The best way to think of a USB host port on a notebook or PC is like a water tap – power and data flow out (downstream) up to a maximum fixed rate and data can flow back (upstream) when it is not flowing downstream (half-duplex – think stop lights controlling traffic).
- USB 1.0 cables have four wires/pins – two for power and two for data. Power flows downstream only at 5V/500mA and data at 1.5Mbit/s. Suffice to say you should throw away older USB 1.0 chargers and cables.
- USB 2.0 cables also have four wires/pins. Power flows downstream only at 5V/500mA and data at 12Mbit/s (shared downstream and upstream). With USB 3.0 now on most devices, USB 2.0 is well past its use-by date too. Plugging a USB-C device into a USB-A cable only delivers USB 2.0 speeds.
- USB 3.0 (usually a blue coloured port) has 9 wires/pins. Power flows downstream at 5V/900mA and data speeds up to 5Gbps (shared downstream and upstream). Upstream power (from a charger to the device) is usually 5V/1.5 or 3A.
- USB 3.1 Gen 2 (usually a turquoise coloured port) has 9 wires/pins. Power is 5V/1.5A downstream and data speeds to 10Gbps (four data lanes allowing quasi full-duplex downstream and upstream). Upstream power (from a charger to the device) is usually a 5V/3A but can also be 20V/5A.
Attached devices cannot draw more current than is provided so it means an external device like a keyboard or mouse that only needs a trickle will work, and some other devices like USB powered hard disks, SSD, and speakers, etc., may not work without a dedicated power pack.
The effect of pouring more mA into a device is twofold. In some cases, like many power banks, it will simply charge faster. Take a 5V/5000mAh power bank. In theory, a 5000mAh battery needs a total of 5A. If using a 500mA charger (small pipe) that takes 10 hours, a 1A charger is 5 hours, and a 2A is 2.5 hours (at 100% efficiency which is never gained).
In other cases where power is regulated, the excess downstream mA may be cut (the tap is turned down) or occasionally in unregulated environments it overflows and is converted into heat. Genuine smartphone adaptors have regulated power and circuitry to deliver what is needed so heat or explosion is generally not an issue. Non-genuine power supplies can be risky.
So, what started this essay?
I was using a powered USB 2.0 hub (where one port is split into typically four) on a Surface Pro 4 (USB 3.0) and having all kinds of reliability issues with a USB powered external hard disk, a Hauppauge USB digital TV Tuner, keyboard and mouse. In short, I was drawing too much power and the downstream and upstream data transfers were too much for the half-duplex data rates.
A quick call to Laser identified that I needed to throw the old hub away and get a USB 3.0, 4 port, powered hub which provides shared four lanes data of up to 5Gbps and discrete power of up to 900mAh per port from its 3500mA power pack. It also was backwards compatible with USB 1.0 and 2.0 standards so everything worked. It uses a USB-A to USB-B nine-wire cable to connect to a host device.
Laser also explained that most current USB-C hubs/dongles were USB-3.0 standard and that is why they only supported one or perhaps two USB-A 3.0 ports (1.5A downstream almost meets 2 x 900mA USB 3.0 standards) and HDMI requires 4.95 Gbps data bandwidth for 1080p leaving precious little for USB ports. HDMI 2.0 (4K) needs 18Gbps and that is why no USB-C 3.0 (5Gbps) or 3.1 (10Gbps) device can drive a 4K monitor via USB let alone support mixed external devices.
Confused? Well, there are now three types of USB-C 3.1 ports (USB-C is simply a reversible connector) – Gen 1, Gen 2 and Thunderbolt 3
USB 3.1 is entirely different and uses nine wires and introduce three port types
- a downstream/upstream (quasi full-duplex) shared data port (10Gbps) and charge port (1.5A) – this is what you plug peripherals into.
- a dedicated data and charging downstream/upstream port – this is what you connect the external power pack and expansion hub/dongle to. Power may also be pumped at 20V and 5A for 100W needed for later Intel Kaby Lake processors.
- Then you may hear of Thunderbolt 3 – this uses the same cable but can transfer up to 40Gbps but needs Thunderbolt peripheral’s and is rare outside the Apple ecosystem.
Whatever you have in your host port (notebook, PC) is the maximum you will be able to get so if you have 3.0 get a 3.0 hub etc.