page_title: Null modem
section_title: Wiring diagrams
hierarchical_section_title: Null modem / Cables and adapters / Wiring diagrams
caption_attribution_description: English: D25 to D25 null-modem cable wiring diagram
context_page_description: Null modem is a communication method to directly connect two DTEs using an RS-232 serial cable. The name stems from the historical use of RS-232 cables to connect two teleprinter devices or two modems in order to communicate with one another; null modem communication refers to using a crossed-over RS-232 cable to connect the teleprinters directly to one another without the modems.
  It is also used to serially connect a computer to a printer, since both are DTE, and is known as a Printer Cable.
  The RS-232 standard is asymmetric as to the definitions of the two ends of the communications link, assuming that one end is a DTE and the other is a DCE, e.g. a modem. With a null modem connection the transmit and receive lines are crosslinked. Depending on the purpose, sometimes also one or more handshake lines are crosslinked. Several wiring layouts are in use because the null modem connection is not covered by the RS-232 standard.
context_section_description: Below is a very common wiring diagram for a null modem cable to interconnect two DTEs (e.g. two PCs) providing full handshaking, which works with software relying on proper assertion of the Data Carrier Detect (DCD) signal:
caption: DB-25 null modem wiring diagram
exif: {}