The RS232 connector was initially developed to use 25 pins. In this DB25 connector pin configuration, provisions were provided for a secondary serial RS232 communication channel. However, practically only one serial communication channel with handshaking is present. There were hardly a few computers manufactured in which provision for both RS232 channels were available. For example : Sun SparcStation 10/20 model and  Dec Alpha Multia. When we talk about a loop back connector, we can say that a loop back connector normally contains only a connector. No external cable is required in it. It only consists inner configuration of wires so that the same signal can be retransmitted to the sender.

This DB9 female connector can be easily attached with a DTE system. When the loop back connector is used, the signal is received back by DTE which has originated it and it will not be able to distinguish even the source of the signal originator. It means that the DTE will not be able to make out that the signal which has been received by it is his own or from other source (DCE).

Now the question comes that if it resend the signal back to the sender, then what is the use of it. The answer is hidden in its basic property i.e. a Loop back connector is used to ascertain whether the serial port of the system is working properly or not. post checking the correctness of the functions of your system’s serial port, you can go ahead with connection of the your serial cable. If you further want to check your serial cable also, then you just have to connect the loop back connector at the cable end and repeat the signal verification.

db9 loopback

As shown in the above mentioned figure, following pins are required to be interconnected to make a loop back connector.

  • Pin no 2 (Receive) and Pin no 3 (transmit)
  • Pin no 7 (Request to send) and Pin no 8 (Clear to send)
  • Pin no 1 (Data carrier detect), Pin no 6 (Data set ready) and Pin no 4 (Data terminal ready)

The below mentioned diagram is the aft view of the female DB-9 connector  in which the red lines and gray dots indicates  the interconnections to be provided.

Loopback db 9

DB-9 loopback has been exclusively designed for data signal testing in RS 232.

To know about RS 232 in detail, visit