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Should I Use Null Modem or Straight Through Serial Cables?

Updated Feb 24, 2020

Reported In

Hardware

  • Serial Cable

Issue Details

I want to connect my serial instrument to my computer’s built-in serial port or to an NI serial interface device. Should I use a null modem or straight through serial cable? 

Solution

The type of cable you should use depends on whether the device you are interfacing with is a Data Terminal Equipment (DTE) or Data Communications Equipment (DCE) device
 

Data Terminal Equipment Device

The null modem cable (crossover cable) should be used when interfacing between a serial port and a DTE device. 

With null modem cables, the transmitting device's Transmit (TX) signal is routed to the receiving device's Receive (RX) line. Similarly, the receiving device's transmission line is routed to the communicating device's receiving line. The effect of the terminal crossover is to simulate a DCE device at either end of the cable. This allows each DTE device to communicate with the other as if it is communicating with a DCE device. Here are two common null modem cable routes: 
 
Simple Null Modem Cable Route
 

To enable handshaking between the two devices, the Request to Send (RTS) pin of one device must be connected to the Clear to Send (CTS) pin of the other device. 
 
Null Modem Cable with Handshaking Route

 

Data Communications Equipment Device

The straight through cable should be used when interfacing with a Data Communications Equipment (DCE) device.  The TX-RX and RTS-CTS pins are not cross-connected in this case, hence the term straight through cable.
 
Simple Straight Through Cable Route
 
 
 

Additional Information

To tell if your NI serial cable is null modem or straight though, search for its part number in NI Serial Hardware Specifications and Features and note the cable type in the description. Alternatively, you can use a handheld DMM to test continuity on the individual pins of your serial cable. If every pin is electrically connected to the corresponding pin on the other end, i.e.: pin 1 to pin1, pin 2 to pin 2, etc. then the cable is straight through.