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This page documents the pin assignments for the RS-232 standard using DB9 and DB25 connectors. Following the table is a set of diagrams to assist in identifying what pin numbers are assigned to each pin inside the different connectors.
| DB-25 | DCE | DB-9 | |||
| 1 | AA | x | Protective Ground | ||
| 2 | TXD | 3 | BA | I | Transmitted Data |
| 3 | RXD | 2 | BB | O | Received Data |
| 4 | RTS | 7 | CA | I | Request To Send |
| 5 | CTS | 8 | CB | O | Clear To Send |
| 6 | DSR | 6 | CC | O | Data Set Ready |
| 7 | GND | 5 | AB | x | Signal Ground |
| 8 | CD | 1 | CF | O | Received Line Signal Detector |
| 9 | -- | x | Reserved for data set testing | ||
| 10 | -- | x | Reserved for data set testing | ||
| 11 | x | Unassigned | |||
| 12 | SCF | O | Secndry Rcvd Line Signl Detctr | ||
| 13 | SCB | O | Secondary Clear to Send | ||
| 14 | SBA | I | Secondary Transmitted Data | ||
| 15 | DB | O | Transmisn Signl Elemnt Timng | ||
| 16 | SBB | O | Secondary Received Data | ||
| 17 | DD | O | Receiver Signal Element Timing | ||
| 18 | x | Unassigned | |||
| 19 | SCA | I | Secondary Request to Send | ||
| 20 | DTR | 4 | CD | I | Data Terminal Ready |
| 21 | CG | O | Signal Quality Detector | ||
| 22 | 9 | CE | O | Ring Indicator | |
| 23 | CH/CI | I/O | Data Signal Rate Selector | ||
| 24 | DA | I | Transmit Signal Element Timing | ||
| 25 | x | Unassigned |
| Connector | FEMALE (Holes) | MALE (Pins) |
| 25 pin DB25 |  |
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| 9 pin DB9 |  |
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