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Electronics => Beginners => Topic started by: giacomo on April 20, 2021, 10:30:48 pm

Title: RS232 Differences/Advantages between D-Sub 9 and 25
Post by: giacomo on April 20, 2021, 10:30:48 pm
Hello everyone, i wanted to ask the difference/advantages between the RS232 that uses the D-Sub 25 pin connector and the 9 pin version
Title: Re: RS232 Differences/Advantages between D-Sub 9 and 25
Post by: mariush on April 20, 2021, 10:51:40 pm
See the pinouts on this page, along with descriptions and text: https://www.virtual-serial-port.org/article/what-is-serial-port/rs232-pinout/ (https://www.virtual-serial-port.org/article/what-is-serial-port/rs232-pinout/)

Title: Re: RS232 Differences/Advantages between D-Sub 9 and 25
Post by: TK on April 20, 2021, 10:54:43 pm
Hello everyone, i wanted to ask the difference/advantages between the RS232 that uses the D-Sub 25 pin connector and the 9 pin version
What devices are you connecting, what distance?
Title: Re: RS232 Differences/Advantages between D-Sub 9 and 25
Post by: giacomo on April 20, 2021, 11:00:37 pm
I have some old instruments with the 25 pin connector, a 25 to 9 pin cable would do the job. Just a couple of meters.

It was just curiosity.
Title: Re: RS232 Differences/Advantages between D-Sub 9 and 25
Post by: mvs on April 20, 2021, 11:15:26 pm
RS232 is very old interface. DB25 connector was original one with some additional signals and reserved pins. DB9 came later, with a rise of the pesonal computers in eighties.
8250 and 16550 UARTs, and obviously later SuperIO chips were able to serve only 8 signals used in DB9. DB25 connectors were used for physical compatibility only and had no advanteges over DB9. I doubt you will find any working equipment with RS232 that use more signals then DB9 can carry.
Title: Re: RS232 Differences/Advantages between D-Sub 9 and 25
Post by: giacomo on April 20, 2021, 11:26:44 pm
Thank you guys

Would you have an use example for the secondary signals ?

From the article posted by mariush:

"They include secondary DTE, RTS, DCD, TxD, and RxD. The secondary signals are used to optionally connect DTE and DCE equipment."
Title: Re: RS232 Differences/Advantages between D-Sub 9 and 25
Post by: T3sl4co1l on April 21, 2021, 12:24:15 am
I don't think I've ever seen anything that used even half the pins in a DB-25 serial port.  Teletype maybe?  I don't know.  It's one of those odd standards that's so old, it's well, well outlived its original purpose.

As far as I know, when it comes to computers, at least since the...70s or so?, all serial signals are present in both, they are functionally identical.

So yep, just dimensions, and whatever cost the bigger connector adds.

Tim
Title: Re: RS232 Differences/Advantages between D-Sub 9 and 25
Post by: guenthert on April 21, 2021, 01:58:19 am
Well, just because a given device has a DB-25 socket, unfortunately no conclusion can be made about its use.  The original IBM PC had (in addition to RS-232 signals), a current loop interface on its DB-25 socket.  I've seen computers (not necessarily 'PC's) which used DB-25 for parallel printers -- where typically a "centronics" (DB-36) socket has been used.  Apple even put SCSI on DB-25 sockets.
Title: Re: RS232 Differences/Advantages between D-Sub 9 and 25
Post by: xrunner on April 21, 2021, 02:33:14 am
I still have a book somewhere ... I think it's titled "The RS-232 Solution" which told you how to interface RS-232 devices that didn't exactly conform to proper standards ...  :-DD
Title: Re: RS232 Differences/Advantages between D-Sub 9 and 25
Post by: giacomo on April 21, 2021, 02:36:18 am
I still have a book somewhere ... I think it's titled "The RS-232 Solution" which told you how to interface RS-232 devices that didn't exactly conform to proper standards ...  :-DD

Is this one ?

https://jbretro.wordpress.com/2018/11/06/rs232/
Title: Re: RS232 Differences/Advantages between D-Sub 9 and 25
Post by: xrunner on April 21, 2021, 02:38:10 am
I still have a book somewhere ... I think it's titled "The RS-232 Solution" which told you how to interface RS-232 devices that didn't exactly conform to proper standards ...  :-DD

Is this one ?

https://jbretro.wordpress.com/2018/11/06/rs232/

YES! I used it a fair amount back in the day.  ;)
Title: Re: RS232 Differences/Advantages between D-Sub 9 and 25
Post by: notsob on April 21, 2021, 03:27:58 am
It took me a while to understand until I started working on synchronous communications over RS232 - that's where the DCE - DTE comes in force where the DCE is providing the clock source and then all the hand shaking control lines come into force
Title: Re: RS232 Differences/Advantages between D-Sub 9 and 25
Post by: CaptDon on April 21, 2021, 02:08:34 pm
Those 'extra' signals were very useful years ago when printers and modems were much slower. For instance the link between modem and computer could have been 9600 baud, but the phone line connection may have only run at 110 or 300 baud so some handshaking would be required. Also on printers such as the ThinkJet with serial comms. You could dump to the printer at 9600 but it couldn't print that fast so DTE/DCE was important. Also remember RS-232 is only one use for a 25 pin cable. How about hooking an external ZIP drive to the printer port? A 25 pin cable isn't only used for RS-232 standards.
Title: Re: RS232 Differences/Advantages between D-Sub 9 and 25
Post by: HB9EVI on April 22, 2021, 04:53:32 am
in the days before FTTH, DSL and LTE/5G we used dedicated modems with Dsub25 ports for dialup. Before the upcoming of NAS-routers like the Ascend MAX series, which were our first all-in-one (analog and ISDN) dialup solution, we had stacks of Elsa modem like the TLV.34. Of course they already didn't use any other signals than a common 9 pin DSub offered, but physically attached they were over bulky 25pin cables.

the fully wired 25pin Dsub cables were quite multi use; either for EIA232, parallel port like the interconnect to a zip drive, or old fashioned SCSI-2 for scanners or SCSI zip drives; to my memory the JAZ drives already used 50pin interconnects like they were used for HSSI as well

yea, the good old times; not that I wish me back the speeds of those days, but the web was at least still free of Google, FB and all those other creepy business makers; I miss these nerdy days of the internet
Title: Re: RS232 Differences/Advantages between D-Sub 9 and 25
Post by: wizard69 on April 22, 2021, 05:20:58 am
You would be better off finding the manuals for the instruments in question.    First to make sure the 25 pin connector is RS232 at all.   Then to make sure there isn't other information on the connector that has nothing to do with RS232.    In the industrial world you often find the connectors overloaded with other unrelated functions so that some pins might have analog signals, trigger inputs or what ever, that have nothing to do with RS232.    If you are lucky you will have Transmit, Receive and Ground.   

I have some old instruments with the 25 pin connector, a 25 to 9 pin cable would do the job. Just a couple of meters.

It was just curiosity.