What's the difference between "COM", "USB", "Serial Port"?
I am confused about the these 3 concepts.
My understanding is, Serial Port usually means RS-232 compatible port (RS = Recommended Standard). USB stands for Universal Serial Bus. So its name contains serial port, does it support RS-232? What does the Universal mean?
And what does COM port mean?
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Some understanding from Hans' answer:
To reduce effort, device manufacturers usually make their device can behave like a serial port device as well. This relies on the the fact that many OS and language libraries have already included serial port communication support. Though such support is no comparable to a real matching device driver.
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A good reference doc about Serial Port HOW-TO.
And btw, the Linux Document Project is really useful.
Answer
Serial port is a type of device that uses an UART chip, a Universal Asynchronous Receiver Transmitter. One of the two basic ways to interface a computer in the olden days, parallel ports were the other way. Serial is simple to hook up, it doesn't need a lot of wires. Parallel was useful if you wanted to go fast, typ 8 times faster than serial, but cables and connectors were expensive. Parallel I/O has completely disappeared from computer designs, caught up by tremendous advances in bus transceivers, the kind of chip that can transmit an electrical signal down a wire.
COM comes from MS-Dos, it is a device name. Short for "COMmunication port". Computers in the 1980's usually had two serial ports, labeled COM1 and COM2 on the back of the machine. This name was carried forward into Windows, most any driver that simulates a serial port will create a device with "COM" in its name. LPT was the device name for parallel ports, short for "Line PrinTer".
RS-232 was an electrical signaling standard for serial ports. It is the simplest one with very low demands on the device, supporting just a point-to-point connection. RS-422 and RS-485 were not uncommon, using a twisted pair for each signal, providing much higher noise immunity and allowing multiple devices connected to each other.
USB means Universal Serial Bus. Empowered by the ability to integrate a micro-processor into devices that's a few millimeters in size and costs a few dimes. It replaced legacy devices in the latter 1990s. It is Universal because it can support many different kinds of devices, from coffee-pot warmers to disk drives to wifi adapters to audio playback. It is Serial, it only requires 4 wires. And it is a Bus, you can plug a USB device into an arbitrary port. It competed with FireWire, a very similar approach and championed by Apple, but won by a land-slide.
The only reason that serial ports are still relevant in on Windows these days is because a USB device requires a custom device driver. Device manufacturers do not like writing and supporting drivers, they often take a shortcut in their driver that makes it emulate a legacy serial port device. So programmers can use the legacy support for serial ports built into the operating system and about any language runtime library. Rather imperfect support btw, these emulators never support plug-and-play well. Discovering the specific serial port to open is very difficult. And these drivers often misbehave in impossible to diagnose ways when you jerk a USB device while your program is using it.