Programmer 7500/MAX_PROGRAMMERS.iso/PROGRAMS/UTILS/MOUSE/RS.ZIP/RS.DOC

Text File | 1987-03-26 | 13.9 KB | 529 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




            Fred E. Davis                                   26 June 1986








            An RS-232 Port Status Utility                              2


                 Protocol lists  six  items:    Data  Bits,  Stop  Bits,

            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.





            Fred E. Davis                                   26 June 1986








            An RS-232 Port Status Utility                              3


                 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



            Fred E. Davis                                   26 June 1986








            An RS-232 Port Status Utility                              4


            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.





            Fred E. Davis                                   26 June 1986








            An RS-232 Port Status Utility                              5


                 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.



            Fred E. Davis                                   26 June 1986








            An RS-232 Port Status Utility                              6


                 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



            Fred E. Davis                                   26 June 1986








            An RS-232 Port Status Utility                              7


                 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.



            Fred E. Davis                                   26 June 1986








            An RS-232 Port Status Utility                              8


                 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