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