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.
Simple Null Modem Cable
Null Modem Cable with Handshaking
A straight-through cable is used to connect a DTE device to a DCE device. The TXD-RXD and RTS-CTS pins are not cross-connected in this case, hence the term straight through cable.
Simple Straight Through Cable
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.
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