Is there an easy way to get low-level data from a Mac USB serial port, from devices that do not show up in the /dev/ directory?

Question

What I want to achieve is that I plug in a device, for example a mouse, but in general any device that outputs serial data via USB, and then either screen it in terminal, or store it in a file and, if possible, use python to do things with it.

As a concrete example, let's say I want access to the data from a trackpad plugged into a USB port on a macbook. When I run /dev/{tty,cu}.* in terminal, I get a bunch of devices, but none of them is the one I want. When I run ioreg -p IOUSB -l -b | grep -E "@|PortNum|USB Serial Number" ioreg -p IOUSB -l -b | grep -E "@|PortNum|USB Serial Number" , I see that the device is plugged in, at a specific address (ie @14200000 ), as well as <class AppleUSBDevice, id 0x100002c38, registered, matched, active, busy 0 (10 ms), retain 14> and "PortNum" = 2 .

Yes, the device is plugged in, no it does not show up in /dev/{tty,cu}.* . I don't want to have to write a device driver, I have heard that it is not the easiest thing to do. Are there any solutions for accessing low-level data from devices that don't show up in the /dev/ directory?

Answer 1

Devices showing up as /dev/tty... and /dev/cu... are devices implementing the asynchronous serial data communication protocol derived from RS-232. There used to be dedicated serial ports on computers. Today on a Mac, they are usually connected via USB.

Few USB devices implement the serial protocol. Instead they implement the HID protocol (keyboard, mouse etc.), USB mass storage protocol (external drives of all kind), USB ECM (for network adapters) or a custom protocol. Except for the ones with a custom protocol, macOS provides device drivers for all of them.

USB isn't a simple serial communication. Instead, devices configure several endpoints. Each endpoint uses a separate communication channel. Even USB CDC ACM for RS-232 like communication uses three endpoints. In addition, USB devices can pretend to be several devices at once. This is call composite device.

My understanding is that you want to observe and record USB communication. The main approaches are:

1. Use Wireshark. It is well known for observing and recording network traffic but it can do the same for USB. Unfortunately, with the ever tighter security on macOS, it isn't so easy to set up and probably doesn't work on Apple Silicon Macs anymore.

2. Use a dedicated USB protocol analyzer. They are very powerful and start at about USD 300.