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Why do old connectors have so many unused pins?


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Sometimes old computers included ports that only used a portion of the pins on a connector, leaving many pins doing nothing. Why did they do this?

 

Just to save money using a standard port? Or are there technical reasons for choosing a port (DB25, DB23) knowing that many pins will go unused?

 

 

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I think a lot of them do it because they had a minimum number of pins that were required for the buss and picked the first connector size that could accommodate that many signals.
With slots, keep in mind there weren't as many connectors available when some machines were designed as there are today and they may have wanted to leave some room for future expansion.

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Just to save money using a standard port? Or are there technical reasons for choosing a port (DB25, DB23) knowing that many pins will go unused?

 

A little from column A, a little from column B.

 

Just like USB or similar ports today, even back in the early days of computing there were connectors that were basically standard across the industry. This did drive down costs and it also let people swap cables between different computers for a lot of things (the video output port on the Atari 8 bit line, C64, and TI 99/4A is standard 5 pin DIN and if you're connecting composite, you can use the same cable). But different computers might use more or fewer pins depending on the features they would support over that port. For example, some computers would output both composite and S-video over their video output port, while others would only output composite. So there's an extra unused pin.

 

I think some ports were also chosen in case of future upgrades, so they wouldn't need to replace the port later and potentially make for all sorts of incompatibilities. But I can't think of a specific example of that, it's one of those things I just remember reading somewhere, sometime and it's now in my head. It makes sense to me, though.

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As a child, still playing with Legos and Tonka Trucks, I never could understand why there were big and small RS-232 connectors.

 

 

well the biguns actually had the cabling for 2 RS232 lines and all its various detect and handshake lines, over the years its now 3 as almost no one runs a 300 meter rs232 line anymore

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Some computers like the Amiga, used an extra pin or two for voltage. Could power certain peripherals without needing to add another brick to the wall. :P

 

I for one, am glad the connectors are as large or wide as they are. Much more robust and reliable than today's chinsey shit. Especially hate HDMI at this point. Finding I need to reseat various connections a few times a month out of necessity when the signal drops out. sigh

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Pretty much every pin in a DB25 serial port has a use. However, in most applications, only a few are really needed.

 

 

ONE Connector, to Rule them ALL!!!

 

 

The DE-9 was just the Most Common Pins used for Modems, Printers and Barcode Readers/Scanners/Strip Readers..

 

 

MarkO

Edited by MarkO
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Sometimes old computers included ports that only used a portion of the pins on a connector, leaving many pins doing nothing. Why did they do this?

 

Just to save money using a standard port? Or are there technical reasons for choosing a port (DB25, DB23) knowing that many pins will go unused?

I think a lot of them were reserved for future use. You could do basic RS232 with just two wires. In college my friend and I did just that. ran two small thin wires that nobody noticed between our rooms to play games that worked with RS232

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The design specs for those ancient and large ports were hammered out in detail. No doubt by committees. :lol: Every line on a Centronics parallel or RS232 serial had a defined purpose. When it came to supporting different peripherals like printers, modems, plotters, data acquisition, etc. over the same port, it was good to have that built-in and ready. Back then, computers were larger and it just wasn't necessary to reconfigure the ports down to what was actually used in the majority of cases. That, and most of the peripherals themselves used those same ports, so it was a case of one hand washing the other. And then there were those companies like Apple, who simply had to be different, and that included their choice of connectors. ;) Forcing third-party builders to make two versions to support Apple and the Rest of Us. And us poor Mac folk to have to upgrade because the old forn factor got dropped like so much old news. My old homebrewed TI99 to Mac RS232 cable has no less than three connectors/adapters I added to the Mac end, to suit their ever-evolving sense of purpose and form.

-Ed

Edited by Ed in SoDak
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In many cases they used whatever was available and cheap. According to wikipedia, "Wang happened to have a surplus stock of 20,000 Amphenol 36-pin micro ribbon connectors that were originally used for one of their early calculators.". Only 21 pins were used, seven for data, for their centronics printer port. IBM likely used the female db25 for their IBM PC printer port because it was already commonly available, used with serial devices for many years. How many pins on an rs232 db25 are defined, less than half?

Edited by mr_me
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