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 |