Null Modem
In serial communications, there exists a type of adapter that is similar in appearance to a gender changer, and in fact may provide the function of changing the gender, but is not the same thing. This device is called a null modem. Despite the word modem appearing in the name, it is not a modem. Rather, it is a device with the appearance of a gender changer that imitates the presence of a modem.
A normal gender changer simply changes the gender without rerouting any pins. Therefore, pin 1 connects to pin 1 on the other side, pin 2 connects to 2, pin 3 connects to 3, and so forth.
A null modem reroutes wiring so that two serial devices may connect directly to one another without any communications hardware (see RS-232). For example, a null modem crosses pins 2 and 3, so that pin 2 on one side is connected to pin 3 on the other. This causes the "transmit" from each side to be routed into the "receive" side of the other, similar to how a crossover cable is used in Ethernet to connect two devices directly without a hub or switch. Two to three other pairs are similarly crossed in a null modem.
A null modem may also act as a gender changer if it has the same gender connector on both sides. Other null modems have a different gender on each side. It is important to recognize that a null modem used in place of a gender changer (or a gender changer used in place of a null modem) will result in an inoperable connection, even if the connectors physically fit.
Because of the significance of this distinction, gender changers and null modems for serial communications are typically labeled with the words Gender Changer or Null Modem to clarify the internal wiring.
Read more about this topic: Gender Changer
Famous quotes containing the word null:
“A strong person makes the law and custom null before his own will.”
—Ralph Waldo Emerson (1803–1882)