If you only have one device connected to your Linux box, your controller may well always be assigned the same port when the system is rebooted. In the real world, multiple connected devices make this unlikely. We use udev rules to create symlinks used within the OpenHAB configuration. The udev rule essentially uses attributes of the device to identify the real port and creates a statically named symlink to that port on boot. So the first thing you need to identify is something unique about the device. We have several video capture cards and a Z-Wave/ZigBee combo controller attached to our server. If you do not have udevinfo, you can use lsusb to find details about the device. First list the devices, then identify the proper one and use the -d switch with the ????:???? formatted ID number. The -v switch outputs verbose information. Find a unique attribute or set of attributes that create a unique identifier. In this case, the interface name is unique and we can stop there.

[lisa@server ~]#  lsusb
Bus 001 Device 004: ID 0bda:0111 Realtek Semiconductor Corp. RTS5111 Card Reader Controller
Bus 001 Device 003: ID 1058:1230 Western Digital Technologies, Inc. My Book (WDBFJK0030HBK)
Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 002 Device 002: ID 10c4:8a2a Cygnal Integrated Products, Inc.
Bus 002 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
[lisa@server ~]# lsusb -d 10c4:8a2a -v
Bus 002 Device 002: ID 10c4:8a2a Cygnal Integrated Products, Inc.
Device Descriptor:
 bLength 18
 bDescriptorType 1
 bcdUSB 2.00
 bDeviceClass 0
 bDeviceSubClass 0
 bDeviceProtocol 0
 bMaxPacketSize0 64
 idVendor 0x10c4 Cygnal Integrated Products, Inc.
 idProduct 0x8a2a
 bcdDevice 1.00
 iManufacturer 1 Silicon Labs
 iProduct 2 HubZ Smart Home Controller
 iSerial 5 90F0016B
 bNumConfigurations 1
 Configuration Descriptor:
 bLength 9
 bDescriptorType 2
 wTotalLength 55
 bNumInterfaces 2
 bConfigurationValue 1
 iConfiguration 0
 bmAttributes 0x80
 (Bus Powered)
 MaxPower 100mA
 Interface Descriptor:
 bLength 9
 bDescriptorType 4
 bInterfaceNumber 0
 bAlternateSetting 0
 bNumEndpoints 2
 bInterfaceClass 255 Vendor Specific Class
 bInterfaceSubClass 0
 bInterfaceProtocol 0
 iInterface 3 HubZ Z-Wave Com Port
 Endpoint Descriptor:
 bLength 7
 bDescriptorType 5
 bEndpointAddress 0x81 EP 1 IN
 bmAttributes 2
 Transfer Type Bulk
 Synch Type None
 Usage Type Data
 wMaxPacketSize 0x0040 1x 64 bytes
 bInterval 0
 Endpoint Descriptor:
 bLength 7
 bDescriptorType 5
 bEndpointAddress 0x01 EP 1 OUT
 bmAttributes 2
 Transfer Type Bulk
 Synch Type None
 Usage Type Data
 wMaxPacketSize 0x0040 1x 64 bytes
 bInterval 0
 Interface Descriptor:
 bLength 9
 bDescriptorType 4
 bInterfaceNumber 1
 bAlternateSetting 0
 bNumEndpoints 2
 bInterfaceClass 255 Vendor Specific Class
 bInterfaceSubClass 0
 bInterfaceProtocol 0
 iInterface 4 HubZ ZigBee Com Port
 Endpoint Descriptor:
 bLength 7
 bDescriptorType 5
 bEndpointAddress 0x82 EP 2 IN
 bmAttributes 2
 Transfer Type Bulk
 Synch Type None
 Usage Type Data
 wMaxPacketSize 0x0020 1x 32 bytes
 bInterval 0
 Endpoint Descriptor:
 bLength 7
 bDescriptorType 5
 bEndpointAddress 0x02 EP 2 OUT
 bmAttributes 2
 Transfer Type Bulk
 Synch Type None
 Usage Type Data
 wMaxPacketSize 0x0020 1x 32 bytes
 bInterval 0

We then need to create a file under /etc/udev/rules.d. The file name begins with a number that is used for a load order – I generally number my custom files 99 to avoid interfering with system operations. The bit in “ATTRS” is the attribute name and value to match. The KERNEL section contains the search domain (i.e. look at all of the ttyUSB### devices and find ones where the interface is this). The symlink bit is the name you want to use (more on this later). Set the group and mode to ensure OpenHAB is able to use the symlink.

[lisa@server rules.d]# cat 99-server.rules
KERNEL=="ttyUSB[0-9]*", ATTRS{interface}=="HubZ Z-Wave Com Port", SYMLINK+="ttyUSB-5", GROUP="dialout", MODE="0666"
KERNEL=="ttyUSB[0-9]*", ATTRS{interface}=="HubZ ZigBee Com Port", SYMLINK+="ttyUSB-55", GROUP="dialout", MODE="0666"

The symlink name can be anything – when I created udev rules for our video capture cards, I named them something immediately obvious: video-hauppauge250 is the Hauppauge 250 card. Tried to do the same thing here, naming the ports controller-zigbee; while the symlink appeared and had the expected ownership and permissions … OpenHAB couldn’t use it.

Turns out there’s a nuance to Java RxTx where non-standard port names need to be accommodated in the java options. So I could have added -Dgnu.io.rxtxSerialPorts=/dev/controller-zigbee and -Dgnu.io.rxtxSerialPorts=/dev/controller-zwave to the OpenHAB startup and been OK (in theory), it was far easier to name the symlinks using the Linux standard conventions. Hence I have symlinks named ttyUSBsomethingsomethingsomething.