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I always thought that the COM port on a classic PC implements the RS-232C serial interface. But I just noticed that the COM port is a DB-25, whereas RS-232C only uses 8 or 9 ports, and therefore tends to use DE-9. Does the DB-25 serial port implement RS-232C, or does it implement some other serial interface, and if so, what standard is this interface?

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  • \$\begingroup\$ Let me google "DB25 rs232" for you and I will come back with the result in a moment \$\endgroup\$ – PlasmaHH Nov 29 '16 at 15:32
  • \$\begingroup\$ You are correct and COM ports earlier than the IBM PC did use RS232c over DB25. But they never used all the pins, and so IBM decided to use a smaller connector and it's been DB9 ever since (until replaced by USB). \$\endgroup\$ – Brian Drummond Nov 29 '16 at 15:34
  • \$\begingroup\$ @BrianDrummond What is the difference between DB-9 and DE-9? \$\endgroup\$ – user628544 Nov 29 '16 at 15:35
  • \$\begingroup\$ Don't know that one! But Wiki says "DE-9 connector, a common type of D-subminiature electrical connector (often called DB-9)" so ... quite possibly, none. \$\endgroup\$ – Brian Drummond Nov 29 '16 at 15:35
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    \$\begingroup\$ The E and B are the shell size, so DB-9 is incorrect, if you want to be picky. But the term is commonly used \$\endgroup\$ – user28910 Nov 29 '16 at 15:53
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All this question requires is a simple Google search to understand the original standards in place.
https://en.wikipedia.org/wiki/RS-232

https://en.wikipedia.org/wiki/D-subminiature

Cannon's part-numbering system uses D as the prefix for the whole series, followed by one of A, B, C, D, or E denoting the shell size, followed by the number of pins or sockets, followed by either P (plug or pins[2]) or S (socket) denoting the gender of the part.
The second letter (A, B, C, D, E) did not denote the number of connections, but the shell size. There was single, high and double density configurations in all shell sizes.

So on a PC you would see DB-25P or DE-9P connectors for RS232 and you see a DB-25S used for the parallel port.

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An early version of the RS-232 spec defines a 25 pin interface, including a second serial channel with full handshaking, and a few unassigned pins. It doesn't appear to specify the type of connector to be used, but DB-25 was common. The early IBM PCs used the DE-9 for the serial port, perhaps due to limited space on the connector panel.

Cannon, who invented the D-subminture connector series, uses the second letter to show the shell size: DA is 15 pin, DB-25, DC-36(or so), DD-50(or so), and DE-9. (I've used the C and D sizes, but am not certain of the pin count.)

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