Some serial devices are rare and nonstandard implementations of RS-232 ports. The problem with the standard implementation of RS-232 serial devices is that it's difficult to add ports beyond the two included on most motherboards. Adding third and fourth ports presents only minor difficulties. Most add-on ports come configured to "share" interrupts with the first two ports, but it's necessary to reconfigure these boards to use unique IRQs because Linux doesn't normally work well with this arrangement.
Note Internal modems traditionally include serial port hardware along with modem hardware. Adding such a modem works just like adding a serial port. It's sometimes helpful to disable an unused motherboard serial port before adding an internal modem, so that the internal modem can take over the identity of one of the first two regular ports. This can normally be done in the computer's BIOS setup screens. Many modern internal modems don't have serial port hardware. These devices are often called WinModems, controllerless modems, or software modems (although the first term technically applies only to 3Com products). There are Linux drivers for a few such devices—check http://www.linmodems.org/ for details. Many don't work under Linux, however, and so should be avoided.
The problem with using a unique IRQ for each serial port is that the x86 architecture allows for only 15 interrupts, and some are required for other devices, so it's not possible to assign unique IRQs to more than a handful of serial ports. To overcome this problem, some companies have developed multiport serial cards, which place several RS-232 ports on one card and one IRQ. Because these devices use nonstandard hardware, they require their own unique drivers. If you have need of such a device, check the Linux kernel configuration area for character devices (Figure 11.2). Depending upon the type of device, you may need to select the Extended Dumb Serial Driver Options item or Nonstandard Serial Port Support. In either case, you must select appropriate options for your specific board. You should study these options before buying the hardware, so you're sure you get something supported in Linux. Another option if you need many RS-232 serial ports is to use USB-to-RS-232 adapters, which can provide many RS-232 ports on a single IRQ.
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