drivers/usb/serial/ftdi_sio.h

   1 /*
   2  * Driver definitions for the FTDI USB Single Port Serial Converter -
   3  * known as FTDI_SIO (Serial Input/Output application of the chipset)
   4  *
   5  * For USB vendor/product IDs (VID/PID), please see ftdi_sio_ids.h
   6  *
   7  *
   8  * The example I have is known as the USC-1000 which is available from
   9  * http://www.dse.co.nz - cat no XH4214 It looks similar to this:
  10  * http://www.dansdata.com/usbser.htm but I can't be sure There are other
  11  * USC-1000s which don't look like my device though so beware!
  12  *
  13  * The device is based on the FTDI FT8U100AX chip. It has a DB25 on one side,
  14  * USB on the other.
  15  *
  16  * Thanx to FTDI (http://www.ftdichip.com) for so kindly providing details
  17  * of the protocol required to talk to the device and ongoing assistence
  18  * during development.
  19  *
  20  * Bill Ryder - bryder@sgi.com formerly of Silicon Graphics, Inc.- wrote the
  21  * FTDI_SIO implementation.
  22  *
  23  */
  24
  25 /* Commands */
  26 #define FTDI_SIO_RESET                  0 /* Reset the port */
  27 #define FTDI_SIO_MODEM_CTRL             1 /* Set the modem control register */
  28 #define FTDI_SIO_SET_FLOW_CTRL          2 /* Set flow control register */
  29 #define FTDI_SIO_SET_BAUD_RATE          3 /* Set baud rate */
  30 #define FTDI_SIO_SET_DATA               4 /* Set the data characteristics of
  31                                              the port */
  32 #define FTDI_SIO_GET_MODEM_STATUS       5 /* Retrieve current value of modem
  33                                              status register */
  34 #define FTDI_SIO_SET_EVENT_CHAR         6 /* Set the event character */
  35 #define FTDI_SIO_SET_ERROR_CHAR         7 /* Set the error character */
  36 #define FTDI_SIO_SET_LATENCY_TIMER      9 /* Set the latency timer */
  37 #define FTDI_SIO_GET_LATENCY_TIMER      10 /* Get the latency timer */
  38
  39 /* Interface indices for FT2232, FT2232H and FT4232H devices */
  40 #define INTERFACE_A             1
  41 #define INTERFACE_B             2
  42 #define INTERFACE_C             3
  43 #define INTERFACE_D             4
  44
  45
  46 /*
  47  *   BmRequestType:  1100 0000b
  48  *   bRequest:       FTDI_E2_READ
  49  *   wValue:         0
  50  *   wIndex:         Address of word to read
  51  *   wLength:        2
  52  *   Data:           Will return a word of data from E2Address
  53  *
  54  */
  55
  56 /* Port Identifier Table */
  57 #define PIT_DEFAULT             0 /* SIOA */
  58 #define PIT_SIOA                1 /* SIOA */
  59 /* The device this driver is tested with one has only one port */
  60 #define PIT_SIOB                2 /* SIOB */
  61 #define PIT_PARALLEL            3 /* Parallel */
  62
  63 /* FTDI_SIO_RESET */
  64 #define FTDI_SIO_RESET_REQUEST FTDI_SIO_RESET
  65 #define FTDI_SIO_RESET_REQUEST_TYPE 0x40
  66 #define FTDI_SIO_RESET_SIO 0
  67 #define FTDI_SIO_RESET_PURGE_RX 1
  68 #define FTDI_SIO_RESET_PURGE_TX 2
  69
  70 /*
  71  * BmRequestType:  0100 0000B
  72  * bRequest:       FTDI_SIO_RESET
  73  * wValue:         Control Value
  74  *                   0 = Reset SIO
  75  *                   1 = Purge RX buffer
  76  *                   2 = Purge TX buffer
  77  * wIndex:         Port
  78  * wLength:        0
  79  * Data:           None
  80  *
  81  * The Reset SIO command has this effect:
  82  *
  83  *    Sets flow control set to 'none'
  84  *    Event char = $0D
  85  *    Event trigger = disabled
  86  *    Purge RX buffer
  87  *    Purge TX buffer
  88  *    Clear DTR
  89  *    Clear RTS
  90  *    baud and data format not reset
  91  *
  92  * The Purge RX and TX buffer commands affect nothing except the buffers
  93  *
  94    */
  95
  96 /* FTDI_SIO_SET_BAUDRATE */
  97 #define FTDI_SIO_SET_BAUDRATE_REQUEST_TYPE 0x40
  98 #define FTDI_SIO_SET_BAUDRATE_REQUEST 3
  99
 100 /*
 101  * BmRequestType:  0100 0000B
 102  * bRequest:       FTDI_SIO_SET_BAUDRATE
 103  * wValue:         BaudDivisor value - see below
 104  * wIndex:         Port
 105  * wLength:        0
 106  * Data:           None
 107  * The BaudDivisor values are calculated as follows:
 108  * - BaseClock is either 12000000 or 48000000 depending on the device.
 109  *   FIXME: I wish I knew how to detect old chips to select proper base clock!
 110  * - BaudDivisor is a fixed point number encoded in a funny way.
 111  *   (--WRONG WAY OF THINKING--)
 112  *   BaudDivisor is a fixed point number encoded with following bit weighs:
 113  *   (-2)(-1)(13..0). It is a radical with a denominator of 4, so values
 114  *   end with 0.0 (00...), 0.25 (10...), 0.5 (01...), and 0.75 (11...).
 115  *   (--THE REALITY--)
 116  *   The both-bits-set has quite different meaning from 0.75 - the chip
 117  *   designers have decided it to mean 0.125 instead of 0.75.
 118  *   This info looked up in FTDI application note "FT8U232 DEVICES \ Data Rates
 119  *   and Flow Control Consideration for USB to RS232".
 120  * - BaudDivisor = (BaseClock / 16) / BaudRate, where the (=) operation should
 121  *   automagically re-encode the resulting value to take fractions into
 122  *   consideration.
 123  * As all values are integers, some bit twiddling is in order:
 124  *   BaudDivisor = (BaseClock / 16 / BaudRate) |
 125  *   (((BaseClock / 2 / BaudRate) & 4) ? 0x4000    // 0.5
 126  *    : ((BaseClock / 2 / BaudRate) & 2) ? 0x8000  // 0.25
 127  *    : ((BaseClock / 2 / BaudRate) & 1) ? 0xc000  // 0.125
 128  *    : 0)
 129  *
 130  * For the FT232BM, a 17th divisor bit was introduced to encode the multiples
 131  * of 0.125 missing from the FT8U232AM.  Bits 16 to 14 are coded as follows
 132  * (the first four codes are the same as for the FT8U232AM, where bit 16 is
 133  * always 0):
 134  *   000 - add .000 to divisor
 135  *   001 - add .500 to divisor
 136  *   010 - add .250 to divisor
 137  *   011 - add .125 to divisor
 138  *   100 - add .375 to divisor
 139  *   101 - add .625 to divisor
 140  *   110 - add .750 to divisor
 141  *   111 - add .875 to divisor
 142  * Bits 15 to 0 of the 17-bit divisor are placed in the urb value.  Bit 16 is
 143  * placed in bit 0 of the urb index.
 144  *
 145  * Note that there are a couple of special cases to support the highest baud
 146  * rates.  If the calculated divisor value is 1, this needs to be replaced with
 147  * 0.  Additionally for the FT232BM, if the calculated divisor value is 0x4001
 148  * (1.5), this needs to be replaced with 0x0001 (1) (but this divisor value is
 149  * not supported by the FT8U232AM).
 150  */
 151
 152 enum ftdi_chip_type {
 153         SIO = 1,
 154         FT8U232AM = 2,
 155         FT232BM = 3,
 156         FT2232C = 4,
 157         FT232RL = 5,
 158         FT2232H = 6,
 159         FT4232H = 7
 160 };
 161
 162 enum ftdi_sio_baudrate {
 163         ftdi_sio_b300 = 0,
 164         ftdi_sio_b600 = 1,
 165         ftdi_sio_b1200 = 2,
 166         ftdi_sio_b2400 = 3,
 167         ftdi_sio_b4800 = 4,
 168         ftdi_sio_b9600 = 5,
 169         ftdi_sio_b19200 = 6,
 170         ftdi_sio_b38400 = 7,
 171         ftdi_sio_b57600 = 8,
 172         ftdi_sio_b115200 = 9
 173 };
 174
 175 /*
 176  * The ftdi_8U232AM_xxMHz_byyy constants have been removed. The encoded divisor
 177  * values are calculated internally.
 178  */
 179 #define FTDI_SIO_SET_DATA_REQUEST       FTDI_SIO_SET_DATA
 180 #define FTDI_SIO_SET_DATA_REQUEST_TYPE  0x40
 181 #define FTDI_SIO_SET_DATA_PARITY_NONE   (0x0 << 8)
 182 #define FTDI_SIO_SET_DATA_PARITY_ODD    (0x1 << 8)
 183 #define FTDI_SIO_SET_DATA_PARITY_EVEN   (0x2 << 8)
 184 #define FTDI_SIO_SET_DATA_PARITY_MARK   (0x3 << 8)
 185 #define FTDI_SIO_SET_DATA_PARITY_SPACE  (0x4 << 8)
 186 #define FTDI_SIO_SET_DATA_STOP_BITS_1   (0x0 << 11)
 187 #define FTDI_SIO_SET_DATA_STOP_BITS_15  (0x1 << 11)
 188 #define FTDI_SIO_SET_DATA_STOP_BITS_2   (0x2 << 11)
 189 #define FTDI_SIO_SET_BREAK              (0x1 << 14)
 190 /* FTDI_SIO_SET_DATA */
 191
 192 /*
 193  * BmRequestType:  0100 0000B
 194  * bRequest:       FTDI_SIO_SET_DATA
 195  * wValue:         Data characteristics (see below)
 196  * wIndex:         Port
 197  * wLength:        0
 198  * Data:           No
 199  *
 200  * Data characteristics
 201  *
 202  *   B0..7   Number of data bits
 203  *   B8..10  Parity
 204  *           0 = None
 205  *           1 = Odd
 206  *           2 = Even
 207  *           3 = Mark
 208  *           4 = Space
 209  *   B11..13 Stop Bits
 210  *           0 = 1
 211  *           1 = 1.5
 212  *           2 = 2
 213  *   B14
 214  *           1 = TX ON (break)
 215  *           0 = TX OFF (normal state)
 216  *   B15 Reserved
 217  *
 218  */
 219
 220
 221
 222 /* FTDI_SIO_MODEM_CTRL */
 223 #define FTDI_SIO_SET_MODEM_CTRL_REQUEST_TYPE 0x40
 224 #define FTDI_SIO_SET_MODEM_CTRL_REQUEST FTDI_SIO_MODEM_CTRL
 225
 226 /*
 227  * BmRequestType:   0100 0000B
 228  * bRequest:        FTDI_SIO_MODEM_CTRL
 229  * wValue:          ControlValue (see below)
 230  * wIndex:          Port
 231  * wLength:         0
 232  * Data:            None
 233  *
 234  * NOTE: If the device is in RTS/CTS flow control, the RTS set by this
 235  * command will be IGNORED without an error being returned
 236  * Also - you can not set DTR and RTS with one control message
 237  */
 238
 239 #define FTDI_SIO_SET_DTR_MASK 0x1
 240 #define FTDI_SIO_SET_DTR_HIGH (1 | (FTDI_SIO_SET_DTR_MASK  << 8))
 241 #define FTDI_SIO_SET_DTR_LOW  (0 | (FTDI_SIO_SET_DTR_MASK  << 8))
 242 #define FTDI_SIO_SET_RTS_MASK 0x2
 243 #define FTDI_SIO_SET_RTS_HIGH (2 | (FTDI_SIO_SET_RTS_MASK << 8))
 244 #define FTDI_SIO_SET_RTS_LOW (0 | (FTDI_SIO_SET_RTS_MASK << 8))
 245
 246 /*
 247  * ControlValue
 248  * B0    DTR state
 249  *          0 = reset
 250  *          1 = set
 251  * B1    RTS state
 252  *          0 = reset
 253  *          1 = set
 254  * B2..7 Reserved
 255  * B8    DTR state enable
 256  *          0 = ignore
 257  *          1 = use DTR state
 258  * B9    RTS state enable
 259  *          0 = ignore
 260  *          1 = use RTS state
 261  * B10..15 Reserved
 262  */
 263
 264 /* FTDI_SIO_SET_FLOW_CTRL */
 265 #define FTDI_SIO_SET_FLOW_CTRL_REQUEST_TYPE 0x40
 266 #define FTDI_SIO_SET_FLOW_CTRL_REQUEST FTDI_SIO_SET_FLOW_CTRL
 267 #define FTDI_SIO_DISABLE_FLOW_CTRL 0x0
 268 #define FTDI_SIO_RTS_CTS_HS (0x1 << 8)
 269 #define FTDI_SIO_DTR_DSR_HS (0x2 << 8)
 270 #define FTDI_SIO_XON_XOFF_HS (0x4 << 8)
 271 /*
 272  *   BmRequestType:  0100 0000b
 273  *   bRequest:       FTDI_SIO_SET_FLOW_CTRL
 274  *   wValue:         Xoff/Xon
 275  *   wIndex:         Protocol/Port - hIndex is protocol / lIndex is port
 276  *   wLength:        0
 277  *   Data:           None
 278  *
 279  * hIndex protocol is:
 280  *   B0 Output handshaking using RTS/CTS
 281  *       0 = disabled
 282  *       1 = enabled
 283  *   B1 Output handshaking using DTR/DSR
 284  *       0 = disabled
 285  *       1 = enabled
 286  *   B2 Xon/Xoff handshaking
 287  *       0 = disabled
 288  *       1 = enabled
 289  *
 290  * A value of zero in the hIndex field disables handshaking
 291  *
 292  * If Xon/Xoff handshaking is specified, the hValue field should contain the
 293  * XOFF character and the lValue field contains the XON character.
 294  */
 295
 296 /*
 297  * FTDI_SIO_GET_LATENCY_TIMER
 298  *
 299  * Set the timeout interval. The FTDI collects data from the slave
 300  * device, transmitting it to the host when either A) 62 bytes are
 301  * received, or B) the timeout interval has elapsed and the buffer
 302  * contains at least 1 byte.  Setting this value to a small number
 303  * can dramatically improve performance for applications which send
 304  * small packets, since the default value is 16ms.
 305  */
 306 #define  FTDI_SIO_GET_LATENCY_TIMER_REQUEST FTDI_SIO_GET_LATENCY_TIMER
 307 #define  FTDI_SIO_GET_LATENCY_TIMER_REQUEST_TYPE 0xC0
 308
 309 /*
 310  *  BmRequestType:   1100 0000b
 311  *  bRequest:        FTDI_SIO_GET_LATENCY_TIMER
 312  *  wValue:          0
 313  *  wIndex:          Port
 314  *  wLength:         0
 315  *  Data:            latency (on return)
 316  */
 317
 318 /*
 319  * FTDI_SIO_SET_LATENCY_TIMER
 320  *
 321  * Set the timeout interval. The FTDI collects data from the slave
 322  * device, transmitting it to the host when either A) 62 bytes are
 323  * received, or B) the timeout interval has elapsed and the buffer
 324  * contains at least 1 byte.  Setting this value to a small number
 325  * can dramatically improve performance for applications which send
 326  * small packets, since the default value is 16ms.
 327  */
 328 #define  FTDI_SIO_SET_LATENCY_TIMER_REQUEST FTDI_SIO_SET_LATENCY_TIMER
 329 #define  FTDI_SIO_SET_LATENCY_TIMER_REQUEST_TYPE 0x40
 330
 331 /*
 332  *  BmRequestType:   0100 0000b
 333  *  bRequest:        FTDI_SIO_SET_LATENCY_TIMER
 334  *  wValue:          Latency (milliseconds)
 335  *  wIndex:          Port
 336  *  wLength:         0
 337  *  Data:            None
 338  *
 339  * wValue:
 340  *   B0..7   Latency timer
 341  *   B8..15  0
 342  *
 343  */
 344
 345 /*
 346  * FTDI_SIO_SET_EVENT_CHAR
 347  *
 348  * Set the special event character for the specified communications port.
 349  * If the device sees this character it will immediately return the
 350  * data read so far - rather than wait 40ms or until 62 bytes are read
 351  * which is what normally happens.
 352  */
 353
 354
 355 #define  FTDI_SIO_SET_EVENT_CHAR_REQUEST FTDI_SIO_SET_EVENT_CHAR
 356 #define  FTDI_SIO_SET_EVENT_CHAR_REQUEST_TYPE 0x40
 357
 358
 359 /*
 360  *  BmRequestType:   0100 0000b
 361  *  bRequest:        FTDI_SIO_SET_EVENT_CHAR
 362  *  wValue:          EventChar
 363  *  wIndex:          Port
 364  *  wLength:         0
 365  *  Data:            None
 366  *
 367  * wValue:
 368  *   B0..7   Event Character
 369  *   B8      Event Character Processing
 370  *             0 = disabled
 371  *             1 = enabled
 372  *   B9..15  Reserved
 373  *
 374  */
 375
 376 /* FTDI_SIO_SET_ERROR_CHAR */
 377
 378 /*
 379  * Set the parity error replacement character for the specified communications
 380  * port
 381  */
 382
 383 /*
 384  *  BmRequestType:  0100 0000b
 385  *  bRequest:       FTDI_SIO_SET_EVENT_CHAR
 386  *  wValue:         Error Char
 387  *  wIndex:         Port
 388  *  wLength:        0
 389  *  Data:           None
 390  *
 391  *Error Char
 392  *  B0..7  Error Character
 393  *  B8     Error Character Processing
 394  *           0 = disabled
 395  *           1 = enabled
 396  *  B9..15 Reserved
 397  *
 398  */
 399
 400 /* FTDI_SIO_GET_MODEM_STATUS */
 401 /* Retrieve the current value of the modem status register */
 402
 403 #define FTDI_SIO_GET_MODEM_STATUS_REQUEST_TYPE 0xc0
 404 #define FTDI_SIO_GET_MODEM_STATUS_REQUEST FTDI_SIO_GET_MODEM_STATUS
 405 #define FTDI_SIO_CTS_MASK 0x10
 406 #define FTDI_SIO_DSR_MASK 0x20
 407 #define FTDI_SIO_RI_MASK  0x40
 408 #define FTDI_SIO_RLSD_MASK 0x80
 409 /*
 410  *   BmRequestType:   1100 0000b
 411  *   bRequest:        FTDI_SIO_GET_MODEM_STATUS
 412  *   wValue:          zero
 413  *   wIndex:          Port
 414  *   wLength:         1
 415  *   Data:            Status
 416  *
 417  * One byte of data is returned
 418  * B0..3 0
 419  * B4    CTS
 420  *         0 = inactive
 421  *         1 = active
 422  * B5    DSR
 423  *         0 = inactive
 424  *         1 = active
 425  * B6    Ring Indicator (RI)
 426  *         0 = inactive
 427  *         1 = active
 428  * B7    Receive Line Signal Detect (RLSD)
 429  *         0 = inactive
 430  *         1 = active
 431  */
 432
 433
 434
 435 /* Descriptors returned by the device
 436  *
 437  *  Device Descriptor
 438  *
 439  * Offset       Field           Size    Value   Description
 440  * 0    bLength         1       0x12    Size of descriptor in bytes
 441  * 1    bDescriptorType 1       0x01    DEVICE Descriptor Type
 442  * 2    bcdUSB          2       0x0110  USB Spec Release Number
 443  * 4    bDeviceClass    1       0x00    Class Code
 444  * 5    bDeviceSubClass 1       0x00    SubClass Code
 445  * 6    bDeviceProtocol 1       0x00    Protocol Code
 446  * 7    bMaxPacketSize0 1       0x08    Maximum packet size for endpoint 0
 447  * 8    idVendor        2       0x0403  Vendor ID
 448  * 10   idProduct       2       0x8372  Product ID (FTDI_SIO_PID)
 449  * 12   bcdDevice       2       0x0001  Device release number
 450  * 14   iManufacturer   1       0x01    Index of man. string desc
 451  * 15   iProduct        1       0x02    Index of prod string desc
 452  * 16   iSerialNumber   1       0x02    Index of serial nmr string desc
 453  * 17   bNumConfigurations 1    0x01    Number of possible configurations
 454  *
 455  * Configuration Descriptor
 456  *
 457  * Offset       Field                   Size    Value
 458  * 0    bLength                 1       0x09    Size of descriptor in bytes
 459  * 1    bDescriptorType         1       0x02    CONFIGURATION Descriptor Type
 460  * 2    wTotalLength            2       0x0020  Total length of data
 461  * 4    bNumInterfaces          1       0x01    Number of interfaces supported
 462  * 5    bConfigurationValue     1       0x01    Argument for SetCOnfiguration() req
 463  * 6    iConfiguration          1       0x02    Index of config string descriptor
 464  * 7    bmAttributes            1       0x20    Config characteristics Remote Wakeup
 465  * 8    MaxPower                1       0x1E    Max power consumption
 466  *
 467  * Interface Descriptor
 468  *
 469  * Offset       Field                   Size    Value
 470  * 0    bLength                 1       0x09    Size of descriptor in bytes
 471  * 1    bDescriptorType         1       0x04    INTERFACE Descriptor Type
 472  * 2    bInterfaceNumber        1       0x00    Number of interface
 473  * 3    bAlternateSetting       1       0x00    Value used to select alternate
 474  * 4    bNumEndpoints           1       0x02    Number of endpoints
 475  * 5    bInterfaceClass         1       0xFF    Class Code
 476  * 6    bInterfaceSubClass      1       0xFF    Subclass Code
 477  * 7    bInterfaceProtocol      1       0xFF    Protocol Code
 478  * 8    iInterface              1       0x02    Index of interface string description
 479  *
 480  * IN Endpoint Descriptor
 481  *
 482  * Offset       Field                   Size    Value
 483  * 0    bLength                 1       0x07    Size of descriptor in bytes
 484  * 1    bDescriptorType         1       0x05    ENDPOINT descriptor type
 485  * 2    bEndpointAddress        1       0x82    Address of endpoint
 486  * 3    bmAttributes            1       0x02    Endpoint attributes - Bulk
 487  * 4    bNumEndpoints           2       0x0040  maximum packet size
 488  * 5    bInterval               1       0x00    Interval for polling endpoint
 489  *
 490  * OUT Endpoint Descriptor
 491  *
 492  * Offset       Field                   Size    Value
 493  * 0    bLength                 1       0x07    Size of descriptor in bytes
 494  * 1    bDescriptorType         1       0x05    ENDPOINT descriptor type
 495  * 2    bEndpointAddress        1       0x02    Address of endpoint
 496  * 3    bmAttributes            1       0x02    Endpoint attributes - Bulk
 497  * 4    bNumEndpoints           2       0x0040  maximum packet size
 498  * 5    bInterval               1       0x00    Interval for polling endpoint
 499  *
 500  * DATA FORMAT
 501  *
 502  * IN Endpoint
 503  *
 504  * The device reserves the first two bytes of data on this endpoint to contain
 505  * the current values of the modem and line status registers. In the absence of
 506  * data, the device generates a message consisting of these two status bytes
 507  * every 40 ms
 508  *
 509  * Byte 0: Modem Status
 510  *
 511  * Offset       Description
 512  * B0   Reserved - must be 1
 513  * B1   Reserved - must be 0
 514  * B2   Reserved - must be 0
 515  * B3   Reserved - must be 0
 516  * B4   Clear to Send (CTS)
 517  * B5   Data Set Ready (DSR)
 518  * B6   Ring Indicator (RI)
 519  * B7   Receive Line Signal Detect (RLSD)
 520  *
 521  * Byte 1: Line Status
 522  *
 523  * Offset       Description
 524  * B0   Data Ready (DR)
 525  * B1   Overrun Error (OE)
 526  * B2   Parity Error (PE)
 527  * B3   Framing Error (FE)
 528  * B4   Break Interrupt (BI)
 529  * B5   Transmitter Holding Register (THRE)
 530  * B6   Transmitter Empty (TEMT)
 531  * B7   Error in RCVR FIFO
 532  *
 533  */
 534 #define FTDI_RS0_CTS    (1 << 4)
 535 #define FTDI_RS0_DSR    (1 << 5)
 536 #define FTDI_RS0_RI     (1 << 6)
 537 #define FTDI_RS0_RLSD   (1 << 7)
 538
 539 #define FTDI_RS_DR      1
 540 #define FTDI_RS_OE      (1<<1)
 541 #define FTDI_RS_PE      (1<<2)
 542 #define FTDI_RS_FE      (1<<3)
 543 #define FTDI_RS_BI      (1<<4)
 544 #define FTDI_RS_THRE    (1<<5)
 545 #define FTDI_RS_TEMT    (1<<6)
 546 #define FTDI_RS_FIFO    (1<<7)
 547
 548 /*
 549  * OUT Endpoint
 550  *
 551  * This device reserves the first bytes of data on this endpoint contain the
 552  * length and port identifier of the message. For the FTDI USB Serial converter
 553  * the port identifier is always 1.
 554  *
 555  * Byte 0: Line Status
 556  *
 557  * Offset       Description
 558  * B0   Reserved - must be 1
 559  * B1   Reserved - must be 0
 560  * B2..7        Length of message - (not including Byte 0)
 561  *
 562  */