RS232 Differences/Advantages between D-Sub 9 and 25 - Page 1
Forum section: electronics / beginners

author: giacomo
author_rank: Regular Contributor
timestamp: April 20, 2021, 10:30:48 pm
content: 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

author: mariush
author_rank: Super Contributor
timestamp: April 20, 2021, 10:51:40 pm
content: See the pinouts on this page, along with descriptions and text:https://www.virtual-serial-port.org/article/what-is-serial-port/rs232-pinout/

author: TK
author_rank: Super Contributor
timestamp: April 20, 2021, 10:54:43 pm
content: Quote from: giacomo on April 20, 2021, 10:30:48 pmWhat devices are you connecting, what distance?

author: giacomo
author_rank: Regular Contributor
timestamp: April 20, 2021, 11:00:37 pm
content: 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.

author: mvs
author_rank: Frequent Contributor
timestamp: April 20, 2021, 11:15:26 pm
content: 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.

author: giacomo
author_rank: Regular Contributor
timestamp: April 20, 2021, 11:26:44 pm
content: Thank you guysWould 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."

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 21, 2021, 12:24:15 am
content: 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

author: guenthert
author_rank: Frequent Contributor
timestamp: April 21, 2021, 01:58:19 am
content: 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.

author: xrunner
author_rank: Super Contributor
timestamp: April 21, 2021, 02:33:14 am
content: 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 ...

author: giacomo
author_rank: Regular Contributor
timestamp: April 21, 2021, 02:36:18 am
content: Quote from: xrunner on April 21, 2021, 02:33:14 amIs this one ?https://jbretro.wordpress.com/2018/11/06/rs232/

author: xrunner
author_rank: Super Contributor
timestamp: April 21, 2021, 02:38:10 am
content: Quote from: giacomo on April 21, 2021, 02:36:18 amYES! I used it a fair amount back in the day.

author: notsob
author_rank: Frequent Contributor
timestamp: April 21, 2021, 03:27:58 am
content: 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

author: CaptDon
author_rank: Super Contributor
timestamp: April 21, 2021, 02:08:34 pm
content: 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.

author: HB9EVI
author_rank: Frequent Contributor
timestamp: April 22, 2021, 04:53:32 am
content: 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 wellyea, 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

author: wizard69
author_rank: Super Contributor
timestamp: April 22, 2021, 05:20:58 am
content: 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.Quote from: giacomo on April 20, 2021, 11:00:37 pm

author: jeremybh1
author_rank: Contributor
timestamp: July 15, 2024, 05:06:13 am
content: Hi everyone. Kind person Christopher S. emailed me directly and reminds me to refresh links to my personal scans of RS232 books.This thread 'comes up top in google' so I figured it was best to reply despite the topic age as clearly people are searching.The good news I have archived further titles and made them available online. Let's start with the root directory link:https://drive.google.com/drive/folders/0BweO6Y13Cmc4VlFKYkx2MWxWU1k?resourcekey=0-ZImjHQr1ZkyLMpwZoW68XA&usp=sharing1) The RS-232 Solution by Joe Campbell.pdfISBN 0895881403Browse folders to: Scanned Book and Magazine Collection > Sybexor search (may require a Google account sign in) "The RS-232 Solution by Joe Campbell"2) Computer Peripherals That You Can Build by Gordon Wolfe.pdfChapter 3 "Parallel Interface Standards — Serial Interface Standards— Data Input Methods—A Universal ParallelInterface for All Busses—Interfacing to RS-232 Serial Lines"Browse folders to: Scanned Book and Magazine Collection > General Vintage Computing Books - huge collectionor search (may require a Google account sign in) "Computer Peripherals That You Can Build by Gordon Wolfe.pdf"Then just over on my sister archive:https://drive.google.com/drive/folders/1w1-LPNvI2xK8q6V-RK-CpwR4FsHrR6do?usp=sharing3) Essentials of Data Communications - Tektronix.pdf4) Data Communications Handbook by Alan J Weissberger (1977).pdf

author: Andy Chee
author_rank: Super Contributor
timestamp: July 15, 2024, 10:47:02 am
content: Also for anyone who hasn't seen it, Ben Eater has a great video:https://youtu.be/AHYNxpqKqwo?t=89

author: tggzzz
author_rank: Super Contributor
timestamp: July 15, 2024, 11:25:46 am
content: Quote from: jeremybh1 on July 15, 2024, 05:06:13 amNot possible to browse; trying to download folder leads to the folder being 5% zipped after 60s.Not going to bother to sign in with a gurgle account.

author: m k
author_rank: Super Contributor
timestamp: July 15, 2024, 04:30:00 pm
content: 9 pin is most likely new compared to the possibility of 25 pin.(D25 can have alternative channels)There fundamental difference can be accepted or required voltage levels.Old machines had 1488/1489 driver/receiver chips that require higher signal voltages and so can't go very far.That was a real problem and so the standard was upgraded, new one is not 232 anymore.

author: Andy Chee
author_rank: Super Contributor
timestamp: July 15, 2024, 04:37:45 pm
content: Quote from: tggzzz on July 15, 2024, 11:25:46 amI initially clicked on the download arrow icon, but that didn't do anything.Turns out you need to click on the actual title text of the folder, and eventually the title text of the document.