A gender changer is a hardware device placed between two cable connectors of the same type and gender. An example is a cable connector shell with either two female or two male connectors on it (male-to-male or female-to-female), used to correct the mismatches that result when interconnecting two devices or cables with the same gender of connector.
Gender changers are used for RS-232C ports in either the original DB-25 or the IBM AT DE-9 format. They are also used when extending any sort of cable that normally has plugs on both ends (rather than a socket on one and plug on the other), however in this case it is usually called just an "extender", such as with F connectors, BNC connectors, and various RJ connectors used in telephony and computer networking.
Gender changers are used in professional audio to adapt XLR connectors, RCA connectors, Speakon connectors and TRS connectors.
Alternative names for a gender changer are "gender bender", "gender blender", and "gender mender".
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.
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