Serial cable is used to interconnect DTEs and DCEs. The DCEs are data communication equipment that generates or receives data, and the DCEs are data communication equipment that only relays data. There are two types of cables: one for connecting a DTE to a DCE and another for connecting two DTEs directly. 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, the function for each of the signaling lines in the interface, and the sequence of events for transmitting data.
Figure 1 illustrates the original RS-232 standard, defined as the connection between DTEs and DCEs (modems). However, we also need 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.
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