PC Online 1996 October/PCO_10.ISO/filesbbs/rs203.arj/RS.DOC

Text File | 1986-06-23 | 9.4 KB | 222 lines

             RS - An RS-232 Port Status Utility
                   Author: Fred E. Davis

                       What RS does.

     RS is  an uncomplicated  utility  that  prints  a  full
status report  of a selected RS-232 "COM" port.  It provides
information about  the communications  protocol,  handshake,
line, error and interrupt status.

                      How to call RS.

     RS is  called by  entering "RS"  followed by  either no
argument  or  a  "1"  or  "2"  then  <enter>.  The  argument
specifies which  COM port  you wish to view; "1" will select
COM1 and "2" will select COM2.  When no argument is given or
an illegal  value is  used, the  result will  be the same as
entering "1".  If the  specified COM port is not present (or
not responding),  RS will notify you with a "not installed!"
message.  The command syntax is:

              [d:][pathspec]RS [1 | 2]<enter>

                     What it all means.

     The report  is divided into five sections: A. Protocol,
B. Handshake  Status, C. Line Status, D. Error Status and E.
Interrupt Status.

A. Protocol

Parity, Rate,  Stick and  Break. The  first four  items  are
those values set by the DOS "mode COMx:" command.

     1. Data  Bits refers  to the number of binary bits used
     to send  data. The  choices are 5, 6, 7 and 8. Five and
     six are rarely used. Most text can be communicated with
     7 bits.  An exception is a Wordstar formatted text file
     that  uses  the  eighth  bit.  Most  binary  files  and
     programs are transmitted with 8 data bits.

     2.  Stop   Bits  are   used  to  identify  the  end  of
     transmission of one character. The possibilities are 1,
     1.5 and  2 stop  bits. Most  protocols request one stop
     bit; some  older (and slower) protocols required 1.5 or
     2 stop bits.

     3.  Parity is used  in an  error detection  function to
     identify a character distorted during transmission. The
     choices are none, even and odd. All of these are likely
     to be  encountered. None  is most often used with eight
     data  bits;   odd  and   even  with  seven  data  bits.
     Practically any  combination is possible and you should
     refer  to   the  specific   protocol  requested  by  an
     application to determine which is right.

     4.  Rate  refers  to  the  timing  of  the  bits  being
     transmitted. The  choices are  many,  though  the  more
     common ones are 1200, 2400, 4800 and 9600 baud, or bits
     per second.  If the  rate isn't  right you'll soon find
     out! You'll receive either nothing or garbage.

     5. Stick  refers to  making the parity bit (if used) to
     "stick" at   logical  1 or  0.  If "stick" is "normal",
     the parity bit function will follow the message in item
     3. If  "stick" is  "set" and  "parity" is  "even",  the
     parity bit will be forced to a logical zero. If "stick"
     is "set"  and "parity" is "odd", the parity bit will be
     forced to logical one.

     6. Break forces the transmitted data output to stay on.
     It is  used mostly  in computer terminal communications
     to grab  the attention  of a computer. If set, any data
     to be transmitted is ignored.

B. Handshake Status

     There are  six handshake  lines: DTR, RTS, CTS, DSR, CD
and RI.  These handshake lines are often used to control the
transfer of  information between  the communicating systems,
but not  always.  They  are  more  frequently  used  between
computers and  modems, especially  CD and RI. The directions
"In" and  "Out" used here are relative to the COM port. Some
communications rely  on software handshake protocols such as
XON/XOFF,   ETX/ACK    and   XMODEM   (which   goes   beyond
handshaking). If  hardware handshaking  is used,  DTR and/or
RTS must  be high  to enable the external device to receive.
Likewise CTS  and DSR  should be high to enable the COM port
to transmit.

     1.  DTR   Out  is  often  used  in  hardware  handshake
     protocols  to   indicate  Data   Terminal  Ready.  Data
     Terminal is  an old  term referring in this case to the
     computer (PC).

     2. RTS  Out notifies  the external  device that the COM
     port is Ready To Send.

     3. CTS  In is  the Clear To Send line. It is used often
     in hardware handshake protocols with the DTR line.

     4. DSR In refers to Data Set Ready. It is often used to
     inform a  Data Terminal  (or computer)  that a Data Set
     (such as a modem) is ready to communicate.

     5. CD  In is  the Carrier Detect input from a modem. It
     is asserted  when a  communications link is established
     with another modem.

     6. RI  In refers  to Ring  Indicate from a modem. It is
     used  primarily   for  auto-answer  operations  so  the
     computer can accept incoming telephone calls (even on a
     specific number of rings by counting RI transitions).

C. Line Status

     The four  Line Status  bits, Receive  Buffer,  Transmit
Buffer, Break  Status and  Loop Back,  refer to the state of
internal registers and a test function.

     1. Receive  Buffer is  the register that holds incoming
     data. When holding data that hasn't been read, a "full"
     message  will   be  displayed,  otherwise  it  will  be
     "empty".

     2. Transmit  Buffer holds  data to  be sent.  When  not
     currently holding  information waiting  to be  sent  it
     will indicate "empty"; if data is waiting to be sent it
     will show "full".

     3. Break  Status indicates  the presence  of a  "Break"
     signal from  the sender. This often initiates some form
     of interrupt to normal communications. (See A.6.)

     4. Loop  Back is  a special  state for internal testing
     and verification  of hardware  and  register  function.
     When "set", the internal output registers are connected
     to the  input registers  (and  vice  versa)  such  that
     transmitted  data  is  immediately  received.  It  must
     indicate "normal" if communications are to take place.

D. Error Status

     The three  error  status  lines,  Overrun,  Parity  and
Frame, are  set by  the COM  ports receiver  to  indicate  a
communications problem.

     1. Overrun  "set" means that another character has been
     received before  the first one has been read by the PC.
     A possible cause for this is a baud rate too high.

     2.  Parity   "set"  means   that  the   last  character
     transmission was  corrupted somehow.  It  obviously  is
     significant only  when parity checking has been enabled
     (See  A.3.).  Possible  causes  can  be  noise  in  the
     communications lines  or improper  parity polarity (odd
     or even).

     3. Frame  errors occur  when a  character was  received
     without a  valid  stop  bit.  Possible  causes  include
     improper communications protocol (See A.).

E. Interrupt Status

     Several  conditions   in  the   COM  port  cause  cause
interrupts to the PCs Microprocessor to demand some service.
The four  conditions are:  Received Data  Ready (See  C.1.),
Transmit Buffer  Empty (See  C.2.), Line Status (See D.) and
Modem Status (See B.).

     1. Output  refers to  an interrupt  control signal that
     can enable  or disable  all interrupt requests from the
     COM port.  If interrupts  are to  be used, it must show
     "enable".

     2. Data Ready, if enabled, causes an interrupt when the
     Received Data Buffer is full (See C.1.).

     3. Xmit Empty, if enabled, causes an interrupt when the
     Transmit Data Buffer is empty and another character can
     be sent.

     4. Line  Status would cause an interrupt, when enabled,
     if C.1., C.2., C.3., D.1., D.2. or D.3. are asserted.

     5. Modem  Status, like Line Status, causes an interrupt
     if B.3., B.4., B.5. or B.6 are asserted.

                       How RS works.

     RS is  written in assembly language, which explains its
small size. The speed limitation on its execution is imposed
by the  BIOS and  DOS display  routines. RS  is not a memory
resident program;  it does  not trap any interrupts; it uses
only BIOS  and DOS display interfaces; it does not alter the
port configuration.

     After displaying  the  title  message,  RS  checks  the
"command  tail"  (any  characters  entered  after  the  "RS"
command) to  see if any arguments are being passed. If a "2"
is present, RS will access COM2: starting at port 2F8h. If a
"1", no  argument or an illegal value is passed, RS will use
COM1: starting at port 3F8h.

     RS then  reads all  of the  port registers  and buffers
into temporary  RAM buffers  used by  RS. The  Line  Control
Register is read, altered, and restored. RS checks to see if
a port  really exists  there  by  verifying  the  read/write
operation. Once  all the  registers are  read, RS then scans
the values  in the  RAM buffers  to generate the messages to
the display.  This minimizes  interruption of port activity.
In the  case of  the baud  rate, RS  first compares the rate
divisor to a table of standard values. If the port's rate is
not found, the actual baud rate is calculated and displayed.

     RS terminates  with a type 4Ch function call to DOS Int
21h. If  the  program  executed  normally  a  value  00h  is
returned. If  the COM port is not installed or functional, a
value of 01h is returned.

Fred E. Davis                                   26 June 1986