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Serial Port Complete
COM Ports, USB Virtual COM Ports, and Ports for Embedded Systems
380 pages; ISBN 9781931448147
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This chapter explores the options for COM ports on PCs, including ports in USB/serial converters and ports in serial servers on networks.
As Chapter 1 explained, COM ports on PCs can include ports on motherboards, expansion cards, USB converters, and serial servers. Other names for COM ports are communications port and Comm port. For each COM port, an operating-system driver assigns a symbolic link name such as COM1, COM2, and so on, which applications use to detect and access the port. Recent Windows editions don’t limit the number of COM ports. Of course, every system has finite resources that limit how many COM ports can be in use at the same time.
The Windows Device Manager shows information about each COM port. To access the Device Manager, right click on My Computer, click Manage, and in the Computer Management pane, select Device Manager. Or click Start and select Settings > Control Panel > System > Hardware > Device Manager. Or save some clicks by creating a shortcut to the file devmgmt.msc in WindowsSystem32.
To view a COM port in the Device Manager, click Ports (COM & LPT), right-click a COM port, and select Properties. The Properties window has several tabs that display the port’s property pages. A vendor-provided co-installer can supply custom property pages for vendor-specific device properties. The pages shown below are typical.
[screenshot] The General tab has basic information about the port.
[screenshot] The Port Settings tab displays the default settings for the port. A USB/serial converter doesn’t use the bit rate, parity, and Stop bits in communications between the PC and the converter, but a driver can send these values to a device that uses the settings on its serial port. For example, a USB/RS-232 converter can set the parameters of its RS-232 port to match values specified on the PC. The Advanced Settings window enables setting buffer properties and the COM-port number. Applications can change the parameters from the default values set in the Device Manager.
[screenshot] The Driver tab enables updating, disabling, and uninstalling the drivers assigned to the port. The Driver Details window lists the drivers the device uses.
[screenshot] The Details tab has a drop-down Property box that provides access to a variety of information about a port, including device instance, hardware, and compatible IDs, class installers and co-installers, and power-state information. In the window shown, the Value pane shows the “Device description” property.
[screenshot] Ports on motherboards or expansion cards have a Resources tab that displays the port addresses and IRQ line the port uses. If you uncheck Use automatic settings, you may be able to change the resources.
A typical COM port on a motherboard or expansion card contains a UART that interfaces to the system bus, typically a PCI bus. Each UART uses a series of eight port addresses. The first address in the series is the port’s base address. Most of these ports also have an assigned IRQ line to carry interrupt requests. A port can use any addresses and IRQ lines supported by the system hardware. These are the addresses and IRQ lines allocated to COM ports in early PCs: [table]
Older ports often have jumpers, switches, or configuration utilities that enable selecting a base address and IRQ line. The setup screens that you can access on boot up can enable configuring motherboard ports. With some hardware, multiple ports can share an IRQ line.
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