4 ms·

In 1990, RS-232 used 25-pin ports (DB25), and the DB9 ports were "new". Why they used 25-pin ports for RS-232 I'm not sure, since only a fraction of the lines

In 1990, RS-232 used 25-pin ports (DB25), and the DB9 ports were "new". Why they used 25-pin ports for RS-232 I'm not sure, since only a fraction of the lines (less than 9) were actually used, probably some silly historical reason. 25-to-9-pin adapters were very common then. The DB25 connector was also used for parallel ports as you said, which caused confusion sometimes. But on a standard PC, the printer port (parallel) was a 25-pin female port, whereas the serial ports (25 or 9 pin) were male.

parker_mountain 4y ago
I guess around 1995 then? Whenever they switched to DB25 to DE9 for serial. > probably some silly historical reason original rs232 spec had a lot more signals and capabilities that very few consumer products implemented. So you were expected to have the full connector, even if the lines weren't hooked up.
- midoridensha 4y ago
  >I guess around 1995 then? Whenever they switched to DB25 to DE9 for serial. There was a transition during the 90s where the DB25s were phased out and replaced with DE9s. Computers became more commonplace and popular, and they also became smaller, so the bulky 25-pin port fell out of favor.
  - hulitu 4y ago
    The bulky 25 pin fell out of favor because some of its pins function was implemented in SW (new SW appeared which used SW flow control).
    - II2II 4y ago
      Hardware flow control was retained with DE-9. The lines that were dropped included an entire secondary communications channel and timing signals, the sorts of features that I doubt saw much use in personal computers in the 1980's (when it was mostly used for things like modems and printers) and would see even less use today (when it seems to be mostly used for diagnostics ports).
- Gordonjcp 4y ago
  > I guess around 1995 then? Whenever they switched to DB25 to DE9 for serial. 1984.
deleted 4y ago
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joezydeco 4y ago
I own a wacky serial cable that has both DB25 and DB9 connectors on both ends and it has saved my ass more than once in the last decade or so. I'm never giving it up.
fsckboy 4y ago
> DB9 port trivia: believe it or not, it's actually called DE-9
- teddyh 4y ago
  Yes, but usage of the incorrect “DB-9” terminology was very widespread, almost to the extent that it could be considered an alternate term.
jchw 4y ago
As far as I understand there were actually lots of different "RS232" variations. Nowadays I believe everything is asynchronous, which makes many of the pins that used to be connected redundant; lines that carried things like clock or additional flow control related signals. I'm not an expert, I just noticed this looking at old NEC computers.
- retrac 4y ago
  RS232 itself is a well-defined spec. But you're right. An lot of equipment existed that was only vaguely RS232 compatible. Some of it even predates RS232. For example, the old electromechanical teletype machines send and receive something close enough to RS232 line signalling that you can connect them to a modern computer's RS232 port, if you wire up a tiny analog circuit to convert from telegraph-style current impulses to RS232-style voltage levels. Just set the UART to 50 bps with 5 bit data and 2 stop bits. And don't send anything too soon after a carriage return! Not all UARTs can handle those settings, but that's why such options were in terminal programs back in the day.
  - ghaff 4y ago
    DB25 ports were also used for different (although related) standards like RS-422.
  - dfox 4y ago
    One particular thing to note is that on anything IBM PC related the DB25 RS-232 connector has the wrong gender (apparently so that it can be connected directly to modem with DB25 extension cable instead of 1:1 M-M cable as would be expected for RS-232).
Gordonjcp 4y ago
DE9 ports - get the terminology right! - were introduced on the IBM PC/AT in 1984 so they weren't especially new in 1990. Indeed, by 1990 DB25 serial had pretty much vanished from PCs, and only existed as a thing that was a pain in the arse to connect to over existing cabling.
- HeyLaughingBoy 4y ago
  > DE9 ports - get the terminology right! Warms my heart to see that I'm not the only person who's a stickler for this :-)

parker_mountain 4y ago

I guess around 1995 then? Whenever they switched to DB25 to DE9 for serial. > probably some silly historical reason original rs232 spec had a lot more signals and capabilities that very few consumer products implemented. So you were expected to have the full connector, even if the lines weren't hooked up.

midoridensha 4y ago
>I guess around 1995 then? Whenever they switched to DB25 to DE9 for serial. There was a transition during the 90s where the DB25s were phased out and replaced with DE9s. Computers became more commonplace and popular, and they also became smaller, so the bulky 25-pin port fell out of favor.
- hulitu 4y ago
  The bulky 25 pin fell out of favor because some of its pins function was implemented in SW (new SW appeared which used SW flow control).
  - II2II 4y ago
    Hardware flow control was retained with DE-9. The lines that were dropped included an entire secondary communications channel and timing signals, the sorts of features that I doubt saw much use in personal computers in the 1980's (when it was mostly used for things like modems and printers) and would see even less use today (when it seems to be mostly used for diagnostics ports).
Gordonjcp 4y ago
> I guess around 1995 then? Whenever they switched to DB25 to DE9 for serial. 1984.

deleted 4y ago

[deleted]

joezydeco 4y ago

I own a wacky serial cable that has both DB25 and DB9 connectors on both ends and it has saved my ass more than once in the last decade or so. I'm never giving it up.

fsckboy 4y ago

> DB9 port trivia: believe it or not, it's actually called DE-9

teddyh 4y ago
Yes, but usage of the incorrect “DB-9” terminology was very widespread, almost to the extent that it could be considered an alternate term.

jchw 4y ago

As far as I understand there were actually lots of different "RS232" variations. Nowadays I believe everything is asynchronous, which makes many of the pins that used to be connected redundant; lines that carried things like clock or additional flow control related signals. I'm not an expert, I just noticed this looking at old NEC computers.

retrac 4y ago
RS232 itself is a well-defined spec. But you're right. An lot of equipment existed that was only vaguely RS232 compatible. Some of it even predates RS232. For example, the old electromechanical teletype machines send and receive something close enough to RS232 line signalling that you can connect them to a modern computer's RS232 port, if you wire up a tiny analog circuit to convert from telegraph-style current impulses to RS232-style voltage levels. Just set the UART to 50 bps with 5 bit data and 2 stop bits. And don't send anything too soon after a carriage return! Not all UARTs can handle those settings, but that's why such options were in terminal programs back in the day.
- ghaff 4y ago
  DB25 ports were also used for different (although related) standards like RS-422.
- dfox 4y ago
  One particular thing to note is that on anything IBM PC related the DB25 RS-232 connector has the wrong gender (apparently so that it can be connected directly to modem with DB25 extension cable instead of 1:1 M-M cable as would be expected for RS-232).

Gordonjcp 4y ago

DE9 ports - get the terminology right! - were introduced on the IBM PC/AT in 1984 so they weren't especially new in 1990. Indeed, by 1990 DB25 serial had pretty much vanished from PCs, and only existed as a thing that was a pain in the arse to connect to over existing cabling.

HeyLaughingBoy 4y ago
> DE9 ports - get the terminology right! Warms my heart to see that I'm not the only person who's a stickler for this :-)