RS-232 Pinout
| RS-232 (EIA-232-A) Function |
DB-25 pin # |
DB-9 pin # |
| Shield | 1 | |
| Transmit Data (TD) from DTE to DCE | 2 | 3 |
| Receive Data (RD) from DCE to DTE | 3 | 2 |
| Request to Send (RTS) | 4 | 7 |
| Clear to Send (CTS) | 5 | 8 |
| DCE Ready (DSR) | 6 | 6 |
| Signal Ground (SG) | 7 | 5 |
| Received Line Signal Detector (DCD) | 8 | 1 |
| DTE Ready (DTR) | 20 | 4 |
| Ring Indicator | 22 | 9 |
DTE - Data Terminal Equipment (computers, terminals, PLC's, ...)
DCE - Data Communication Equipment (modems, scanners, ...)
RS-232 cable lengths
| Data Rate (bps) |
Maximum Length (meters) |
Maximum Length (feet) |
| 2400 | 120 | 400 |
| 4800 | 60 | 200 |
| 9600 | 30 | 100 |
| 19200 | 15 | 50 |
| 38400 | 7.5 | 25 |
| 57600 | 5 | 16 |
| 115200 | 2.5 | 8 |
Common interrupt numbers and port addresses
| Device |
Interrupt |
Port number |
| COM1 | IRQ 4 | 3F8h |
| COM2 | IRQ 3 | 2F8h |
| COM3 | IRQ 4 | 3E8h |
| COM4 | IRQ 3 | 2E8h |
| LPT1 | IRQ 7 | 378h |
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