CNC Services Northwest

How To Make a Null Modem Cable

  1. Verify that you need a null modem cable. A null modem cable is used to connect two independent computers together. Locate the serial (RS232) port on each computer. It is a safe bet that if they are both male connectors, you need a null modem cable. If one of the ports has a female connector, either it is already set up to receive data, or you are looking at the wrong port (for example, a parallel printer port).
  2. Determine what kind of plugs you will need. There are 9-pin serial ports and there are 25-pin serial ports. Both kinds work equally well. You may be making a 25-to-25 cable, a 25-to-9 cable, or a 9-to-9 cable.
  3. Purchase some multi-conductor cable with at least seven conductors, and the appropriate female DB9 and/or DB25 connectors. Ethernet cable is readily available and works fine.
  4. Refer to the table below for pin assignments. For example, if you are making a 9-to-9 cable, wire one plug according to the leftmost column, and the other plug according to the rightmost column. If you are making a 9-to-25 cable, wire the 9-pin plug according to the leftmost column, and the 25-pin plug according to the second column from the right.
  5. If you are making a straight extension cable (not a null modem cable), just stay on the same side of the table. For example, to make a 9-to-25 straight cable, wire the 9-pin plug according to the leftmost column, and the 25-pin plug according to the second column from the left.

Fill in the wire color column for your own reference.

9 pin25 pinWire Color25 pin9 pinMeaning
23 23RX-TX
32 32TX-RX
420 66DTR-DSR
57 75GND-GND
66 204DSR-DTR
74 58RTS-CTS
85 47CTS-RTS

Note: If you got here searching for information to help you get DNC working on a Fanuc control, try www.i-logic.com/serial/FanComm.htm for good information on that subject.

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Last updated 30-Apr-2007 MBL