Serial cable is used to interconnect DTEs and DCEs. The DCEs are data communication equipment that generated or received data, and the DCEs are data communication equipment that only relayed data. There are two different types of cables, one for connecting a DTE to a DCE, and another for connecting two DTEs directly to each other. The DTE/DCE interface for a particular standard defines the standards including the number of pins and connector types, voltage levels for 0 and 1, defines the function for each of the signalling lines in the interface and sequence of events for transmitting data.
Figure-1 illustrates the original RS-232 standard that was defined as the connection between DTEs and DCEs (modems). However, we have also the requirement to connect two DTEs, for example, two routers or two computers. For this purpose, a special cable called a null modem can be used which eliminates the requirement of DCEs. With a null-modem connection, the Trans and receive lines are cross-linked between both ends of the cable. Figure 2 illustrates the null-modem cable.