Hi All. We have a PC-in-a-box unit, with 2 COM-ports, 2 USB 2.0 ports and 1 USB 3.0 port. We are testing the COM-ports (COM1 and COM2) and the USB-ports. We proceeded as follows :: a) In the setup User-App <=> COM1 <=> RS232-data <=> RS232-to-RS485 converter <=> RS485-data <=> Modbus-Device when we send a modbus-command from user-app ==> modbus-device, we receive the modbus-response fine. (Of course, RS232 is enabled for COM1 in BIOS). b) If we modify the setup just a bit as User-App <=> USB-Port <=> USB-Serial-to-RS232 converter <=> Rs232-data <=> RS232-RS485 converter <=> RS485-data <=> Modbus-Device and then send a modbus-command from user-app ==> modbus-device, we DO NOT receive even a single byte as response. Very surprisingly, if we use the above setup in a USB-port of my personal laptop, we get the response fine (thereby signifying that that all elements from USB-Serial-to-RS232 converter <=> Rs232-data <=> RS232-RS485 converter <=> RS485-data <=> Modbus-Device are fine). Some more facts :: 1) uname -a gives identical output for my-laptop and the pc-in-a-box unit :: uname -a Linux blah-blah-login 3.16.0-30-generic #40~14.04.1-Ubuntu SMP Thu Jan 15 17:45:15 UTC 2015 i686 i686 i686 GNU/Linux 2) On the pc-in-a-box-unit, following are some diagnostics that I could think of :: lsmod | grep usb usbnet 37829 1 qmi_wwan mii 13654 1 usbnet usb_wwan 19306 1 qcserial usbhid 47035 0 hid 95946 3 i2c_hid,hid_generic,usbhid usbserial 38972 8 qcserial,pl2303,usb_wwan lsmod | grep pl2303 pl2303 18516 0 usbserial 38972 8 qcserial,pl2303,usb_wwan 3) The USB-Serial-to-RS232 converter information :: lsusb Bus 002 Device 001: ID 1d6b:0003 Linux Foundation 3.0 root hub Bus 001 Device 007: ID 1199:68a2 Sierra Wireless, Inc. Bus 001 Device 005: ID 05e3:0608 Genesys Logic, Inc. USB-2.0 4-Port HUB Bus 001 Device 004: ID 413c:2107 Dell Computer Corp. Bus 001 Device 003: ID 046d:c05a Logitech, Inc. M90/M100 Optical Mouse *Bus 001 Device 002: ID 067b:2303 Prolific Technology, Inc. PL2303 Serial Port* Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub 4) The checksums of the following drivers are identical on the pc-in-a-box unit and my-laptop :: cksum /lib/modules/3.16.0-30-generic/kernel/drivers/usb/serial/usbserial.ko 425603799 59904 /lib/modules/3.16.0-30-generic/kernel/drivers/usb/serial/usbserial.ko cksum /lib/modules/3.16.0-30-generic/kernel/drivers/usb/serial/pl2303.ko 1987618250 25036 /lib/modules/3.16.0-30-generic/kernel/drivers/usb/serial/pl2303.ko Behaviour remains same if I disable all USB 3.0 support in pc-in-a-box unit via :: USB3.0 Support [Disable] XHCI Hand-Off [Disable] EHCI Hand-Off [Enabled] XHCI Mode [Disable] USB 2.0(EHCI) Support [Enabled] *So, now my question is that given the OS is identical and all "OS-drivers" same, what could be the difference in behaviour on pc-in-a-box and my-laptop? Are some usb-drivers present at hardware/motherboard level too, thus making the "difference in hardware" the root-cause (rather than "difference in software")?* I have witheld pc-in-a-box unit details, kindly ask me if specifying the brand would make any difference. Will be thankful for any architectural pointers. Thanks and Regards, Ajay -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html