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Serial Port Signals and Pin Assignments

Serial ports consist of two signal types: data signals and control signals. To support these signal types, as well as the signal ground, the RS-232 standard defines a 25-pin connection. However, most PC's and UNIX platforms use a 9-pin connection. In fact, only three pins are required for serial port communications: one for receiving data, one for transmitting data, and one for the signal ground.

The pin assignment scheme for a 9-pin male connector on a DTE is given below.

The pins and signals associated with the 9-pin connector are described below. Refer to the RS-232 standard for a description of the signals and pin assignments used for a 25-pin connector.

Table 8-1: Serial Port Pin and Signal Assignments 
Pin
 Label
 Signal Name
 Signal Type
1
 CD
 Carrier Detect
 Control
2
 RD
 Received Data
 Data
3
 TD
 Transmitted Data
 Data
4
 DTR
 Data Terminal Ready
 Control
5
 GND
 Signal Ground
 Ground
6
 DSR
 Data Set Ready
 Control
7
 RTS
 Request to Send
Control
8
 CTS
 Clear to Send
 Control
9
 RI
 Ring Indicator
 Control

The term "data set" is synonymous with "modem" or "device," while the term "data terminal" is synonymous with "computer."

Signal States

Signals can be in either an active state or an inactive state. An active state corresponds to the binary value 1, while an inactive state corresponds to the binary value 0. An active signal state is often described as logic 1, on, true, or a mark. An inactive signal state is often described as logic 0, off, false, or a space.

For data signals, the "on" state occurs when the received signal voltage is more negative than -3 volts, while the "off" state occurs for voltages more positive than 3 volts. For control signals, the "on" state occurs when the received signal voltage is more positive than 3 volts, while the "off" state occurs for voltages more negative than -3 volts. The voltage between -3 volts and +3 volts is considered a transition region, and the signal state is undefined.

To bring the signal to the "on" state, the controlling device unasserts (or lowers) the value for data pins and asserts (or raises) the value for control pins. Conversely, to bring the signal to the "off" state, the controlling device asserts the value for data pins and unasserts the value for control pins.

The "on" and "off" states for a data signal and for a control signal are shown below.

The Data Pins

Most serial port devices support full-duplex communication meaning that they can send and receive data at the same time. Therefore, separate pins are used for transmitting and receiving data. For these devices, the TD, RD, and GND pins are used. However, some types of serial port devices support only one-way or half-duplex communications. For these devices, only the TD and GND pins are used. In this guide, it is assumed that a full-duplex serial port is connected to your device.

The TD pin carries data transmitted by a DTE to a DCE. The RD pin carries data that is received by a DTE from a DCE.

The Control Pins

9-pin serial ports provide several control pins that:

The control pins include RTS and CTS, DTR and DSR, CD, and RI.

The RTS and CTS Pins.   The RTS and CTS pins are used to signal whether the devices are ready to send or receive data. This type of data flow control - called hardware handshaking - is used to prevent data loss during transmission. When enabled for both the DTE and DCE, hardware handshaking using RTS and CTS follows these steps:

  1. The DTE asserts the RTS pin to instruct the DCE that it is ready to receive data.
  2. The DCE asserts the CTS pin indicating that it is clear to send data over the TD pin. If data can no longer be sent, the CTS pin is unasserted.
  3. The data is transmitted to the DTE over the TD pin. If data can no longer be accepted, the RTS pin is unasserted by the DTE and the data transmission is stopped.

To enable hardware handshaking in MATLAB, refer to Controlling the Flow of Data: Handshaking.

The DTR and DSR Pins.   Many devices use the DSR and DTR pins to signal if they are connected and powered. Signaling the presence of connected devices using DTR and DSR follows these steps:

  1. The DTE asserts the DTR pin to request that the DCE connect to the communication line.
  2. The DCE asserts the DSR pin to indicate it's connected.
  3. DCE unasserts the DSR pin when it's disconnected from the communication line.

The DTR and DSR pins were originally designed to provide an alternative method of hardware handshaking. However, the RTS and CTS pins are usually used in this way, and not the DSR and DTR pins. However, you should refer to your device documentation to determine its specific pin behavior.

The CD and RI Pins.   The CD and RI pins are typically used to indicate the presence of certain signals during modem-modem connections.

CD is used by a modem to signal that it has made a connection with another modem, or has detected a carrier tone. CD is asserted when the DCE is receiving a signal of a suitable frequency. CD is unasserted if the DCE is not receiving a suitable signal.

RI is used to indicate the presence of an audible ringing signal. RI is asserted when the DCE is receiving a ringing signal. RI is unasserted when the DCE is not receiving a ringing signal (for example, it's between rings).


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