We only use 4 wires (pwr, common, and two data), so why does rs232 use such a big connector?

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    \$\begingroup\$ Because it's smaller than the original DB-25 connector? \$\endgroup\$ – Dave Tweed Aug 29 '18 at 23:11
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    \$\begingroup\$ It is an old, old connector. It does NOT use only 4 contacts. There are other signals which were important in the old days but less so now. Check wikipedia for a list of all the signals. \$\endgroup\$ – mkeith Aug 29 '18 at 23:18
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    \$\begingroup\$ Pwr? RS232 doesn’t have a power pin. \$\endgroup\$ – Blair Fonville Aug 29 '18 at 23:57
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    \$\begingroup\$ The 9 pin connector is a DE-9. The DB shell normally carries 25 contacts. </pedant> \$\endgroup\$ – Peter Bennett Aug 29 '18 at 23:57
  • \$\begingroup\$ @BlairFonville, yes, some DB9 connectors do have one of the pins dedicated to supplying power .... that configuration is used mostly in retail environments .... used to power barcode scanners and debit terminals, etc... \$\endgroup\$ – jsotola Aug 30 '18 at 2:35

why does rs232 use such a big connector?

Assuming you mean the standard 25-pin connector, then there is a simple answer: The RS-232 specification states the defined function for every one of the 25 pins bar three. There are lots of features of RS-232 which are little used these days!

This diagram is a brief overview of the pinout of the 25-pin connector:

25-pin RS-232 connector showing usage of each pin

(Figure above courtesy of Dallas Semiconductor Application Note 83, Fundamentals of RS–232 Serial Communications)

Why do we use a DB9 connector for serial com?

It's a similar explanation for the 9-pin connector made popular by its use on the original IBM PC/AT (the earlier PC and PC/XT used the full 25-pin RS-232 connector). All 9 pins have a defined function, whether you use that function or not.

(Strictly, it's a DE-9 connector, not a DB-9. The first letter, D, refers to the D-shaped connector's metal shell. The 2nd letter is the size of the connector shell, and this smaller connector is size "E", whereas the larger 25-pin connector is size "B"; other sizes exist too. See the Wikipedia article on the D-subminiature connectors for more details.)

9-pin RS-232 connector showing usage of each pin

(Figure above also courtesy of Dallas Semiconductor Application Note 83, Fundamentals of RS–232 Serial Communications)

We only use 4 wires (pwr, common, and two data)

In fact there is no power pin in the RS-232 specification. Of course people can adapt an RS-232 connector to include a power connection, but then it doesn't fully meet the RS-232 specification.


RS232 specifies a number of out of band control signals as well, which were highly useful when devices of varying capabilities, often with limited memory, processing power, or other resources, needed to reliably interoperate with flow control and the need to avoid dropped characters.

As it turns out, there were nine signals specified within the DB9 serial standard, including a ground. The list of signals included handshaking signals such as DTR and DSR, flow control signals (RTS and CTS), an out-of-band ring indicator that was useful for modems, and (of course) the actual data signals themselves, as well as two grounds. They mapped nicely to the nine pins of a DB9 connector when one of the grounds was removed (RTR and RTS shared the same pin and only one was in use at a time). The DB9 connector and serial cable did not supply power (but as pericynthion mentioned in the comments, some devices tried to steal power from the RTS pin).

The fact that only TXD+RXD+Power+Ground are used today is likely related to the availability of more capable hardware with larger transmit/receive buffers and higher clock speeds, which meant that data could be sent with less handshaking requirements. Modems have also fallen out of fashion, so we also don't see many of the issues with flow control that they originally required.

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    \$\begingroup\$ There's no standard power pin defined for the DE-9 connector in the RS-232 / TIA-232-F pinouts. Some devices sketchily try to steal power from RTS but that can't really be relied upon. \$\endgroup\$ – pericynthion Aug 29 '18 at 23:13
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    \$\begingroup\$ I see 11 signals listed, including two grounds. \$\endgroup\$ – Dave Tweed Aug 29 '18 at 23:13
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    \$\begingroup\$ @DaveTweed Right, there are two signals that share DB25 pin 4 and two grounds, one of which was removed for DB9. I've updated the answer. \$\endgroup\$ – Andrey Akhmetov Aug 29 '18 at 23:38

We only use 4 wires (pwr, common, and two data)

As has been pointed out, this was not always the case, although the extent of signal pruning has been significantly understated.

so why does rs232 use such a big connector?

What do you mean, "such a big connector"? 9 pins? This only started on the IBM AT. Back in the dark ages (say, the 1980s) RS232 connectors were normally 25 pin connectors, and a common part of PC construction was a 25 pin to 9 pin adapter cable for running from the input connector on the chassis to the AT-compatible header on the serial board or motherboard. The original RS232 spec (ca 1969) called for 22 signals. See, for instance, https://www.camiresearch.com/Data_Com_Basics/RS232_standard.html#anchor1155404 for a discussion of all the pins. Here is an illustration


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