Options for Adding Serial Ports The serial port is prevalent on PC's
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
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.