TEMPERATURE CONTROLLER SERIAL COMMUNICATION SPECIFICATION



        1.0 - INTRODUCTION

        This specification provides all the information required for
        developing  software that communicates with the Temperature
        Controller  models  89000-10,-15 and 689-0010,-0015. All  of  the
        Temperature Controller features that are available via the keypad
        can also be performed through its serial port by sending the
        appropriate  command. Any programmable device that has an  RS-232
        serial port can be used to communicate with the Temperature
        Controller.  In  this specification the device connected  to  the
        Temperature  Controller will be referred to as the  PC  (Personal
        Computer  or  Programmable Controller).


        2.0 - SERIAL PORT CONFIGURATION

        The serial data format is full duplex, 1 start bit, 8 data  bits,
        no  parity, and one stop bit.  The BAUD rate is  user  selectable
        to 9600, 4800, 2400, 1200, 600, or 300 bits per  second.


        2.1 - RS-232 CONNECTIONS

        The  Temperature Controller RS-232 connections are made through a
        9 pin  male  DB connector.  Only 3 of the 9 pins are used for the
        RS-232 port.

                          Temp.     |
                          Controller|
                          DB-9      | Function
                          ----------+----------------------
                          Pin 2     | Receive data from PC
                          Pin 3     | Send data to PC
                          Pin 5     | Ground

                                RS-232 connector


        The following two tables show the required connections for inter-
        facing  to an IBM compatible computer with either a 9 pin  or  25
        pin plug.

               Temp.     |                   Temp.     |
               Controller| Computer          Controller| Computer
               DB-9      | DB-9              DB-9      |  DB-25
               ----------+---------          ----------+---------
               Pin 2     |  Pin 3            Pin 2     |  Pin 2
               Pin 3     |  Pin 2            Pin 3     |  Pin 3
               Pin 5     |  Pin 5            Pin 5     |  Pin 7




                                PAGE 1




        2.2 - HANDSHAKING

        Software handshaking is implemented in the Temperature Controller
        using  <XON> (11 hex) and <XOFF> (13 hex).  When the  Temperature
        Controller  receives an <XOFF>, it will  stop transmitting  until
        it receives an <XON> or a new  command.  The Temperature Control-
        ler  does  not send <XOFF> since its buffer is  large  enough  to
        receive the longest command string. Hardware  handshaking is  not
        provided.


        3.0 - LOCAL/REMOTE MODE

        The  Temperature  Controller will always power up  in  the  local
        mode.  In local mode it is controlled entirely by the front panel
        keypad.  It  will  automatically switch to remote  mode  when  it
        receives any valid command from the PC except for the exit remote
        mode  command.  While  in the remote mode, all  keys  except  for
        RUN/STOP  are  disabled on the Temperature  Controller.  "REMOTE"
        will  appear  on the upper display after the  actual  temperature
        when the unit is in remote mode. Only the actual temperature  and
        error  messages  will be updated on the display, with  all  other
        display  functions  disabled. The lower 16 character  display  is
        controlled  by using the remote display command. It is up to  the
        PC  to  put any alarm or run status messages on  the  Temperature
        Controller  lower  display. The LED annunciators  and  all  other
        Temperature  Controller  functions will operate the  same  as  in
        local  mode. Remote mode is disabled by sending the  exit  remote
        mode command or by turning the power off and then back on.


        4.0 - COMMAND FORMAT

        All   commands  to the Temperature Controller are  preceded  with
        three   characters: the ASCII start of text character  <STX>  (02
        hex),  a  T,  and a  1. Following  this header are  one  or  more
        command  letters,   optional data,   and then a  carriage  return
        <CR> (0D hex). Note  that  the <STX> and <CR> are each one  ASCII
        character and all alpha characters must be upper case.


                                 <STX>T1UC<CR>
                                   |  | || |
                                   |  | || |
          Start of Text (02 hex) -/   | ||  \-- Carriage Return (0D hex)
                          Header ----/  | \---- Command data
          Command character -----------/

                              Sample Command string







                                PAGE 2



        All  of  the Temperature Controller serial commands are  used  to
        either  request  data or set a Temperature Controller  parameter.
        When  data follows a command, the  data will  set a parameter  in
        the  Temperature  Controller. To request a  parameter   from  the
        Temperature  Controller,  the same command is sent  without  data
        following   it. When a request data command is sent the  Tempera-
        ture Controller responds with  the requested  data, preceding  it
        with  <STX>  and the  command  letter and  following  it  with  a
        <CR><LF>.   The  Temperature Controller will respond   with   the
        ASCII  acknowledge  character <ACK> (06 hex) when it  receives  a
        valid set command. It will send a negative acknowledge  character
        <NAK> (15 hex) if the set command data is invalid or a communica-
        tion error occurred. The request status command (section 5.0) can
        be  used after a <NAK> is received to determine the cause of  the
        error.   All temperature values sent and received will be in  the
        currently selected units. When the temperature units are changed,
        all  temperature values will be converted to the new  units.  For
        example,  if an alarm was set for 100 Celsius and the  units  are
        changed to Fahrenheit, the alarm value will now be 212.


               |  DATA    |
        COMMAND|  FIELD   |            FUNCTION
        -------+----------+----------------------------------------------
          AA   | x        | set or request Audible Alarm status
               |          | x = 0 = alarm off, 1 = on
               |          |
          AC   | vwxyz    | request Alarm Conditions
               |          | v = 0 = alarm inactive, 1 = active
               |          |     for all alarms except low in process hi/lo
               |          | w = 0 = alarm inactive, 1 = active
               |          |     for low alarm in process hi/lo
               |          | x = 0 = alarm acknowledged, 1 = new alarm
               |          | y = 0 = alarm relay off, 1 = on
               |          | z = 0 = no Stop alarms
               |          |     1 = Run Time Stop
               |          |     2 = Loop Break Stop
               |          |     3 = Over Temperature Stop
               |          |     4 = Output Error Stop
               |          |     5 = Open Probe Stop
               |          |     6 = Over Probe Stop
               |          |     7 = Under Probe Stop
               |          |     8 = Heater fuse blown
               |          |
          AE   | x        | set or request Audible Alarm Enable status
               |          | x = 0 = disabled, 1 = enabled
          AH   | xx.x     | set or request Alarm Hysteresis
               |          | xx.x = temperature (0.1 to 99.9)
          AK   | none     | Acknowledge Alarm condition
               |          |

                         Temperature Controller commands





                                PAGE 3



               |  DATA    |
        COMMAND|  FIELD   |            FUNCTION
        -------+----------+----------------------------------------------
          AM   | x        | set or request Alarm Mode
               |          | x = 0 = process high alarm
               |          |     1 = process low alarm
               |          |     2 = process high/low
               |          |     3 = deviation high
               |          |     4 = deviation low
               |          |     5 = deviation high/low
               |          |     6 = deviation band
               |          |
          AS   | xxxx.x   | set or request Alarm Setpoint
               |          | setpoint temperature for all alarm modes,
               |          | high setpoint if process high/low mode
          AL   | xxxx.x   | set or request Low Alarm Setpoint for
               |          | setpoint temperature for low alarm
               |          | in process high/low
               |          |
          AR   | x        | set or request Alarm Reset mode
               |          | x = 0 = alarms disabled
               |          |     1 = auto reset
               |          |     2 = manual reset
               |          |
          B    | xxxx     | set or request BAUD rate
               |          | xxxx = 300, 600, 1200, 2400, 4800, or 9600
               |          |
          CA   | x        | set or request Control Action
               |          | x = 0 = heat, 1= cool
          CC   | xxx      | set or request Control output Cycle time
               |          | xxx = seconds (1 to 300)
          CD   | xxxx     | set or request Derivative Time for current
               |          | control parameter number (CN command)
               |          | xxxx = seconds (0 to 3600)
               |          |
          CE   | x        | set or request auto tune enable status
               |          | x = 0 = disabled, 1 = enabled
          CH   | xx.x     | set or request on/off Control Hysteresis
               |          | xx.x = temperature (0.1 to 99.9)
          CI   | xxxx     | set or request Integral Time for current
               |          | control parameter number (CN command)
               |          | xxxx = seconds (0 to 3600)
               |          |
          CM   | x        | set or request Control Mode
               |          | x = 0 = on/off control
               |          |     1 = PID
               |          |     2 = ramp/soak
          CN   | x        | set or request Control parameter Number
               |          | for accessing PID data
               |          | x = CP number (0 to 9), 0 = auto tune data
          CP   | xxxx     | set or request Proportional Band for current
               |          | control parameter number (CN command)
               |          | xxxx = temperature (1 to 1000)

                         Temperature Controller commands


                                PAGE 4



               |  DATA    |
        COMMAND|  FIELD   |            FUNCTION
        -------+----------+----------------------------------------------
          CR   | x        | set or request Control Run status
               |          | x = 0 = stop, 1 = running or continue run
               |          |     2 = restart run, 3 = auto tune
          CU   | x        | set or request Power Up run status
               |          | x = 0 = always stopped, 1 = last state
               |          |
          D    | xxx...x  | set or request Display data
               |          | x = up to 16 alphanumeric characters
               |          |
          F    | t xx.x   | set or request Field cal probe offset
               |          | t = sensor type (see T command)
               |          | xx.x = temperature offset
               |          |
          H    | xx:xx    | set or request run time Hours & minutes
               |          | xx:xx = hours:minutes (00:00 to 99:59)
               |          |         00:00 = continuous
               |          |
          I    | x        | request communication Info (see section 5.0)
               |          |
          K    | x        | request Keypad status (see section 6.0)
               |          |
          L    | hcta     | request LED annunciators status
               |          | hcta = Heat, Cool, auto Tune, Alarm
               |          |        0 = off, 1 = on
               |          |
          OL   | xxxx.x   | set or request recorder Output at 4 mA
          OH   | xxxx.x   | set or request recorder Output at 20 mA
               |          | xxxx.x = temperature at 4 and 20 mA
               |          |
          P    | xxx      | request control output Power
               |          | xxx = percent output on (0 to 100)
          PV   | xxxx.x   | request Process Variable (actual temperature)
               |          | xxxx.x = temperature or OPEN, UNDER, OVER
               |          |
          RA   | x        | set or request Assured Soak status for all
               |          | segments in current profile number
               |          | x = 0 = No, 1 = Yes
          RC   | x        | set or request Control Parameter number
               |          | for current profile and segment number
               |          | x = control parameter number (0 to 9)
          RE   | xxxx.x   | set or request Ramp/soak End temperature for
               |          | current profile and segment number
               |          | xxxx.x = temperature
               |          |
          RI   | x        | request Ramp/soak Info
               |          | x = 0 = done, 1 = waiting for assured soak,
               |          |     2 = ramping up, 3 = ramping down, 4 =soaking
          RP   | x        | set or request Ramp/soak Profile number
               |          | x = profile (1 to 9)

                         Temperature Controller commands



                                PAGE 5



               |  DATA    |
        COMMAND|  FIELD   |            FUNCTION
        -------+----------+----------------------------------------------
          RR   | xx:xx:xx | request Remaining Run time
               |          | xx:xx:xx = hr:min:sec actual time remaining
               |          |
          RS   | xx       | set or request Ramp/soak Segment number
               |          | xx = segment(1 to 16)
          RT   | xx:xx    | set or request Ramp/soak time
               |          | xx:xx = hours:minutes (00:00 to 99:59)
               |          |         for current profile & segment number
               |          |
          SB   | xxx.x    | set or request loop Break Stop time
               |          | xxx.x = minutes (0 to 300.0), 0 = disabled
          SP   | xxxx.x   | set or request SetPoint temperature
               |          | xxxx.x = temperature
               |          |
          ST   | xxx      | set or request over Temperature Stop
               |          | xxx = temperature (1 to 999)
               |          |
          T    | x        | Set or request sensor type
               |          | x = 0=B, 1=E, 2=J, 3=K, 4=N, 5=R, 6=S, 7=T,
               |          |     8=YSI 400 thermistor, 9=YSI 700
               |          |     A=RTD alpha .00385, B=alpha .003916
               |          |
          U    | x        | set or request temperature Units
               |          | x = 0 = Fahrenheit
               |          |     1 = Celsius
               |          |     2 = Kelvin
               |          |     3 = Rankine
               |          |     4 = Reaumur
               |          |
          V    | xx.xx    | set or request ROM software Version number
               |          | xx.xx = version number (1.00 to 99.99)
               |          |
          W    | none     | write all setup data to non-volatile memory
          X    | none     | eXit remote mode
               |          |
          ZK   | none     | Clear Key press status
          ZS   | none     | Clear communication Status


                         Temperature Controller commands














                                PAGE 6



        COMMAND STRING TO | TEMP. CONTROLLER  |
        TEMP. CONTROLLER  | REPLY             |        FUNCTION
        ------------------+-------------------+-------------------------------
        <STX>T1AA1<CR>    |<ACK>              | turn on Audible Alarm
        <STX>T1AA<CR>     |<STX>AA0<CRLF>     | get Audible Alarm status
        <STX>T1AC<CR>     |<STX>AC01100<CRLF> | get Alarm Conditions
                          |                   |
        <STX>T1AE1<CR>    |<ACK>              | set audible Alarm Enable
        <STX>T1AH1.0<CR>  |<ACK>              | set Alarm Hysteresis to 1.0
        <STX>T1AH<CR>     |<STX>AH   1.0<CRLF>| get Alarm Hysteresis
                          |                   |
        <STX>T1AK<CR>     |<ACK>              | Acknowledge alarm condition
        <STX>T1AM<CR>     |<STX>AM1<CRLF>     | get Alarm Mode
        <STX>T1AM6<CR>    |<ACK>              | set Alarm Mode to deviation band
                          |                   |
        <STX>T1AS<CR>     |<STX>AS 100.0<CRLF>| get Alarm Setpoint
        <STX>T1AS200<CR>  |<ACK>              | set Alarm Setpoint to 200
        <STX>T1AL<CR>     |<STX>AL   0.0<CRLF>| get Low Alarm setpoint
                          |                   |
        <STX>T1AL-10<CR>  |<ACK>              | set Low Alarm setpoint to -10
        <STX>T1AR<CR>     |<STX>AR1<CRLF>     | get Alarm Reset mode
        <STX>T1AR2<CR>    |<ACK>              | set Alarm Reset mode to manual
                          |                   |
        <STX>T1B<CR>      |<STX>B 300<CRLF>   | get BAUD rate
        <STX>T1B9600<CR>  |<ACK>              | set BAUD rate 9600 BPS
        <STX>T1CA<CR>     |<STX>CA1<CRLF>     | get Control Action
                          |                   |
        <STX>T1CA0<CR>    |<ACK>              | set Control Action to heat
        <STX>T1CC<CR>     |<STX>CC  1<CRLF>   | get Control output Cycle time
        <STX>T1CC10<CR>   |<ACK>              | set Control output Cycle time
                          |                   |
        <STX>T1CE<CR>     |<STX>T1CE0<CRLF>   | get auto tune Enable status
        <STX>T1CE1<CR>    |<ACK>              | Enable auto tune
        <STX>T1CH<CR>     |<STX>CH 5.0<CRLF>  | get on/off Control Hysteresis
                          |                   |
        <STX>T1CH2.5<CR>  |<ACK>              | set on/off Control Hysteresis
        <STX>T1CM<CR>     |<STX>CM0<CRLF>     | get Control Mode
        <STX>T1CM1<CR>    |<ACK>              | set Control Mode to PID
                          |                   |
        <STX>T1CN0<CR>    |<ACK>              | set Control Parameter number
        <STX>T1CD<CR>     |<STX>CD  60<CRLF>  | get Derivative time
        <STX>T1CI<CR>     |<STX>CI 240<CRLF>  | get Integral time
                          |                   |
        <STX>T1CP<CR>     |<STX>CP  50<CRLF>  | get Prop. Band
        <STX>T1CN1<CR>    |<ACK>              | set Control Parameter number
        <STX>T1CD3<CR>    |<ACK>              | set Derivative time
                          |                   |
        <STX>T1CI24<CR>   |<ACK>              | set Integral time
        <STX>T1CP12<CR>   |<ACK>              | set Prop. Band
        <STX>T1CM1<CR>    |<ACK>              | set Control Mode to PID

                         Example command strings and responses





                                PAGE 7



        COMMAND STRING TO | TEMP. CONTROLLER  |
        TEMP. CONTROLLER  | REPLY             |        FUNCTION
        ------------------+-------------------+-------------------------------
        <STX>T1CR<CR>     |<STX>CR0<CRLF>     | get Control Run status
        <STX>T1CR1<CR>    |<ACK>              | set Control Run to continue
        <STX>T1CR3<CR>    |<ACK>              | start Auto Tune cycle
                          |                   |
        <STX>T1CU1<CR>    |<ACK>              | set Power Up status to last state
        <STX>T1D TESTING  |<ACK>              | put TESTING on lower display
        <STX>T1F3<CR>     |<STX>F3 0.0<CRLF>  | get cal offset for type K sensor
                          |                   |
        <STX>T1FA1.2<CR   |<ACK>              | set cal offset to 1.2 for
                          |                   | RTD .003850 alpha sensor
        <STX>T1H<CR>      |<STX>H00:00<CRLF>  | get run time Hours & minutes
        <STX>T1H2:00<CR>  |<ACK>              | set run time to 2 Hours
        <STX>T1I<CR>      |<STX>I0<CRLF>      | get communication Info
                          |                   |
        <STX>T1K<CR>      |<STX>K1<CRLF>      | get keypad status
        <STX>T1L<CR>      |<STX>L1000<CRLF>   | get LED annunciator status
        <STX>T1OL<CR>     |<STX>OL   0.0<CRLF>| get 4mA recorder Output temp.
                          |                   |
        <STX>T1OL32<CR>   |<ACK>              | set 4mA recorder Output to 32
        <STX>T1OH<CR>     |<STX>OH1000.0<CRLF>| get 20mA recorder Output temp.
        <STX>T1OH100<CR>  |<ACK>              | set 20mA recorder Output to 100
                          |                   |
        <STX>T1P<CR>      |<STX>P100<CRLF>    | get control output Power %
        <STX>T1PV<CR>     |<STX>PV 208.3<CRLF>| get Process Variable temperature
        <STX>T1RA<CR>     |<STX>RA0<CRLF>     | get Assured soak status
                          |                   |
        <STX>T1RA1<CR>    |<STX>RA0<CRLF>     | set Assured soak status
        <STX>T1RC<CR>     |<STX>RC0<CRLF>     | get Control Parameter #
        <STX>T1RC3<CR>    |<ACK>              | set Control Parameter # to 3
                          |                   |
        <STX>T1RE<CR>     |<STX>RE 100.0<CRLF>| get Ramp/soak End temperature
        <STX>T1RE200<CR>  |<ACK>              | set Ramp/soak End temperature
        <STX>T1RI<CR>     |<STX>RI00<CRLF>    | get Ramp/soak status Info
                          |                   |
        <STX>T1RP<CR>     |<STX>RP1<CRLF>     | get Ramp/soak Profile number
        <STX>T1RP2<CR>    |<ACK>              | set Ramp/soak Profile to 2
        <STX>T1RR<CR>     |<STX>RR00:08:21<CR>| get Remaining Run time
                          |                   |
        <STX>T1RS<CR>     |<STX>RS 2<CRLF>    | get Ramp/soak Segment number
        <STX>T1RS5<CR>    |<ACK>              | set Ramp/soak Segment to 5
        <STX>T1SB<CR>     |<STX>SB 10.0<CRLF> | get loop Break Stop time
                          |                   |
        <STX>T1SB5<CR>    |<ACK>              | set loop Break Stop to 5 minutes
        <STX>T1SP<CR>     |<STX>SP 100.0<CRLF>| get control SetPoint temperature
        <STX>T1SP120<CR>  |<ACK>              | set control SetPoint to 120
                          |                   |
        <STX>T1ST<CR>     |<STX>ST100<CRLF>   | get over Temperature Stop
        <STX>T1ST150<CR>  |<ACK>              | set over Temperature Stop to 150
        <STX>T1T<CR>      |<STX>T3<CRLF>      | get sensor Type

                      Example command strings and responses



                                PAGE 8



        COMMAND STRING TO | TEMP. CONTROLLER  |
        TEMP. CONTROLLER  | REPLY             |        FUNCTION
        ------------------+-------------------+-------------------------------
        <STX>T1TA<CR>     |<ACK>              | set sensor to RTD alpha .00385
        <STX>T1U<CR>      |<STX>U0<CRLF>      | get temperature units
        <STX>T1U1<CR>     |<ACK>              | set temperature units to Celsius
                          |                   |
        <STX>T1V<CR>      |<STX>V 1.00<CRLF>  | get ROM software version number
        <STX>T1W<CR>      |<ACK>              | Write all setup data to EEPROM
        <STX>T1X<CR>      |<ACK>              | eXit remote mode
                          |                   |
        <STX>T1ZK<CR>     |<ACK>              | clear Keypad status
        <STX>T1ZS<CR>     |<ACK>              | clear communication Status


                      Example command strings and responses


        4.1 - TEMPERATURE CONTROLLER DATA FIELDS

        Any  data  that the Temperature Controller sends to the PC   will
        have   a  fixed number of characters.  The number  of  characters
        sent will  depend on  the  command. Temperature data has a  fixed
        length  of  six characters, counting the decimal  point.  Leading
        zeros  in display data will be replaced with spaces to  get   the
        fixed number  of  characters needed for the data. See the command
        tables  to determine the number of characters sent for each  com-
        mand.


        4.2 - PC DATA FIELDS

        Data   that  the PC sends to the Temperature  Controller  can  be
        variable  in length.  Leading zeros or spaces can be included  or
        left   out  of the  data  field.  Data after a decimal  point  is
        ignored  if  it is greater  than the resolution specified for the
        command.  The   command  strings shown below are  all  valid  and
        would be used to set the control setpoint to 100.


                               <STX>T1SP100<CR>
                               <STX>T1SP0100<CR>
                               <STX>T1SP 100<CR>
                               <STX>T1SP+100.0<CR>













                                PAGE 9



        5.0 - COMMUNICATION STATUS REQUEST

        The communication status request command is used to determine why
        the Temperature Controller has sent a negative acknowledge  <NAK>
        in  response  to  a command.  The   Temperature  Controller  will
        respond with the following  string  in response to the I command:


                            <STX>Ix<CR>

                        x = 0 = No error
                            1 = Framing error
                            2 = Overrun error
                            3 = Invalid command
                            4 = Data out of range
                            5 = Invalid character in data
                            6 = Noise detected
                            7 = Error saving setup data



        5.1 - HANDLING COMMUNICATION ERRORS

        Whenever  a communication error occurs, its status is latched  in
        the  I command until the PC clears it with the ZS command.  If  a
        new error occurs before the status is cleared, the new error will
        overwrite  the old one. Therefore, it is not  necessary  to  send
        the ZS command after every I command. It is best to  include  the
        ability  to  resend a command a limited number of times  when   a
        communications error occurs before aborting the command. A  typi-
        cal command sequence for handling communication errors would be:

             1. Retry count = 0
             2. PC sends command to Temperature Controller
             3. If  PC receives an <ACK> or valid data, then  all
                done, else goto step 4
             4. PC receives a <NAK> from Temperature Controller or  noth-
                                                   *
                ing  after waiting 800 milliseconds .
             5. Increment retry count
             6. If retry count < 4, goto step 2, else goto step 7
             7. PC  sends I command to Temperature Controller to  try  to
                determine cause of error and reports Temperature Control-
                ler reply.
             *
               The minimum time to wait for a reply from the  Temperature
                Controller will vary with the selected BAUD rate:

                                300 BAUD = 800 mS
                                600 BAUD = 400 mS
                               1200 BAUD = 200 mS
                               2400 BAUD = 100 mS
                               4800 BAUD =  50 mS
                               9600 BAUD =  25 mS



                                PAGE 10




        6.0 - KEYPAD STATUS

        The  PC can read the Temperature Controller keypad by issuing the
        K   command.   The Temperature Controller will respond  with  one
        character  indicating  the last  key  pressed  since the last  ZK
        command.  A  key  press  is latched until the ZK command is  used
        to   reset  the  status to no keys pressed. The  following  table
        shows   the   character code returned by the K  command  and  the
        corresponding Temperature Controller key.

                        0 = No key pressed       5 = Down arrow
                        1 = Run/Stop             6 = Menu
                        2 = Setpoint             7 = Alarm
                        3 = Left arrow           8 = Tune
                        4 = Up arrow

                 Key codes for Temperature Controller K command


        7.0 - ASCII CONTROL CHARACTERS USED



          DECIMAL | HEX |       FUNCTION             | CONTROL KEY
          --------+-----+----------------------------+----------------
             2    | 02  | <STX> Start of Text        | (CTRL-B)
             6    | 06  | <ACK> Acknowledge          | (CTRL-F)
            10    | 0A  | <LF>  Line Feed            | (CTRL-J)
            13    | 0D  | <CR>  Carriage Return      | (CTRL-M)(Enter)
            17    | 11  | <XON> enable transmit      | (CTRL-Q)
            19    | 13  | <XOFF> disable transmit    | (CTRL-S)
            21    | 15  | <NAK> Negative Acknowledge | (CTRL-U)

                            ASCII control codes used






















                                PAGE 11