Building a DTE to DTE (Null Modem) Cable

This page provides information for connecting two asynchronous serial (RS-232) Data Terminal Equipment (DTE) devices together (example: two PC's or a dumb terminal and a ImageStream router). Assuming both DTEs have DB-25 interfaces, each DTE transmits data on pin 2 and receives data on pin 3. Consequently, both DTE devices cannot transmit or receive data on the same pin.  A straight thru cable does not allow data to be transmitted or received.  Therefore, a null modem cable is required to allow both ends to transmit and receive data. 

pin 1  (GND) goes to pin 1  (GND)
pin 2  (TD)  goes to pin 3  (RD)
pin 3  (RD)  goes to pin 2  (TD)
pin 4  (RTS) goes to pin 5  (CTS)
pin 5  (CTS) goes to pin 4  (RTS)
pin 6  (DSR) goes to pin 6  (DSR)
pin 7  (GND) goes to pin 7  (GND)
pin 8  (DCD) goes to pin 20 (DTR)
pin 20 (DTR) goes to pin 8  (DCD)

  • Null modem cables and cable adapters may be purchased from any retailer handling computer products and supplies or computer games.
  • The minimum number of pin outs for a null modem cable for a basic configuration is illustrated below:
pin 2 (TD)  goes to pin 3 (RD)
pin 3 (RD)  goes to pin 2 (TD)
pin 7 (GND) goes to pin 7 (GND)
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Null Modem

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Envoy: 4.4.1-30
Others: 4.4.1-30

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