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Serial Port Pin Configurations

Document Version 1.5

On this page:

  1. DB-25 and DB-9 diagrams.
  2. DB-25 and DB-9 numbering scheme.
  3. DB-25 and DB-9 Null-Modem pinouts.
  4. DB-25 to DB-9 Null-Modem pinouts.
  5. Wire name for each pin number.

Here are several diagrams and tables explaining serial port and null-modem configurations. If you are seeking further detail regarding serial port hardware settings, then find out More about com ports.

A serial port can consist of either a 25 pin port adapter called a DB-25 or 9 pin adapter called a DB-9 port adapter. Whether the port is a 9 pin or 25 pin it can accomplish all of the same tasks that serial port communications have been designed for.

Each adapter can be a male type connector or a female type adapter. Generally a com port on the back of a computer is male for the serial ports but it may not necessarily be. Below are diagrams of a DB-25 and DB-9. (Each diagram on this page is the view you see when you look into the end of the cable from the outside of the cable.)

The "o" characters represent holes, the "." characters represent pins.
A DB-25 looks like this... 

Diagram #1
Female:                        Male:
 
_____________________________  _____________________________
\ o o o o o o o o o o o o o /  \ . . . . . . . . . . . . . /
 \ o o o o o o o o o o o o /    \ . . . . . . . . . . . . /
  -------------------------      -------------------------


And a DB-9 is like this... 

Diagram #2
Female:        Male:
_____________  _____________
\ o o o o o /  \ . . . . . /
 \ o o o o /    \ . . . . /
  ---------      ---------


Pin numbers

Each pin has a number assigned to it. When connecting null modem, for example, it is important to know these numbers in order to select the correct cables, or when making your own cables. Image of the cable I made.

DB-25 Connector
Chart #1 
  13              <------- 1 
 _____________________________
 \ o o o o o o o o o o o o o /  
  \ o o o o o o o o o o o o /
   -------------------------
    25             <----- 14
Chart #2 
  1 ------->              13      
 _____________________________
 \ . . . . . . . . . . . . . /
  \ . . . . . . . . . . . . /
   -------------------------
   14 ----->             25
DB-9 Connector
Chart #3 
   5 4 3 2 1
 _____________
 \ o o o o o /
  \ o o o o /
   ---------
    9 8 7 6
Chart #4 
   1 2 3 4 5
 _____________
 \ . . . . . /
  \ . . . . /
   ---------
    6 7 8 9

Null Modem

If you are seeking to buy a null modem adapter, or trying to make your own cable, you should know what pins need to be switched in order to make null modem. Below is a chart of what pins go to what on the other end. Only 8 pins are used in a null modem, therefore I will only show those eight here.

Chart #5
Chart #6
Chart #7
Null-Modem Pinouts
DB-25 to DB-25
2 to 3
3 to 2
4 to 5
5 to 4
6 and 8 to 20
7 to 7
20 to 6 and 8
Null-Modem Pinouts
DB-9 to DB-9
1 and 6 to 4
2 to 3
3 to 2
4 to 1 and 6
5 to 5
7 to 8
8 to 7
Null-Modem Pinouts
DB-25 to DB-9
2 to 2
3 to 3
4 to 8
5 to 7
6 and 8 to 4
7 to 5
20 to 6 and 1


Below translates the pin number with the wire name.

Chart #8 Chart #9
Pin assignments
DB-25
1 Not Used
2 Transmit Data
3 Receive Data
4 Request to send
5 Clear to send
6 Data Set Ready
7 Signal Ground
8 Data Carrier Detect
9 Not Used
10 Not Used
11 Not Used
12 Not Used
13 Not Used
14 Not Used
15 Not Used
16 Not Used
17 Not Used
18 Not Used
19 Not Used
20 Data Terminal Ready
21 Not Used
22 Ring Indicator
23 Not Used
24 Not Used
25 Not Used
Pin Assignments
DB-9
1 Data Carrier Detect
2 Receive Data
3 Transmit Data
4 Data Terminal Ready
5 Signal Ground
6 Data Set Ready
7 Request to Send
8 Clear to Send
9 Ring Indicator

For further questions, spelling corrections, or other, mail Acclaim@geocities.com.

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