What do you do when
you need to make a dedicated low speed data connection between two
places? Need to monitor a PLC on the factory floor from an office area,
connect a host PC to a time clock or alarm system, or make a connection
between buildings?
The asynchronous serial connection has been the
workhorse of low bandwidth communications for decades. For control,
monitoring and low volume data transfer asynchronous serial provides a
low cost, low development solution. The RS-232 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, many applications require
that it be adapted to fit requirements. This article summarizes the
choices a system designer has when selecting a serial communications
system.
RS-232 or, more currently, EIA-232 uses a single ended, bipolar
voltage signal. Voltages typically swing from -12V to +12V with respect
to signal ground. Suitable for low noise environments and distances
below 30.5 meters (100 feet), RS-232 is commonly used for the desktop
modem and mouse. An increase in modem speeds has spurred an effort to
increase RS-232 data rates by chip vendors. Transceivers capable of 460k
baud and higher are now available, although the actual throughput gains
of running higher data rates than 115.2k baud on interrupt based systems
is questionable at best.
RS-422 is suited to longer distance communications, up to 1200 meters
(4000 feet) without repeaters. Using a balanced differential pair
results in higher noise immunity than EIA-232. The differential voltage
provides a valid signal down to 200 mV. Two wires are required for each
signal in addition to a signal ground conductor. RS-422 is most commonly
used for point-to-point communications, although up to 10 receivers may
be connected to a single transmitter.
RS-485 is also suited to longer distance communications, up to 1200
meters (4000 feet) without repeaters. Again, a balanced differential
pair is used for higher noise immunity than EIA-232. Voltage levels are
identical to RS-422. In addition, RS-485 offers a multidrop capability,
up to 32 nodes can be connected. The multidrop feature also allows
"two-wire" (in addition to signal ground) half-duplex data connection to
be made.
Current Loop is the oldest method of connecting serial devices,
dating back to Teletype machines. Typically a loop current of 20 mA
indicates a marking condition and 0 mA represents a space.
Unfortunately, there is no true standard for current loop, so switching
thresholds, voltage requirements and connections vary widely. A well
designed current loop system has high noise immunity, and is inherently
optically isolated. However, speeds are generally low and the lack of a
standard makes connectivity between manufacturers spotty.
Fiber Optic communications is growing in popularity as another low
bandwidth serial solution. While costs are still higher than copper
solutions, fiber optic links benefit from optimum isolation, noise
immunity, and distances up to several miles. Installation of fiber optic
cabling requires more care than copper, and repairing damaged cabling is
difficult.
RF wireless has become more affordable in recent years and the
adoption of spread spectrum technology has further improved performance.
Modules to convert RS-232 signals to RF can be used for low to medium
data rates. Range is limited, typically several hundred feet, although
units are available that reach several miles with appropriate antennas.
Higher power units are also available but require an FCC site license to
operate. The range and performance of RF wireless is highly dependent on
the physical and electrical environment and costs are high. If mobility
is required or wire isn't possible, wireless has become a viable
solution.
|
COM Type |
Pros |
Cons |
|
RS-232 |
Low cost
Widely available |
Limited distance
Poor noise immunity |
|
RS-422 |
Good noise immunity
Long distance |
May require additional isolation to prevent
ground loops |
|
RS-485 |
Good noise immunity
Long distance
Multidrop capability |
May require additional isolation to prevent
ground loops |
|
Current Loop |
High noise immunity
Built-in isolation |
Low speed
Compatibility problems
Rarely used in new designs |
|
Fiber Optics |
Ideal noise immunity
Long distance |
More care required in installation
Higher initial cost |
|
RF Wireless |
High mobility |
High cost
Sensitive to environmental variables |
For more information on serial ports, we recommend
Serial Port Complete and
C Programmer's Guide to Serial Communication.
RS232, RS422, RS485
|
