It is often useful to control the memory layout of fields in a data structure to match an interface control definition, or to interface with hardware. Define a data structure matching the RS-232 Plug Definition. Use the 9-pin definition for brevity.
 Pin Settings for Plug
 (Reverse order for socket.)
 __________________________________________
 1  2  3  4  5  6  7  8  9  10 11 12 13
  14 15 16 17 18 19 20 21 22 23 24 25
 _________________
 1  2  3  4  5
 6  7  8  9
 25 pin                        9 pin
 1 - PG   Protective ground
 2 - TD   Transmitted data     3
 3 - RD   Received data        2
 4 - RTS  Request to send      7
 5 - CTS  Clear to send        8
 6 - DSR  Data set ready       6
 7 - SG   Signal ground        5
 8 - CD   Carrier detect       1
 9 - + voltage (testing)
 10 - - voltage (testing)
 11 -
 12 - SCD  Secondary CD
 13 - SCS  Secondary CTS
 14 - STD  Secondary TD
 15 - TC   Transmit clock
 16 - SRD  Secondary RD
 17 - RC   Receiver clock
 18 -
 19 - SRS  Secondary RTS            
 20 - DTR  Data terminal ready      4
 21 - SQD  Signal quality detector
 22 - RI   Ring indicator           9
 23 - DRS  Data rate select
 24 - XTC  External clock
 25 -
