DTE & DCE / DB25 & DB9 Port

DTE & DCE / DB25 & DB9 Port

Let's talk about DCE (Data Communications Equipment) and DTE (Data Terminal Equipment) devices. The difference between DCE and DTE is largely in the Plug and the direction if each pin (input or output). Your desktop PC is termed as a DTE device.

DCE devices use a 25-pin female connector while a DTE device uses a 25 pin male connector. Also, complimentary signals lines like transmit and receive are "swapped" between the two types. Thus, a straight-through cable can be used to connect a DCE device to a DTE device.

DTE	DCE
25 pin Male pinout	25 pin Female pinout
Pin 1 - Shield Ground	Pin 1 - Shield Ground
Pin 2 - Transmitted Data (TD) output	Pin 2 - Transmitted Data (TD) input
Pin 3 - Receive Data (RD) input	Pin 3 - Receive Data (RD) output
Pin 4 - Request To Send (RTS) output	Pin 4 - Request To Send (RTS) input
Pin 5 - Clear To Send (CTS) input	Pin 5 - Clear To Send (CTS) output
Pin 6 - Data Set Ready (DSR) input	Pin 6 - Data Set Ready (DSR) output
Pin 7 - Signal Ground	Pin 7 - Signal Ground
Pin 8 - Carrier Detect (CD) input	Pin 8 - Carrier Detect (CD) output
Pin 20 - Data Terminal Ready (DTR) output	Pin 20 - Data Terminal Ready (DTR) input
Pin 22 - Ring Indicator (RI) input	Pin 22 - Ring Indicator (RI) output

You can effectively convert DCE/DTE devices by using a NULL Modem cable. The null modem cable swaps the complimentary signals and allows a DCE device to act like a DTE and vice-versa.

The following chart depicts the cabling of the DB9 connector found on an IBM-PC type computer.

DTE

9 pin Male pinout

Pin 1 - Carrier Detect (CD) input

Pin 2 - Receive Data (RD) input

Pin 3 - Transmitted Data (TD) output

Pin 4 - Data Terminal Ready (DTR) output

Pin 5 - Signal Ground

Pin 6 - Data Set Ready (DSR) input

Pin 7 - Request To Send (RTS) output

Pin 8 - Clear To Send (CTS) input

Pin 9 - Ring Indicator (RI) input

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