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RS232 DB9 pinout
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[edit] RS232 on DE9 (miscalled DB9) (EIA/TIA 574)
the original Serial connectors used DB-25 connectors, it seems people, not knowing the significance of the letter "B" (the shell size), began to call all D-sub connectors "DB" connectors. Instead of correctly saying DB-25, and DE-9 we have called all serial connectors DB-. Today "DB-9" really means DE-9. Today you will see DE-9 connectors sold as DB-9 more often than the correct terminology.
The letter after the D refers to the shell size, with DB referring to the common 25 pin size and DE for the 9 pin size. The error is so often repeated that there is no fixing it.
[edit] 
[edit] Note that the numbering is not circular!
| Pin | Name | Notes/Description |
|---|---|---|
| 1 | DCD | Data Carrier Detect |
| 2 | RD | Receive Data (a.k.a RxD, Rx) |
| 3 | TD | Transmit Data (a.k.a TxD, Tx) |
| 4 | DTR | Data Terminal Ready |
| 5 | SGND | Ground |
| 6 | DSR | Data Set Ready |
| 7 | RTS | Request To Send |
| 8 | CTS | Clear To Send |
| 9 | RI | Ring Indicator |
[edit] Null Modem Adapter
| Male | Signal | Male | Signal |
|---|---|---|---|
| 2 | RD | 3 | TD |
| 3 | TD | 2 | RD |
| 4 | DTR | 6,1 | DSR, DCD |
| 6,1 | DSR, DCD | 4 | DTR |
| 7 | RTS | 8 | CTS |
| 8 | CTS | 7 | RTS |
| 5 | SGND | 5 | SGND |
| 9 | RI | 9 | RI |
