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I'm sure at least some of you remember the pre PS/2 (which introduced the DB-9 connector) serial port good old days, where "all" you had to know were the bit rate, number of data bits, number of start bits, number of stop bits, hardware or software flow control, and whether you needed to use a straight through or null-modem cable and which type cable a given cable was. Oh, and whether power on one of those pins was necessary or a guarantee of magic smoke. A set of gender adapters were good to keep around. We won't consider all the reasons 25 (or even 9) wires were needed for a "serial" port.
Wild cards were cables that omitted "unused" wires (including various flow control or modem specific lines) and proprietary hijacking of "unused" pins. Not using those special cables when needed or using them when they weren't could lead to all kinds of puzzlements.
The arrival of the 9 pin serial port not only required the duplication of all the hardware above, but a set of 25 pin to 9 pin adapters with all possible gender combinations.
But, since you already had to know what you were doing and had all the necessary equipment, it was no big deal to use a device as either a peripheral or a host, something it took a long time for USB to sorta kinda get right.
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