Getting From Here to There: Serial Communications Systems
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 timeclock 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.
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