2.2 Identifying RS-232 Connections

Basics

There are two types of RS-232 connectors: 1) DB9, and 2) DB25. The “D” refers to the general shape of the connector and the number refers to the number of pins each supports. You will learn about identifying these and what the pins are responsible for.

You should know the following terms:

  • DCE (Data Communication Equipment)
  • DTE (Data Terminal Equipment)
  • Multimeter
  • RS-232 Connectors: DB9, DB25
  • Signal Functions: 
    • CD (Carrier Detected), 
    • CTS (Clear to Send), 
    • DSR (Data Set Ready), 
    • DTR (Data Terminal Ready), 
    • GND (Ground)
    • RTS (Request to Send), 
    • RXD (Receive Data), and
    • TXD (Transmit Data), 

Pins are assigned within the DTE (Data Terminal Equipment) and DCE (Data Communication Equipment) so that pin number X at one end is connected to pin number X at the other end and there are no crossovers. For example, DTE transmits data on pin 2 and the DCE receives data on pin 2. Similarly, the DTE receives data on pin 3, whereas the DCE transmits on pin 3.

Image Credit: https://www.virtual-serial-port.org/article/what-is-serial-port/rs232-pinout/


NOTE: The 25-pin connector has some pins that are not used for data transmission. These pins are mainly used for loop-back testing of the port. Also notice that the designations for pins 2 and 3 on the 9-pin connector are the exact opposite of what they are for pins 2 and 3 on the 25-pin connector.

How do you tell whether the DB25 connector is DTE or DCE?

Measure the DC voltages between (DB25) pins 2 & 7 and between pins 3 & 7. Be sure the black lead is connected to pin 7 (Signal Ground) and the red lead to whichever pin you are measuring.

If the voltage on pin 2 is more negative than -3 Volts, then it is a DTE, otherwise, it should be near zero volts. If the voltage on pin 3  is more negative than -3 Volts, then it is a DCE.

If both pins 2 & 3 have a voltage of at least 3 volts, then either you are measuring incorrectly, or the device is not a standard EIA-232 device.

In general, a DTE provides a voltage on TXD (Transmit Data), RTS (Request to Send), & DTR (Data Terminal Ready) pin signal functions. A DCE provides voltage on RXD (Receive Data), CTS (Clear to Send), DSR (Data Set Ready), & CD (Carrier Detected) pin signal functions.

If you have to repair a serial cable or make a new one, it is important to be able to identify the ground, the RXD (Receive Data), and the TXD (Transmit Data) pins. Watch the video below to learn how to use a multimeter to identify the wires in a serial cable.

Here are additional resources you may find useful:

Complete the following task or self-assessment:

Identify the types of connectors your RS-232 cables use that you’ve found in your building.

If you have access to a multimeter, test different RS-232 connectors to become familiar with how the meter operates.