A, micro-B or the new USB Type-C with the DisplayLink software installed.
Today, the Targus ACP71/77 with the ACA41 is the only way to make sure your USB Type-C host will be supported for at least video and most often power too. Soon the DOCK177 with the ACA44 will provide these same capabilities with better then TB3 video capability.
Categories: White Paper