The RS-232 standard has several major omissions. It does not specify the connector that should be used on the DTE if the required cable is detachable, and it does not specify how to connect two DTEs directly.
If the cable between the DTE and the DCE is detachable from the DTE, the standard only specifies the connector for the DCE end of the cable: a male 25 pin D shell connector. The connector for the DTE end of the cable is not specified. The RS-232 port on an IBM AT has a male 9 pin D shell connector. This is not in violation of the standard, so long as a cable is provided that makes a 25 pin D shell male connector available at the DCE end of the cable. The RS-232 port on a Hewlett-Packard 98626 Serial Interface has yet another type of connector.
It is however, a de-facto (i.e., "non-standard") standard to use a 25 pin D shell male connector on the DTE. Thus, the DTE to DCE cable has a female connector on one end, a male connector on the other, and corresponding pins in the connectors are connected "straight through:" pin 2 to pin 2, pin 3 to pin 3, etc.
While the RS-232 standard describes a straight forward method for connecting a DTE to a DCE (modem), it does not describe how to connect one DTE directly to another. Unfortunately, many people would like to connect a computer directly to an instrument — both DTEs. Individual manufacturers have addressed this problem in different ways, and have inadvertently created a lot of confusion. The general approach is to make each DTE think it is communicating with a DCE, thus preserving adherence to the RS-232 standard as much as possible. This can be done in several different ways, none of which are completely compatible with the standard.
The problem is that the standard assumes the communication channel is always slower than the DTE. Thus handshaking is present to prevent the DTE from writing too quickly to the DCE, but the standard contains no means for preventing the DCE from writing too quickly to the DTE. The only signal available for the DTE to use to tell the DCE it is not ready to accept data is DTR. But the standard specifies that "the OFF condition [of DTR] causes the DCE to be removed from the communication channel." Obviously, character handshaking with DTR is going to cause problems if the DCE hangs up between every character. Thus, although DTR is commonly used for handshaking, that use is not completely compatible with the standard.
A less common approach to hooking two DTEs together is to implement a full DCE interface in the instrument, plotter, or printer even though it is a data terminating equipment. Again, this is not completely compatible with the standard. The device must always be connected locally to the controlling computer (the DTE), but a standard DTE to DCE cable can then be used.
