USB cables

I have just finished designing a board that uses the FTDI quad serial port USB chip. This board is designed to be mounted inside a PC case, so doesn't have a standard USB connector on it and instead has a 4 pin socket that accepts a crimp header style connector. The board worked fine when connected to my Windows PC, but when I installed it inside a PC running Linux it had problems. Everything seemed fine until I started using the USB serial ports - they would work fine at first, but then report errors and eventually the USB device would be disconnected and the 4 serial ports (/dev/ttyUSB0 to 3) would disappear.
The sort of errors appearing in /var/log/syslog were:

Nov 3 13:49:23 minibox2 kernel: [ 1787.820556] usb 5-8: USB disconnect, address 5
Nov 3 13:49:23 minibox2 kernel: [ 1787.820556] ftdi_sio ttyUSB0: FTDI USB Serial Device converter now disconnected from ttyUSB0
Nov 3 13:49:23 minibox2 kernel: [ 1787.820556] ftdi_sio 5-8:1.0: device disconnected
Nov 3 13:49:23 minibox2 kernel: [ 1787.820556] ftdi_sio 5-8:1.1: device disconnected
Nov 3 13:49:23 minibox2 kernel: [ 1787.820556] ftdi_sio 5-8:1.2: device disconnected
Nov 3 13:49:23 minibox2 kernel: [ 1787.820556] ftdi_sio ttyUSB3: FTDI USB Serial Device converter now disconnected from ttyUSB3
Nov 3 13:49:23 minibox2 kernel: [ 1787.820556] ftdi_sio 5-8:1.3: device disconnected

and

Nov 3 15:37:42 minibox2 kernel: [ 132.047082] hub 5-0:1.0: port 8 disabled by hub (EMI?), re-enabling...
Nov 3 15:37:42 minibox2 kernel: [ 132.047082] usb 5-8: USB disconnect, address 5
Nov 3 15:37:42 minibox2 kernel: [ 132.051081] ftdi_sio ttyUSB0: FTDI USB Serial Device converter now disconnected from ttyUSB0
Nov 3 15:37:42 minibox2 kernel: [ 132.051081] ftdi_sio 5-8:1.0: device disconnected
Nov 3 15:37:42 minibox2 kernel: [ 132.051081] ftdi_sio 5-8:1.1: device disconnected
Nov 3 15:37:42 minibox2 kernel: [ 132.051081] ftdi_sio 5-8:1.2: device disconnected
Nov 3 15:37:42 minibox2 kernel: [ 132.051081] ftdi_sio ttyUSB3: FTDI USB Serial Device converter now disconnected from ttyUSB3
Nov 3 15:37:42 minibox2 kernel: [ 132.051081] ftdi_sio 5-8:1.3: device disconnected
Nov 3 15:37:42 minibox2 kernel: [ 132.293491] ehci_hcd 0000:00:1d.7: port 8 reset error -110
Nov 3 15:37:42 minibox2 kernel: [ 132.293491] hub 5-0:1.0: hub_port_status failed (err = -32)
Nov 3 15:37:42 minibox2 kernel: [ 132.656542] hub 5-0:1.0: unable to enumerate USB device on port 8
Nov 3 15:38:02 minibox2 kernel: [ 165.111640] ftdi_sio: error from flowcontrol urb
Nov 3 15:38:02 minibox2 kernel: [ 165.111658] ftdi_sio: update_mctrl Error from MODEM_CTRL urb: DTR LOW, RTS LOW
Nov 3 15:38:02 minibox2 kernel: [ 165.111834] ftdi_sio ttyUSB1: FTDI USB Serial Device converter now disconnected from ttyUSB1
Nov 3 15:38:28 minibox2 kernel: [ 203.066197] ftdi_sio ttyUSB2: FTDI USB Serial Device converter now disconnected from ttyUSB2

I first suspected the driver (I didn't spot the "port 8 disabled by hub (EMI?), re-enabling..." messages at first) and tried rebuilding it with no luck. Eventually I realised that it was the cable that I had connecting the USB on my board to the USB header on the PC motherboard. I had made up a cable just using 4 pieces of hookup wire - not such a good idea with the speed of USB - so I suspect that the USB connection to my board was getting excessive noise and errors. All I had to do to fix the problem initially was to twist the D+ and D- wires together - that was enough to improve it and get it working! I recommend, however, using a shielded cable (as recommended by the USB 2.0 standard).