A type-A* USB connection is not logically symmetrical: one end is connected to a 'host' and the other end to a 'device'. For example, the host can be a computer and a device might be a keyboard.
When a USB cable is plugged in to host and device, it is the device that initiates communication. It sends a packet of information to the host saying what sort of device it is - for example a keyboard - and the host remembers this in order to send any data from that device to the (software) keyboard driver, and in appropriate cases to send data from that driver to the device.
The protocol was originally aimed at devices which didn't have their own power supply, and the host USB socket and cable provide a 5 volt power supply for the use of the device.
So, as designed, an unpowered device is powered up when both ends of the cable are plugged in and it receives its 5V. The device identifies itself to the host, negotiates the highest speed each end can handle, and communication starts.
This also works if the cable is connected at both ends before the host is powered up.
So in our situation, if the sound desk - the device - is powered up first, it supplies power to its USB socket, which wakes up and sends its initial message to the computer and waits indefinitely for a response. If the computer is then powered up it will provide the 5V supply, which the sound desk ignores as it has its own power, and the computer will then wait for the sound desk to identify itself, which it never does because it has already sent the information while the computer was turned off.
On the other hand, if the computer is powered up first, it will raise the 5V and wait for the sound desk to communicate. Then when the sound desk is powered up, its USB interface will send its identifying message, which the computer is primed to act on, and all is well.
Why is this only a recent problem? Until recently, it was believed that the computer had to be left on all the time, as instructed by a label which is still there. There was a belief that the computer was driving the screens by the front door of the church, but if that was ever the case, it isn't now. So we are now in the state where the sound desk gets turned on when the band arrives to practice, and the computer is turned on later - and that causes the problem.
So the sequence has to be:
- Warn the band, if they are there, that the sound will be off for a minute or two
- Turn off the amplifiers (turn switch anti-clockwise 90 degrees and check lights on the amps are off)
- Turn off the sound desk (power switch on rear at right-hand end)
- Start the computer (red button on box behind the screen) and wait for the logon screen
- Turn on the sound desk
- Turn on the amplifiers (turn switch clockwise)
* A type-A USB plug and socket are what we normally just call USB: a small rectangular plug which can only be inserted one way up. There is also a type-B which is a more complicated shape and often used for printers, and type-C which is the newer reversible one which is smaller and has rounded ends. I have not researched those, but I am guessing that type-B has a different power-up protocol that gets round this problem.
