COMMUNICATION PARAMETERS

B.1 Communication parameters 

baud rate

                        The  transmit  speed between two  serial ports.

parity

The   parity  bit  is  used   in   error detection. Three different settings are used, which are ODD, EVEN, and NONE parity.

data bits

The  number of bits in each  transmitted or  received serial character.  Either 7 or 8 data bits can be specified.

stop bits

The  stop bits is used to  detect  where one character ends and another starts. Either 1  or 2 stop bits can be specified. When stop bits   is   specified  then  stop  bits   are generated  or  checked  in  the  transmit  or

receive data respectively. IRQ specifies   which  interrupt is  emerged when  the character is received  from  serial port.

interrupt vector

The   interrupt   vector  is  a   global interrupt  indicator  which  is  designed  at several  multi-serial adapters (such as  four serial  card, eight serial card ... etc.)  to check  which serial port  emerges  interrupt. Since  several serial ports are  designed  at one  adapter and use the common IRQ, so  that interrupt  vector is useful to  detect  which port emerges interrupt. For example,  suppose interrupt vector address of hardware  adapter is set to 2C2H, the relationship between each port  corresponds to the interrupt  indicator is shown in the follows. If you specify  this interrupt  vector address, when interrupt  is emerged, device driver may check address 2C2H to find which port emerges interrupt. 

The  standard  COM1 and  COM2  need  not interrupt vector. We will list several famous multi-serial adapters in the appendix D. 

modem control register

The   contents  of  the  modem   control register are indicated and described below. 

The  bit  0  (DTR) and bit 1  (RTS)  are hardware  handshaking.  To let PCCOM  in  the normal mode, you must set the bit 0 and bit 1 to  1.  The bit 4 provides a loopback feature for diagnostic testing of the serial port. In this case,  you must set bit 4 to be 0.  OUT1 and  OUT2 are auxiliary user designed  output which is used to enable/disable interrupt  in most adapters,  so that you must control OUT1 and  OUT2  carefully to let your serial  port emerges an interrupt.  You may set OUT1=0 and OUT2=1  to  enable  interrupt  of  the   most standard COM1 and COM2 adapter,  however some adapters  may not work in this case,  you can ask   OUT1  and  OUT2  control  from   origin hardware manufacturer or trial and  error  to find one of four  combination  (0,0),  (0,1), (1,0),  (1,1).  Appendix D will list OUT1 and OUT2 signal of our multi-serial family.  Some 8250/16450  chip will generate OUT1 =  1  and OUT2 = 1 when internal loop is selected  (bit 4 is set to 1), so that they will not  emerge an  interrupt when internal loop  feature  is test. 

active status

The  active  status is used  to  specify activation status of interrupt vector. Either active low or high can be set.  If active low is specified,  when the corresponding bit  of interrupt    vector   is   0,    means    the corresponding  port  emerges  an   interrupt, otherwise if the bit is 1, means no interrupt is active. 

XON/XOFF

XON/XOFF is software handshaking,  which is  used  to prevent the lost of  data  while communication   between  two  serial   ports. Suppose  we connect two computer named  local computer   and  remote  computer,   if  local computer  is  too busy to receive  data  from remote computer,  it send XOFF (S/CTRL  whose code  is 17) to remote computer to notice  he is   in  busy  mode.   When  remote  computer receives  XOFF,  it  will  stop  transmission until  the local computer sends  XON  (Q/CTRL whose  code is 19) to continue  transmission. The  same  manner  is  activated  at   remote computer. 

DTR and DSR

The  DTR/DSR  is  hardware  handshaking. Data Set Ready(DSR) indicates that the modem or   data   set   is   ready   to   establish communications  link and transfer  data  with the    serial   interface.    Data   Terminal Ready(DTR) informs the modem or data set that the    serial   interface   is    ready    to communication. 

RTS and CTS

The  RTS/CTS  is  hardware  handshaking. Clear  To Send(CTS) and Request To  Send(RTS) can   be  used  to  establish   communication protocol.  RTS  informs the modem or data set that   the  serial  interface  is  ready   to transmit data. 

B.2 Serial Ports status 

     The serial ports status are described in the following. 

Carrier Detected

The  modem  status which  provides  Data Carrier Detected(DCD) to indicate the voltage level of the associate RS232 line.

Ring Indicator(RI)

Indicates  that a telephone ring  signal has been received by the modem or data set.

Txmtr SR

                 Transmitter shift register of 8250/16450 chip.

Txmtr HR

Transmitter    holding    register    of  8250/16450 chip.

Break Received

Indicates   whether  the  received  data input is held in the spacing state for longer than a full word transmission time,  that is, the  total  time of start bit + data  bits  + parity bit + stop bits.

     Framing Error

    Indicates  that the  received  character did not have a   

    valid stop bit. 

    Parity Error  

Indicates that the received   data character  does not have the correct even  or odd parity.           

Overrun Error

Indicates  that  data  in  the  receiver buffer   register   was  not  ready  by   the processor  before  the  next  characters  was transferred   into   the   receiver    buffer register,  thereby  destroying  the  previous character. 

Receiver DR full

Received Data Register full. If the flag always  set, it means the RS232 port hang  up because  no  interrupt  generated  when  data arrive.  Please  restart our  computer  when error occurs.  

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