Why are connection adapters even possible?

[ShawnD1]

This has been bugging me for a while. How is it possible to have an adapter change a parallel connection into a serial connection, or a ps2 connection into a USB connection? If it's possible to emulate a 25 pin parallel connection using only 9 pins (serial), why does the 25 pin connection even exist? Wouldn't that mean 16 of those pins do absolutely nothing?

 

[Aluvus]

Well, it depends.

Parallel uses more pins because data is sent in parallel over multiple pins at once. Adapters aren't just changing the pinout, they have some electronics to reformat the data a bit (edit: no pun intended). There are advantages to both serial and parallel transmission, thus the existence of multiple standards.

PS/2 and USB is a bit of a different case, but you should note that the USB device -> PS/2 port converters are specific to mice that are compatible with them (IOW, not all USB mice will work through such an adapter), but like the older P@/s -> serial adapters weren't compatible with absolutely all PS/2 mice. The converter does a little bit of voodoo to make everything work.

Different connection types often exist for historical reasons, and sometimes quirks of a standard are simply due to the limitations of contemporary equipment.

For an interesting case, look at VGA. 15 pins, sure, but 14 conductors in the cable. 1 is left out and used as a key to prevent the connection of the wrong type of device. 5 separate grounds, 4 pins reserved for Monitor ID, 2 sync signals. Of the 15 pins in the connector, 3 carry video data (red/green/blue). It is telling that DVI-I manages to safely compress all of this down to 5 pins: red, green, blue, sync, and 1 ground. But the extra pins in the original connector helped ensure better signal integrity, and at the time made perfect sense.

 

[Zepper]

I guess someone decided the conversion was necessary and just did it. It's not really that hard if you think about it a bit. they have single chip conversion ICs with a memory buffer built in. Ser to Par, you just let the bits store up in the buffer until you have 8 or more and then send 8 out together - vice versa for Par to Ser.

.bh.