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Serial port connector conventions - Why & When did they change? What's the history?

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WimWalther

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For the last 25-30 years, we typically find D-sub 9pin Male connectors used for computer-side serial port connections. Older machines would occasionally have both 9 & 25pin Male connectors though some would only have the 25pin Male.

But further yet, back in time, and it seems that D-Sub 25pin Female connectors were a standard.. I see this setup on the VIC-1011A RS-232 adapter for C64 & VIC-20 as well as the Tandy 1000EX / HX RS-232 PLUS card - despite the fact that those machines are from the late 80s.

So who put these various standards in-place, assuming there were any. Well obviously, there were certainly some as the numeric pinout of the different size & gendered ports seem to usually match up.
 
Back in the day there were Standards related to computers and modems and whether they had male or female connectors and whether they required a straight through or cross-over cable.

However, with the creation of 'computers for the masses', the Standards sort of went out of the window I am afraid.

Dave
 
As usual, I would blame IBM's dominance. The IBM PC's serial port card had a 25-pin male connector, and the rest of the industry eventually followed suit. And then the IBM AT came with a 9-pin male serial port, and that eventually that became the standard, especially for use with a mouse.
 
The original 1960s EIA RS-232 standard was a DB25. The DB9 came later, I believe it's TIA-574.
 
I imagine that with the unused pins or excessive number of grounds in a db25 connection that they came up with a way to consolidate into a smaller db9 connection to save space on the very limited backbone plate of cards and backs of computer cases. I believe the db25 was a carry over from telephone systems much like the centronics connectors and RJ series jacks and plugs were.

Lets all be glad that they didnt decide to go with Cannon and Bendix plugs that military gear tends to use :D
 
Lets start with the 9-pin connecter. Its not an official standard. Its and IBM invention but usually gender is as below.

The RS232 and V24 standards define two interfaces, DCE and DTE.

DCE => Data Communications Equipment => think modem => Should have a FEMALE connector.
DTE => Data Terminal Equipment => Think Terminal => Should have a male.

so in a typical setup your would have:-

Terminal = <DTE> ==== <DCE> <Modem> <---- Comms Line ---> <Modem> <DCE> ======= <DTE> <Mainframe/mini Computer>

Some personal computers may have female and/or DCE interfaces so they could be directly connected to a mini/mainframe with a straight through cable but really this is wrong/non-standard.
 
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On a real standard-conforming D-sub 25 pin connection, very few of the pins are grounds of any sort:
image30.gif


Also, this standard.

But then, I suspect very few readers of this forum have had to struggle with the signals of a Bell 408 modem. (before someone points it out, I know that it's not a "DB-9" connector but rather a "DE-9" one.)
 
About the 25-pin serial connector: I don't believe I ever used a serial device with a 25-pin connector. I never had a mouse that used one. I'm kind of thinking they were more or less in the commercial area. I have a some 25-pin to 9-pin connectors in my do-dad box that have never been used.
 
Agent Orange said:
About the 25-pin serial connector: I don't believe I ever used a serial device with a 25-pin connector. I never had a mouse that used one. I'm kind of thinking they were more or less in the commercial area. I have a some 25-pin to 9-pin connectors in my do-dad box that have never been used.
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My first serial mouse had a 25, I had to use a 25 to 9 adapter to use it later on a 286 clone.
 
Terminals, computers, etc. all used DB25 connections for EIA serial before the PC era. Look at any vintage S100 system, for example. I've still got a bunch of DB25 breakout/jumper boxes, for example--and no DE-9 ones.
A couple of my systems are actually equipped with the secondary channel stuff.

I even have a couple of cables that use the brown bakelite (Bell Standard) backshells.
 
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Any PC prior to the IBM 5170 would have had DB25 RS-232 ports.
 
Plasma said:
Any PC prior to the IBM 5170 would have had DB25 RS-232 ports.
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Not any PC. That was 1984 and I don't recall any Tandy PC's having any type of mouse port back then. Serial ports were usually add-in cards or mobo plug-ins.
 
You are misinterpreting my post. Any PC with RS-232 would have had DB25 ports. Also the Tandy 1000 came out after the 5170.
 
And the first Tandy 1000 serial cards (the original ones, not later or aftermarket ones) had DB-25 ports... which, as the OP noted, were gender-bent compared to “IBM standard” cards. (They followed the gender that Tandy had used on earlier TRS-80 machines.)

Anyway, the 9 pin serial port was invented by IBM and got popular for one simple reason: you can’t quite fit two DB25 connectors onto a PC slot back panel. The smaller plug makes combo cards a lot easier, and while it does omit some of the more obscure functions it has enough for most consumer equipment.
 
Eudimorphodon said:
And the first Tandy 1000 serial cards (the original ones, not later or aftermarket ones) had DB-25 ports... which, as the OP noted, were gender-bent compared to “IBM standard” cards. (They followed the gender that Tandy had used on earlier TRS-80 machines.)

Anyway, the 9 pin serial port was invented by IBM and got popular for one simple reason: you can’t quite fit two DB25 connectors onto a PC slot back panel. The smaller plug makes combo cards a lot easier, and while it does omit some of the more obscure functions it has enough for most consumer equipment.
Click to expand...
The Tandy 1000SX did not come with any serial port. Most, if not all serial ports of that era were add-ons or aftermarket. My 1000SX has never had a serial port but does have a bus mouse.
 
Chuck(G) said:
On a real standard-conforming D-sub 25 pin connection, very few of the pins are grounds of any sort:
...

Also, this standard.

But then, I suspect very few readers of this forum have had to struggle with the signals of a Bell 408 modem. (before someone points it out, I know that it's not a "DB-9" connector but rather a "DE-9" one.)
Click to expand...
I suspect that if the OP would like to attribute "first cause" responsibility then the answer will be found in some Western Electric document, WECo being the hardware supplier to AT&T (Bell Telephone) -- hence the Bell 408 modem and its interface. There has been a lot of evolution since then as the requirements were simplified and shifted. Now just three pins on a 0.1" SIL header is deemed to be enough, and there isn't any standard for ordering them ... and as for voltage levels ... :-}.
 
Well, the other aspect is that many of the early high-speed modems operated in synchronous mode, hence the need for pins dedicated to clocks.

And I had a brain-bubble--the modem is the Bell 208 (4800 bps). The Bell 209 could do a blazing 9600 bps over leased lines (have very deep pockets).
 
Agent Orange said:
The Tandy 1000SX did not come with any serial port. Most, if not all serial ports of that era were add-ons or aftermarket.
Click to expand...

I never said they came with them built in. But if you bought the serial port card Tandy sold in the store for the original 1000, catalog #25-1006, it is how I described, 25 pin with an opposite gender plug from that found on the 25 pin plug on IBM’s original serial option card for the 5150.

(* This also goes for the “Plus” card Tandy sold for the EX/HX.)

Anyway, the SX didn’t come out until late 1986 so if you *had* bought a third party serial card for yours I suppose, yeah, there’s a pretty good chance you’d get a nine pin one. Although 25 pin ports were still around then. It wasn’t unusual into the 1990’s for those I/O plates to plug into on-motherboard or multi-IO cards would have one each of nine and 25 pin ports on them. I’m reasonably confident I found 25 pin dongles in the boxes with Pentium motherboards.
 
By the 386 era it became common for COM1 to be a 9-pin port for use with a mouse, and COM2 to be a 25-pin port for use with a modem (or, rarely, a serial printer).

And the Macintosh 128K had a 9-pin serial port before the IBM AT did, although it was RS-422 and female, not RS-232C and male.
 
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