Connecting Two Devices with a Serial Cable :: Serial Port I/O (External Interfaces)

External Interfaces

Connecting Two Devices with a Serial Cable

The RS-232 standard defines the two devices connected with a serial cable as the Data Terminal Equipment (DTE) and Data Circuit-Terminating Equipment (DCE). This terminology reflects the RS-232 origin as a standard for communication between a computer terminal and a modem.

Throughout this guide, your computer is considered a DTE, while peripheral devices such as modems and printers are considered DCE's. Note that many scientific instruments function as DTE's.

Because RS-232 mainly involves connecting a DTE to a DCE, the pin assignments are defined such that straight-through cabling is used, where pin 1 is connected to pin 1, pin 2 is connected to pin 2, and so on. A DTE to DCE serial connection using the transmit data (TD) pin and the receive data (RD) pin is shown below. Refer to Serial Port Signals and Pin Assignments for more information about serial port pins.

DTE to DCE serial connection using the transmit data (TD) pin and the receive data (RD)

If you connect two DTE's or two DCE's using a straight serial cable, then the TD pin on each device are connected to each other, and the RD pin on each device are connected to each other. Therefore, to connect two like devices, you must use a null modem cable. As shown below, null modem cables cross the transmit and receive lines in the cable.

Figure showing computer to computer connection

Note You can connect multiple RS-422 or RS-485 devices to a serial port. If you have an RS-232/RS-485 adaptor, then you can use the MATLAB serial port object with these devices.

The Serial Port Interface Standard Serial Port Signals and Pin Assignments