The null modem cable is frequently called a Crossover cable. It is used to allow two serial Data Terminal Equipment (DTE)
devices to communicate with each other without using a modem or a Data
Communications Equipment (DCE) device in between. For this to happen, the
Transmit (TXD) pin of one device needs to be connected to the Receive
(RXD) pin of the other device. 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. Because these pins are
"crossed" on the two cable terminals, the name crossover cable is
used.
DB9 - Null Modem Cable (Female - Female Connectors)
DB9 - Null Modem Cable (Female - Female Connectors)
Null
Modem Cable with Handshaking
Simple
Straight Through Cable
DB9 - Straight Through Cable(Male - Female Connectors)
The built-in
serial port on a PC is a DTE device. Modems and printers are examples
of DCE devices. Note that an instrument with serial interface could be
either a DTE or a DCE device. It is best to check the user manual of
the instrument to find out the device type. For more information
regarding DTE and DCE devices, please see the links below.
To tell if your cable is null modem or straight though, you can search the part number at ni.com, the product description will tell if it is null modem. Alternatively you can use a hand held 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.
To tell if your cable is null modem or straight though, you can search the part number at ni.com, the product description will tell if it is null modem. Alternatively you can use a hand held 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.
