Crossover or "Null Modem" vs. Straight Through Serial Cable
Brief Introduction
A straight-through serial cable is the most common type of serial cable, used to connect a Data Terminal Equipment (DTE) device (for example, a computer) to a Data Communications Equipment (DCE) device (for example, a modem). The pinout of the serial connectors on both sides of the cable are exactly the same with a straight-through serial cable.
A null modem serial cable (frequently called a crossover cable) is used to connect two DTE devices together without the use of a DCE device in between. For this to happen, the Transmit (TXD) and Receive (RXD) pins on one of the serial connectors are flipped.
To confirm the type of serial cable that is required for your device or configuration, refer to the information provided by the manufacturer.
The RS-232 specification defines two types of serial devices, DTE and DCE. DTE normally refers to the serial port on a PC or terminal, while DCE refers to a modem or other type of communications device.
Depending on the type of device on each end of the line, you either need a straight-through or a null-modem ("crossover") cable in order to communicate.
Simple Straight Through Cable
A straight-through or "one to one" cable is used to connect a DTE device (PC) to a DCE device (modem or other communications device). The transmit and receive lines are not cross-connected in this case, hence the name.
Simple Null Modem Cable
Null Modem Cable with Handshake
A Null Modem or "crossover" cable is used to connect to DTE devices together. For this to work, the Transmit (TxD) pin of one device needs to be connected to the Receive (RxD) pin of the other device, and vice versa.
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, and the Data Set Ready (DSR) pin is connected to the Data Terminal Ready (DTR) pin of the other.