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Options for Adding Serial Ports
The serial port is prevalent on PC's as well as scores of industrial, scientific and consumer devices, making it a convenient starting point for communications. Since RS-232 itself is only suited to short connections (50 ft. max.) many applications require that it be adapted to fit requirements. RS-422, typically used for extending RS-232 communications, provides full duplex communications between two devices. It can also provide one way communications from one master to several slaves. RS-485 communications standard allows cable lengths up to 4000 feet. It also provides very high noise immunity. Multidrop connections up to 32 different nodes can be made over a single twisted pair cable along with a signal ground connection.
This article summarizes the choices a system designer has when adding a serial (RS-232, RS-422, or RS-485) communications device. There are many different ways to add additional serial ports to your Desktop PC, Palm, or Laptop PC. USB, PCMCIA, PCI and ISA are some of the methods that will be discussed.
The Universal Serial Bus (USB) provides a high data rate and Hot Swap connection for PCs, providing an easy connection to a wide variety of multimedia and network USB devices. Generally considered to be for home or office use, USB is finding its way into Industrial applications.
PCMCIA Serial Ports are yet another way to add serial ports to a PC that has PCMCIA Slots available. Most Laptops today offer only one (if any at all) DB9 type serial ports. A PCMCIA Serial Card is a good way to add a serial port (or ports) to any laptop that is low on available serial ports (RS-232, RS-422 or RS-485).
PCI Cards are becoming more popular as a way to add serial ports to desktop PC's. PCI type slots are becoming the "norm" in standard PC's today, and PCI Serial Cards are an excellent way to add serial ports. The standard Desktop PC may only come with one serial port standard (if any serial ports at all). PCI Serial Cards use Software Drivers to address the ports to the PC.
ISA Cards are yet another way to add serial ports to the older desktop PC's. ISA type slots are becoming scarce in standard PC's today, but are still an excellent way to add serial ports. The ISA Cards are typically addressed using dip switches and shunts or jumpers to set IRQ's for each port. No Software Drivers are required with ISA style serial cards.
Palm OS PDA's may be connected to RS-422 or RS-485 data circuits using Palm adapters. These adapters offer total portability for collecting data and field service chores. The Palm device and the adapter will usually slip into a shirt pocket. Some models use the PDA's connecting cable, some include a cable and some are cradles that allow the PDA to nest in place and connect directly to the Palm base socket. PalmTerm Freeware allows you to transmit and receive data from the Palm serial port.