Are USB Type A data lines unidirectional or bidirectional?

Question

I want to isolate the data between my computer USB and my FT232RL (as usb to UART converter) to communicate with my arduino Like this:

^{simulate this circuit – Schematic created using CircuitLab}

What i want to know is, if usb type A (or usb 3) are bidirectonal in order to select the appropriate Digital Isolator. (Some dc isolators are unidirectional and they are cheaper than the bidirectional ones)

I was whinking USB are unidirectional, but then i saw in wikipedia here:

It says That both lines are driven LOW by the host. So, it is bidirectional?

Answer 1

they are bidirectional. The paragraph explicitly states this only happens for 20ms at reset, not "generally". Also, think about it: If they were unidirectional, where would data flow from device to host.

Answer 2

This is a minimal answer, but it's what I know:

USB lines are bidirectional, and there's some analog handshaking that can happen on startup to negotiate power.

So not even a generic bidirectional digital isolator will do. You need something specific to USB, or you need to power the FT232 from your USB line, and isolate its I/O to the rest of the board (which will be simple and straightforward).

Answer 3

The correct isolator is something like the ADuM4160, designed specifically for this task.

The datasheet explains some of the challenges:

... presents particular challenges when isolation is required. USB lines must automatically switch between actively driving D+/D−, receiving data, and allowing external resistors to set the idle state of the bus. The ADuM4160 provides mechanisms for detecting the direction of data flow and control over the state of the output buffers. Data direction is determined on a packet-by-packet basis.