Lenovo ThinkPad Thunderbolt 3 Dock (40ac) Review - How does it hold up in 2024?

Image of Lenovo ThinkPad Thunderbolt 3 Dock (40ac) showing two angles to show all ports.

What is the Lenovo ThinkPad Thunderbolt 3 Dock (40ac)?

The Lenovo ThinkPad Thunderbolt 3 Dock (40ac) is a Thunderbolt 3 dock based on the Intel DSL6540 controller--Intel's 1st generation of Thunderbolt 3 controller. The Lenovo Thunderbolt 3 Dock (40ac) was commonly purchased by large businesses, which has contributed to large supply and low prices on the used market.

Why review a nearly 8 year old docking station?

Docking station technologies have evolved rather slowly over the past 8 years. This means that old docks can still offer decent capabilities while also achieving lower prices on the used market.

Dock Technical Specifications

The Lenovo Thunderbolt 3 Dock (40ac) featuring the following ports:

Thunderbolt 3 upstream (to laptop) port (40 Gbps, 60W power)
1x Thunderbolt 3 downstream port (40 Gbps, 15W power)
2x DisplayPort 1.2 ports (DisplayPort #2 and the HDMI port cannot be used simultaneously)
1x HDMI port
1x VGA port
5x USB 3.0 ports
1x RJ45 Ethernet port (1 GbE)
1x Combo Audio
1x DC power input

The DisplayPort, HDMI, and VGA ports all share 17.28 Gbps of display bandwidth through a Synaptics VMM3320 MST hub. The Thunderbolt 3 downstream port has a separate 17.28 Gbps of dedicated display bandwidth, enabling a total of 34.56 Gbps of display data.

Due to limitations of the Intel DSL6540 controller the Lenovo Thunderbolt 3 Dock (40ac) does not support compression of display data. This limits the supported resolutions and refresh rates, particularly in setups with 3 or 4 monitors.

The USB 3 ports are driven by the Fresco Logic FL1100EX controller, which is limited to 4 Gbps (+1 Gbps for error correction) shared across the USB ports, audio port, and the Ethernet port. By contrast many other Thunderbolt 3 docks can support USB 9.7 Gbps (+0.3 Gbps for error correction).

The Ethernet port is driven by the RealTek RTL8153 controller.

The Combo Audio port is driven by the Conexant CX20985B CODEC.

Monitor Capabilities

Highest display capabilities of Lenovo Thunderbolt 3 Dock (40ac)

The Lenovo Thunderbolt 3 Dock (40ac) supports two separate DisplayPort connections, each with up to 17.28 Gbps of bandwidth.

One DisplayPort connection is shared between the DisplayPort, HDMI, and VGA ports using the Synaptics VMM3320 MST hub. The other DisplayPort connection is available through the Thunderbolt downstream port.

Lenovo does not officially advertise support for quad displays, however theoretically the hardware is fully capable of it. At the time the dock was released in 2017 most laptops were limited to 3 displays, which is likely why Lenovo doesn't advertise support for 4 displays.

Software Bugs

Failures to connect

In my testing approximately a quarter of the time the dock fails to establish a connection until unplugged and plugged back in. On Windows 11 the failures coincide with a "Display connection might be limited" notification and on Linux the failures result in "GET_CABLE_PROPERTY failed (-5)" being displayed in the logs.

This issue might potentially be solved by updating the firmware of the Thunderbolt 3 controller chip, however unfortunately that is the other bug I encountered.

Firmware update failures

Shortly after receiving this dock I attempted to install a firmware update. The updater successfully updated the firmware of the MST hub and the audio CODEC, however it failed to update the Thunderbolt 3 controller with the error message "Tbt3_Pd FW update fail."

Conclusion

Like other Thunderbolt 3/4 and USB4 docks he Lenovo ThinkPad Thunderbolt 3 Dock (40ac) supports a total bandwidth of 40 Gbps with the ability to use up to 34.56 Gbps for display data.

However, many other docks support Display Stream Compression which enables significantly greater display capabilities. Furthermore other docks typically have more bandwidth available for the non-display outputs such as USB and Ethernet. The Lenovo ThinkPad Thunderbolt 3 Dock (40ac) only has 4 Gbps available for non-display purposes while many other docks support 9.7 Gbps or higher.