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.

Rs232 db9M.gif

Source for cheap cables

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


Null Modem Adapter

Null modem cable


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

Serial debugging software

We all used to suffer with HyperTermanal but today there is a much better option - ctrlterm. pretty much platform independent, free and easily modifiable for custom testing, debugging work. http://ctrlterm.com/

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