Q: What is a “null modem cable”?
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.