similar module at a substantially reduced cost. Upon receipt however I discovered that instead of being a simple serial USB interface it presented USB HID and the Debian system I was running it on has loaded the hiddev driver for me but it did not implement any of the standard HID Usage Pages leaving me with no way to control the device.
I did the obligatoryThis simply showed me what I already knew and surprised me that lsusb did not dump HID report descriptor items. Some searching revealed that teh device had to be unbound so lussb could access the descriptor.
sudo lsusb -d 12bf:ff03 -vvv
Bus 003 Device 019: ID 12bf:ff03 Device Descriptor: bLength 18 bDescriptorType 1 bcdUSB 1.10 bDeviceClass 0 (Defined at Interface level) bDeviceSubClass 0 bDeviceProtocol 0 bMaxPacketSize0 8 idVendor 0x12bf idProduct 0xff03 bcdDevice 1.00 iManufacturer 1 Matrix Multimedia Ltd. iProduct 2 Flowcode USB HID iSerial 0 bNumConfigurations 1 Configuration Descriptor: bLength 9 bDescriptorType 2 wTotalLength 41 bNumInterfaces 1 bConfigurationValue 1 iConfiguration 0 bmAttributes 0x80 (Bus Powered) MaxPower 50mA Interface Descriptor: bLength 9 bDescriptorType 4 bInterfaceNumber 0 bAlternateSetting 0 bNumEndpoints 2 bInterfaceClass 3 Human Interface Device bInterfaceSubClass 0 No Subclass bInterfaceProtocol 0 None iInterface 0 HID Device Descriptor: bLength 9 bDescriptorType 33 bcdHID 1.10 bCountryCode 0 Not supported bNumDescriptors 1 bDescriptorType 34 Report wDescriptorLength 54 Report Descriptors: ** UNAVAILABLE ** Endpoint Descriptor: bLength 7 bDescriptorType 5 bEndpointAddress 0x81 EP 1 IN bmAttributes 3 Transfer Type Interrupt Synch Type None Usage Type Data wMaxPacketSize 0x0008 1x 8 bytes bInterval 5 Endpoint Descriptor: bLength 7 bDescriptorType 5 bEndpointAddress 0x01 EP 1 OUT bmAttributes 3 Transfer Type Interrupt Synch Type None Usage Type Data wMaxPacketSize 0x0008 1x 8 bytes bInterval 5 Device Status: 0x0980 (Bus Powered)
Thus a simple
echo -n 3-1.1.4:1.0 | sudo dd of=/sys/bus/usb/drivers/usbhid/unbindresulted in lussb dumping the descriptor items:
Item(Global): Usage Page, data= [ 0xa0 0xff ] 65440 (null) Item(Local ): Usage, data= [ 0x01 ] 1 (null) Item(Main ): Collection, data= [ 0x01 ] 1 Application Item(Local ): Usage, data= [ 0x02 ] 2 (null) Item(Main ): Collection, data= [ 0x00 ] 0 Physical Item(Global): Usage Page, data= [ 0xa1 0xff ] 65441 (null) Item(Local ): Usage, data= [ 0x03 ] 3 (null) Item(Local ): Usage, data= [ 0x04 ] 4 (null) Item(Global): Logical Minimum, data= [ 0x00 ] 0 Item(Global): Logical Maximum, data= [ 0xff 0x00 ] 255 Item(Global): Physical Minimum, data= [ 0x00 ] 0 Item(Global): Physical Maximum, data= [ 0xff ] 255 Item(Global): Report Size, data= [ 0x08 ] 8 Item(Global): Report Count, data= [ 0x08 ] 8 Item(Main ): Input, data= [ 0x02 ] 2 Data Variable Absolute No_Wrap Linear Preferred_State No_Null_Position Non_Volatile Bitfield Item(Local ): Usage, data= [ 0x05 ] 5 (null) Item(Local ): Usage, data= [ 0x06 ] 6 (null) Item(Global): Logical Minimum, data= [ 0x00 ] 0 Item(Global): Logical Maximum, data= [ 0xff 0x00 ] 255 Item(Global): Physical Minimum, data= [ 0x00 ] 0 Item(Global): Physical Maximum, data= [ 0xff ] 255 Item(Global): Report Size, data= [ 0x08 ] 8 Item(Global): Report Count, data= [ 0x08 ] 8 Item(Main ): Output, data= [ 0x02 ] 2 Data Variable Absolute No_Wrap Linear Preferred_State No_Null_Position Non_Volatile Bitfield Item(Main ): End Collection, data=none Item(Main ): End Collection, data=none
By consulting the device class definitions document I determined the device was using the "Vendor defined" Usage page (0xff00 to 0xffff) so I would definitely have to write a program to control the device.
Linux provides a really easy interface to deal with HID devices called hiddev (gosh, such adventurous naming) which I already had to unbind to get my descriptors decoded so I am fairly sure it works ;-)
The kernel documentation and header for hiddev provide the absolute basic mechanics of the interface but no example code or guidance. The obligatory web search turned up very little and even that had to be retrieved from the internet archive. So It seems I would be forced to work it through myself.
It seems the hiddev interface is orientated around HID devices generating reports which the program is expected to read. Numerous ioctl() are provided so the program can obtain the descriptor information necessary to control and process the received reports.
However in this case we need to be able to send reports to the device, all the descriptor information revealed was that there were eight (Item Report Count = 8) values with eight bits each (Item Report Size = 8) with logical and physical values representing the whole range of the octets.
Fortunately the seller provided a website with some control programs and even source. After some time rummaging through the Visual Basic program I finally found (in FrmMain.vb:2989) that the eight bytes were largely unused and the first was simply a bitmask of the eight relays coil status, set for energised clear for off. With bit 0 controlling relay labelled 1 through to bit 7 for relay 8.
To send a report to a HID device the hiddev interface uses the HIDIOCSREPORT ioctl where the report data is first set using HIDIOCSUSAGE .
The HIDIOCSUSAGE ioctl is passed a hiddev_usage_ref structure which must be initialised with information about the report descriptor identifier (constructed from the Usage Page and Usage as set by the items in the descriptor), the index of the item (named usage) we wish to set in the report (in this case the first which is 0) and the value we actually want to set.
After a great deal of debugging the final program is very short indeed but does the job, my main problem now is that if I switch too many (more than one) relays at once the whole device resets. The scope says the supply rails are behaving very badly when this happens, looks like I need to add a load of capacitance to the power well to stabilise it during the switching events.
Oh and add in the fact Relay 1 LED doest work unless you push on it and I do wonder about the wisdom of the economy in this case. Though yet again Linux makes the software side easy.