Figure 9-9 Principal RS-232-C elect. standards      

 Vo<25 V
 Maximum short circuit current to any wire in cable - 0.5 A
 MARK signal at load < -3 V
 SPACE signal at load < +3 V
 MARK signal out of driver < -5 V and > -15 V
 SPACE signal out of driver > +5 V and < +15 V
 Rl < 7000 ohms when measured with a voltage from 3 to 25 V,
           but > 3000 ohms
 Cl including line capacitance < 2500 pF
 When El=0.5 V < Vi < 15 V
      Ro > 300 ohms under power off conditions
      Co is such that slew rate of the driver's output
         voltage is < 30 V/microsecond, but the transition
         between -3 V and +3 V does not exceed the smaller
         of 1 ms or 4% of the bit time       


         Figure 9-11 Summary of RS-232-C control line definitions 

   Symbol         Pin         Name                      Description

  EIA (CCITT)      1      Protective Ground      Used as an equipment ground

  AA (101)         7      Signal Ground          Used as a common ground
                                                  for all signals

  BA (103)         2      Transmitted Data       For outputting data to modem

  BA (104)         3      Received Data          For inputting data from modem    

  CA (105)         4      Request to Send        To modem - to turn the modem's
                                                  transmitter on and off when
                                                  operating in half duplex

  CB (106)         5      Clear to Send          From modem - indicates modem
                                                  is ready to transmit     

  CC (107)         6      Data Set Ready         From modem - indicates that modem
                                                  power is on and the modem is not
                                                  in test mode

  CF (109)         8      Received Line          From modem - indicates the modem 
                           Signal Detector        is receiving a signal from the 
                                                  modem at the other end of the link

  CD (108/2)      20      Data Terminal Ready    To modem - prepares the modem to
                                                  be connected to the communication 
                                                  link and begin transmitting

  CE (125)        22      Ring Indicator         From modem - indicates ringing 
                                                  signal is detected on link

  CG (110)        21      Signal Quality         From modem - active when there is a
                           Detector               low probability of error in the 
                                                  received data

  CH (111)        23      Data Signal Rate       To modem - indicates to the modem
                           Selector(DTE Source)   one of two synchronous data rates
                                                  or ranges of rates

  CH (112)        23      Data Signal Rate       From modem - indicates to interface
                           Selector(DTE Source)   or terminal one of two synchronous
                                                  data rates or ranges of rates

  DA (113)        24      Transmitter Signal     To modem - provides modem 
                           Element Timing         transmitter with signal timing
                           (DTE) Source  

  DB (114)        15      Transmitter Signal     From modem - provides interface or 
                           Element Timing         terminal with transmitter signal
                           (DTE) Source           timing

  DD (115)        17      Receiver Signal        From modem - provides interface or 
                           Element Timing         terminal receiver with signal
                                                  timing

  SBA (118)       14      Secondary Transmitted  To modem - for outputting low 
                           Data                   rate data  
 
  SBB (119)       16      Secondary Received     From modem - for inputting low 
                           Data                   rate data  

  SCA (120)       19      Secondary Request      To modem - to turn on modem's
                           to Send                secondary transmitter   

  SCB (121)       13      Secondary Clear        From modem - indicates secondary
                           to Send                transmitter is ready              

  SCF (122)       12      Secondary Received     From modem - indicates signal is
                           Line Signal Detector   detected on secondary link