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 -

package main
 
import "fmt"
 
type rs232p9 uint16
 
const (
	CD9  rs232p9 = 1 << iota // Carrier detect
	RD9                      // Received data
	TD9                      // Transmitted data
	DTR9                     // Data terminal ready
	SG9                      // signal ground
	DSR9                     // Data set ready
	RTS9                     // Request to send
	CTS9                     // Clear to send
	RI9                      // Ring indicator
)
 
func main() {
	// set some nonsense bits just for example
	p := RI9 | TD9 | CD9
	fmt.Printf("Type=%T value=%#04x\n", p, p)
}