| 9 pin # | 25 pin# | Acronym | Full name | Direction | Description |
|---|---|---|---|---|---|
| 3 | 2 | TxD | Transmit Data | → | Transmits bytes out of computer or device |
| 2 | 3 | RxD | Receive Data | ← | Receives bytes into computer or device |
| 7 | 4 | RTS | Request To Send | → | RTS/CTS flow control |
| 8 | 5 | CTS | Clear To Send | ← | RTS/CTS flow control |
| 6 | 6 | DSR | Data Set Ready | ← | This device is ready to communicate |
| 4 | 20 | DTR | Data Terminal Ready | → | This device is ready to communicate |
| 1 | 8 | DCD | Data Carrier Detect | ← | Modem connected to another |
| 9 | 22 | RI | Ring Indicator | ← | Ringing on telephone line |
| 5 | 7 | SG | Signal Ground |
If devices RTS and CTS or DTR and DSR pins are connected then it means that the “hardware” flow control can be used. We have already described it in the Flow Control chapter but the pins and voltage signals were not. Advanced Serial Port Monitor supports both RTS/CTS and DTR/DSR flow controls. Both flow control modes work the same way. To get RTS/CTS flow control to work you need to select a corresponding option in the Advanced Serial Port Monitor settings.
Then when a DTE (such as a computer) wants to stop the data sending into it, it sets RTS to LOW. The low state of the RTS (Request To Send) signal equal to -12V means “Do not send to me” (stop sending). When the computer is ready to receive some bytes it sets RTS to HIGH (+12V) and the flow of bytes to it resumes. Flow control signals are always sent in a direction opposite to the flow of bytes that is being controlled. DCE equipment (modems) works the same way but sends the stop signal out the CTS pin. If you don’t need the flow control then you may not connect corresponding pins. In the most simple case you may connect 3 pins only.
RS232 pinout and signals
Serial port pinout and signals
DB9 pinout and signals
DB25 pinout and signals