Since there is no DCE to raise CTS, the outgoing RTS signal is looped in the NULL modem cable to the incoming CTS to satisfy the DTE’s need for this signal. For example, when the DTE raises Request to Send (RTS) it typically expects Clear to Send (CTS) from the DCE. This is called a NULL (no) modem configuration. When PCs are connected back-to-back each end is acting as a DTE (there is no DCE in this case) and consequently certain signals may have to be looped in the connection to satisfy any input signal requirement. In all the pinout diagrams below the signal direction is with respect to the DTE (PC) end. RS-232 signals have a direction (in or out) depending on whether they are with respect to a DTE or a DCE. In serial communications the terminal end (PC) is called the Data Terminal Equipment (DTE) and the modem end is called the Data Communications Equipment (DCE) as shown in the diagram below. RS232 is an Active LOW voltage driven interface and operates at +12V to -12V where: RS-232 is practically identical to ITU V.24 (signal description and names) and V.28 (electrical). RS-232 defines both the physical and electrical characteristics of the interface. RS-232 standards(TIA-232) are defined by TIA (Telecommunications Industry Association).
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